IT8323251A1 - Signal processing network for an automatic kinescope polarization control system - Google Patents

Description

DOCUMENTATION

BOUND

SYSTEM. FOR .AUTOMATIC CONTROL

OF POL ARI.ZZAZIONE_DL. A CINESCOPE

_ COMPENSATION FOR VARIATIONS IN IMPACT

DENCE OF A REVEALING POINT

The present invention relates to a complex signal processor used in a system for automatically controlling the level of a current representative of a black image conveyed by a device, for displaying images associated with video signals and consisting of for example, from the kinescope of a television receiver. Furthermore, the invention relates to a complex for compensating the impedance variations in correspondence with a veiling point from which a signal representative of the nerb current is derived, so that the impedance variations cannot negatively alter the operation. of the subsequent control circuits to which the detection point is coupled.

Television receivers sometimes use an automatic kinescope bias control system AKJ3 _ (Aut.omat i c kKHEHS kinescope bias) to establish, _autimati cameni e, g appropriate levels of the representative black image current for each electron gun. of the kinescope, As a result of this operation, the reproduced images are prevented. from the . kinescope are negatively influenced by changes that may occur in the parameters of the kinescope and caused, for example, by phenomena of aging and by effects associated with temperature changes. A type of AKB system? st disclosed in U.S. Pat. 4. 263. 622 Werner Hinn, filed "Automatic Kinescope Biasin SysfcXa System".

An AKB system operates, typically during image blanking intervals, i.e. when the kinescope carries a small blanking current representative of the black level, in response to a reference voltage representative of the video signal information of the black. This current is monitored by the AKB system, in order to generate the nescope bias correction voltage representing the difference between the detected level of the current. black and a desired level of the black current. la., correction voltage. is applied, to the kinescope, for example through the processing circuits of the video signals preceding. kinescope with a sense to reduce the difference. Typically, the voltage of EEfd & KK correction is applied to a polarization control input of a current coupled kinescope driver amplifier which feeds the output video signals having an appropriate level, in order to directly control an electrod.or cathode of control of the intensity? of the kinescope. Does the correction voltage change the output bias voltage of the? amplify the pilot hours, thereby modifying the bias voltage of the cat in order to obtain the desired level of the black cathode current.

In .one system. AKB of the type described in the Hinn patent referred to above, the control circuits respond to a periodically derived signal, with a sentative amplitude of the black cathode current level. The derived signal has a predicted level different from zer9 when the black current level is correct and different levels (for example, more or less positive) when the black current level is too high. or too low. The derived signal is developed at a detection point which is connected to control circuits including level fixing and sampling networks for the development of a kinescope polarization correction signal. in? accord'o with the fullness of the derived signal. Does the bias correction signal increase or decrease accordingly. to what is required, to maintain a correct level rx? rx of the current of the ner

In the course of the present discussion it is recognized that the control circuits to which the detection point is coupled can be adversely affected when the flat detection point which the signal representative of the black current is derived exhibits variations in ? nBa in function of the polarization level of the kinescope pilot. Consequently, an apparatus is described for substantially canceling the effect of these impedance variations on the control circuits. The apparatus described also advantageously increases the immunity of a fastening network of the? associated with the level of control, with respect to the spurious signals including locally generated interference, such as _po - = _ would otherwise distort and obscure the polarization correction signal.

. In accordance with the present invention, an apparatus for automatic kinescope polarization control includes a capacitor sator for coupling a derivative marker, representative of the black current level of the kinescope. and an auxiliary signal having a predetermined amplitude and direction, at the input of a sampling amplifier. The source of the derived signal has a variable output impedance, proportional to the magnitude of the kinescope polarization level. ac cbpplamento impedance. The coupling impedance was found to be high with respect to the variable output impedance so as to significantly reduce the impedance variations presented at the source of the auxiliary signal from the output of the signal source, derived.

In accordance with a feature. ..of the invention, the ac. coupling capacitor is included. in a fixing network .. The impedance of ????: coupling also increases dsika immunity? of the response of the network, of fixing the level towards the spurious signals.

In the drawing:

1 illustrates a portion of a color television receiver including an AKB system and associated apparatus? H in accordance with the principles of the present invention; And.

Figure 2 illustrates the forms dTon of the signals which are associated with the operation of the system schematized in Figure 1.

In accordance with as shown in Figure 1, the processing circuits IO of the television signals supply the separ components of luminance (Y) and chrominance (C) of a composite color television signal, to a network 12 of: processing of the luminance signals chrominance, the processor 12 includes luminance and chrominance gain control circuits, direct current level setting circuits (including, for example, manipulated black level fixing circuits), color demodulators for. the development of the color difference signals of red, green and <"> blue r-y, g-y b-y, respec / tivamant e and matrix amplifiers to combine these latter signals with the processed luminance signals, so as to allow the generation of the signals representative of the color images, of low level, associated with the color red, the color green and the color blue, r and b, respectively. These signals are amplified and otherwise processed by means of circuits contained in the signal processing networks output video, identified by the references 14 a, 14 b and 14 c, respectively, which feed signals representative of the color images, suitably amplified and of a high level, associated with red, green and blue, R,? and B, respectively, to the respective intensity control cathode electrodes 16a, 16b and I6c of EK color kinescope 15. The networks 14a, 14b and 14c also perform functions related to the AKB operation second Glove will come? described below. The kinescope 15? of the in-line, self-converging electron gun type, having a grid 18, excited in common, associated with each of the electron guns comprising the cathode electrodes 16a, 16b and 16C.

_ Since? in this practical embodiment according to the invention, the processors 14a, 14b and 14c of the output signals are similar, the description which will come? now, reported on the operation of the * computer .14a will result? pu valid for the atife computers 14b and 14C.

The processor 14a includes a kinescope pilot stage comprising an input transistor 20 configured as a common emitter amplifier, which receives the video signal r from the processor 12, through an input resistor 21 and an output transistor 22. , at high voltage, configured as a common-based amplifier which, together with the transistor 20, forms a "casconde" video driver amplifier. The high-level video signal R, suitable for driving the cathode 16a of the kinescope, is developed at the ends of a load resistor 24 present in the rarefied output collector circuit of the transistor 22 An operating supply voltage for the amplifier 20, 22 is supplied by means of a direct voltage source of high value B + (for example, 230 volts). A negative direct current feedback for the driver stage including the transistors 20, 22 is obtained by means of a resistor 25. determined mainly by the relationship between the value of the feedback resistor 25 and the value of the input resistor 21. the feedback network allows to obtain an output impedance of an appropriately low value corresponding to the output of the amplifier 20, 22 and allows to stabilize the operating level, in direct current, in correspondence with the amplifier output.

A detection resistor 30, connected in direct current (DC), in series, with and between the collector-emitter paths of the transist 20, 22, serves to develop a voltage BE:? P in correspondence with a detection node A, relatively low, representing the cathode current level of the kinescope core during CRT cancellation. The re store 30 operates in conjunction with the receiver's AKB system, according to what will come? now described in more detail.

A timing signal generator 40, containing suitable logic control circuits, responds to the periodic signals using the horizontal synchronization frequency (H and the signals, periodic recurring at the vertical synchronization frequency (V), these signals being derived from the deflection circuits of the detector, in order to generate timing signals VA, V ?, Vn, V_ and V capable of controlling the operation of the AKB circuit, during the periodic intervals AKB. Each of the intervals AKB begins just after the term of the vertical retrace interval within the vertical suppression interval and encompasses several horizontal line intervals, also falling within the vertical suppression interval, and during these there is no representative image information associated with the video signal. These temperature signals were represented by the respective waveforms in Figure 2.

Bacando for the moment specific reference to figure 2, can? it can be detected that the reaction time signal Vg, a signal useful for inhibiting the video signals produced by the processor 12, comprises a positive pulse generated just after the end of the vertical retrace interval corresponding to the indicated instant. from ... reference to the form? fc? miax? si o.nda of signal V. The signal d? inhibition V EXISTS for the duration of the interval M B and is applied to an input terminal d? against the suppression of the luminance chrominance processor 12, so that the outputs r, g and b of the processor 12 may have a dc reference level representative of a black image, corresponding to the absence of video signals. This can? to be; are obtained by reducing the gain, as far as the signal of the processor 12 is concerned, to a value equal to zero, through the control circuits of the gain of the processor i? 12, in respect to the signal V ^, and by means of a modification of the direct current output path of the video XH signals, through the direct current level control circuits of the processor 12, in mod so as to produce a reference level representative of the black image at the signal outputs of the processor 12. positive spans three intervals of horizontal line within the vertical blanking interval. The timing signal V_ controls the operation of a circuit testing the level associated with the signal charging function of the SKB system. The timing signal V ^, constituting a sampling control signal, occurs after the signal VC. and the same is used for timing or synchronizing the operation of a sampling and holding circuit, or storage which XXKHE develops a direct current bias control signal, to control the cathode current level of the kinescope. The signal embraces a sampling interval, the beginning of which is slightly delayed with respect to the end of the fixing interval of the level embraced by signal V_, and whose term substantially coincides with the end of the interval AKB . An auxiliary amount V, with a negative trend, coincides with the sampling interval. The signal timing delays shown in Figure 2 are in the order of 200 nanoseconds.

Referring again to figure 1, it must be noted that during the interval AKB, the positive impulse V ^, whose value? of the order of 10 volts polarizes the grid 18 of the kinescope in the direction of AKB intervals, the signal provides the formal polarization, less positive, for the JL8 grid of the kinescope. In response to the positive pulse of the V_ grid, a positive current pulse, phase, analogous, is highlighted in correspondence with cathode 16a, during the corresponding interval 1? grid pulse. The amplitude of the AC Iodic output current pulse thus developed is proportional to the conductivity level of the black current of the cathode, whose value? typically equal to a few microamps.

The positive cathode pulse of u; induced output is highlighted, in correspondence with the collector of transistor 22 and is fed to the base input terminal of transistor 20, through resistor 25, causing an increase

rent of transistor 20, while the cathod pulse of. output current thus developed, proportional to the conduction level of the black color of the cathode, the value of which is typically equal to a few amps. .

The positive cathode pulse of u. induced output occurs in correspondence with the collector of transistor 22 and is fed to the base input terminal. of the transistor 2C, through the resistor 25, causing a proportional increase in the degree of conduction of the current of the transistor 20, while the current pulse is present. The greater current conveyed by the transistor 20 causes the development of voltage across the sensing resistor 30.This voltage takes the form of a voltage variation, with a negative trend, which makes it available at node A and which is proportional, as amplitude, to the amplitude of the? cathode output pulse rpresentative of the black current. Is the amplitude of the voltage variation in correspondence of nod A determined Gtacdmqs? from the product of the v of the resistor 30 and the amplitude of the current flowing through the resistor 30.

The voltage variation corresponding to node A is transferred through a resistor having a low ohmic value, indicated at 31,. to a node B at which a voltage variation VI is developed, essentially corresponding to the voltage variation at node A ... bias control, the network 50 includes an input coupling capacitor 51? an operational amplifier? of inlet 52 fixing? of sampling costit for example, from u? t conductance operational amplifier, with an associated feedback switch 54, sensitive to the level fixing signal V, and also includes a charge storage expander 56 coupled to the output of amplifier 52 through a switch 55? sensitive to the temp

capacitor 56 KXKK is used to feed a kinescope bias correction signal through a network 58 and a resistor network 60, 62, 64, to the kinescope pilot, through a bias control input, at the ba. sse of transistor 20. Network 58 includes signal translation circuits and separation circuits, for a-? Supply the bias control voltage dm, corresponding to an appropriate level and a low level impedance, in accordance with the requirements of the bias control input of transistor 20.

Will come now described the operating principle of the system schematized in Figure 1, with specific reference to the wave forms shown in Figure 2. The auxiliary signal Vp is applied to the circuit node B, shown in Figure 1, through a diode 35 and a voltage translation impedance network, comprising the resistors. 32 and $ 34 showing, for example, values of 200 kilohm and 270 kilohm, respectively. The Vp signal has a positive direct current level of approximately 8.0 volts under all conditions, except during? KXXSH? the sampling interval AKB, in order to keep the diode 35 in the conduction state, so as to develop, in correspondence of the node B, a normal direct current bias voltage. When the positive DC component of the Vp signal is present, the junction point frf.

the resistors! 32 and 34 is fixed at a voltage equal to the direct component of the signal V ^, decreased across the diode 35 * The signal V rm shows a KSBIEHE? E? impulsive component of fixed amplitude, less positive, with negative trend, during the sampling interval AKB, which makes the diode 35 non-conductive and frees the junction of the resistors 32 and 34. The resistor 31 causes an insignificant attenuation of the voltage variation developed at node A with respect to the corresponding voltage variation (V) developed, at node B, since? the value of resistor 31 (in the order of 200./i) is small compared to the values of resistors 32 and 34.

Before the level fixing interval, but during the AEB time interval the pre-existing DC voltage? present at node B, charges the positive terminal of capacitor 51. During the level fixing interval, when the grid pilot pulse V ^ is developed, the voltage at node A decreases in response to the pulse VQ of a quantity? representative of the black current level. This causes a decrease in the voltage at nnd B to a level substantially equal to V? V ^. Furthermore, during the level fixing interval, the timing signal V ^, causes the closing, i.e. the switching to the conducting state of the level fixing switch 54 and, therefore, the inversion input (- of the signal, belonging to the amplifier 52, SXHS is coupled to the output of the considered amplifier, thus obtaining an amplifier 52 operating as a repeater amplifier, of the unity gain type. Consequently, a source of fixed reference voltage, in continuous current V ___ capable of supplying a voltage equal to, for example, 5 volts, applied to a non-inversion input terminal (+) of the amplifier 52, XXKH is coupled, by means of the feedback action, to the signal inversion input terminal of the amplifier 52, through the output of the amplifier 52 and by means of the Switch 54 to the conducting state.

Therefore, during the level fixing interval, the voltage across capacitor 51 represents a function of a set-up voltage determined by the voltage VT

IIP at the negative terminal of the capacitor 51 and a function of a sensor at the positive terminal of the capacitor 51, corresponding to the difference between the nominal level, in direct current

(V Ojfl) pre-existing at node Bi and the variation in tendon V developed in node B during the level fixing interval.

Thus, the voltage across capacitor 51, during the level fixing reference interval represents a function of the level of the change of. voltage V ^, representative of the black current, the value of which can? varial. The tension can? be expressed as

V.

RI IV

During the immediately following camnionaniento interval, the pilot grid impulse is absent. positive causing a positive increase of the t? ns? one on node B, up to the nominal level in direct current pre-existing present before, of the sampling interval. Simultaneously, a negative trend I \ pulse V is highlighted with consequent polarization, in the reverse direction!, Of the diode 35 and perturbing, that is to say by varying! moment aneaajnente the normal ecxobc translation action

and coupling of the voltage carried out by the resistors 32 and 34 and, therefore, the voltage on node B is reduced by an amount? V ^, as indicated in Figure 2. At the same time, the level fixing switch 54 is switched to the non-conducting state, while the sampling switch 55 is closed, i.e., switched to the conducting state, in response to signal V and, therefore, charge storage capacitor 56 is coupled to the output of amplifier 52.

Therefore, during the sampling interval, the input voltage applies Ita to the inversion input for the signals, i.e. to the (-) input of amplifier 52,? equal to the difference between the voltage at node B and the voltage across the input capacitor 51. The input voltage applied to the amplifier 52 represents a function of the amplitude of the voltage variation V, which can? vary according to the variations found in the level of the black current of the kinescope.

The voltage on the output storage capacitor 56 remains unchanged during the sampling interval, by altering the amplitude of the voltage variation developed during the level fixing interval, it is equal to the amplitude of the variation in the voltage V developed during the sampling interval, indicating a correct level of the black coIrent of the kinescope. This arises from the fact that during the sampling interval, the voltage change at node B increases in the positive direction (from the level fixing tuning reference level), when the pulse is eliminated. grid pilot, while the voltage variation causes a simultaneous voltage perturbation, with a negative trend, at node B.

When the polarization of the cine? scope? correct, the variation d? voltage, with a positive trend, and the voltage variation with a negative trend V have equal amplitudes and, therefore, will result? evident that. these voltage variations will cancel each other out during the sampling interval, leaving unchanged the voltage value at node B

When the amplitude of. voltage variation is less than the amplitude of the voltage variation, the amplifier 52 loads, proportionally, the storage capacitor 56 in a direction such as to cause an increase of? & ?????? conduction of the black cathode current. Conversely, the amplifier 52 proportionally discharges the charge storage capacitor 56 to cause a decrease in the conduction of the black cathode current when the amplitude of the voltage change is increased. greater than the amplitude of the voltage change

. In accordance with to what is represented, in a more? specific from the waveforms shown in Figure 2, the "A" magnitude of the voltage variation is assumed to be approximately three m? llivolts, when the black cathode current level is correct, while it varies within a range of a few millivolts (+?) when the level of the black cathode current increases and decreases with respect to the correct level, as the operating or work parameters of the kinescope vary. Therefore, the peak reference voltage of the level fixing interval, present at the ends of the capacitor, varies as a function of the variations in the amplitude of the voltage V ^, when the level of the black cathode current varies. The voltage variation at node B has an "A" amplitude of about millivolts, corresponding to the "A" amplitude associated with the voltage variation when the black current level stands out correctly

As indicated by the waveform shown in Figure 2, the voltage I present in correspondence with the inversion input (-) of the amplifier 52 remains unchanged during the sampling interval, when the voltages have both, one amplitude "A". However, as indicated by the waveform V ^, the input voltage of the amplifier 52 increases by an amount. ^ when the voltage variation has an amplitude

"? ^", corresponding to a high level of the black current. In this case, the amplifier 52 discharges the storage capacitor! output 56 and, therefore, the voltage d :. control of the bias, applied to the b2-se of the transistor 20, causes an increase in the voltage. of the collector of transistor 22 and, therefore, the black cathode current decreases towards the correct level.

On the contrary, and according to ouanto in-? given by the waveform V, the input voltage of amplifier 52 decreases by one. How many T-t? A during the sampling interval, when the voltage variation shows an amplitude "A - A M, corresponding to a" low level of the black current. In this case, the amplifier 52 loads the output storage capacitor 56, with a consequent decrease in voltage. sion of the collector of transistor 22 and, therefore, will occur? an increase in the black cathode current, towards the correct level. In both cases, several sampling intervals may be required to obtain the correct black current level.

Therefore, the described system automatically produces an output current of the amplifier having a value equal to zero, for the storage capacitor 56 when the non-zero amplitude of the voltage variation V_ ^ corresponds to the level corrected black current. Consequently, the use of manual controls from the pre-set polarization type are not required, n? the associated regulation, with slower consumption of times, to offset the response as regards the conduction of the sampling amplifier, for the production of an output current flow of the amplifier equal to zero. for the storage capacitor, when the sampled signal has an amplitude other than zero for the correct bias conditions. The previous sampling amplifier's input signal coupling complex -? previously described, using an auxiliary pulse V is advantageous in a system in which the sampling amplifier 52KEUXD & MX comprises a differential input amplifier of the coupled emitter type, as described in the correlative US patent application? n, if filed at the same time, having the serial number (RCA. 76.583) in the name of R.P. Parker, entitled "Signal Processing Network for an Automatic Kmnescope Bias Control System", referred to in the above context by way of reference. This correlative application also describes additional information concerning the assembly including an auxiliary control signal V ^, and also describes a suitable assembly for the empiiication signal generator and 40 illustrated in FIG. In some systems. AKB can be desirable to develop a voltage variation: representative of the black current, durant l? sampling interval, instead? during the previous level fixing interval, as previously described. In an alternative system of this type, the pilot impulse of grids V ^ is timed in such a way that it can occur during the? sampling interval, coinciding with a positive auxiliary pulse

The negative trend voltage variation and the positive trend voltage variation V2, sivlunpated in response to the auxiliary signal Vp, therefore occur simultaneously and combine directly at node B, so that they cancel each other out. when the black current level? correct (i.e., no voltage change is produced at node B). Details of this alternative technique can be found in the aforementioned correlative patent application of R.P. Parker

The voltage developed at node B during the level fixing and sampling intervals AKB represents a function of the values of resistors 31, 32 and 34 and of the value of an output impedance ZQ, present on node A. When the Vp signal shows the positive level, in direct current (+ 8 volts) during the level fixing interval, the junction point between the resistors 32 and 34 is "fixed" as a voltage, while a current carried by the resistor 31, from node A to node B, represents a function of the values of 2 ^, of the value of resistor 31 and of the value of resistor 34. During the next sampling interval, when the component of the signal Vp is present, represented by a pulse with a negative trend, the diode 35 is in the non-conduction state, while the junction point of the resistors 32 and 34 does not remain "fixed". In this time, a current of different value is conveyed by p art of resistor 31, from node A to node B, in fxxsra as a function of the value of resistor 32, in addition to the values of the impedance and of the resistors 31, 34. the voltage variation V ^, developed at node B in response to Conoonent impulsive, a negative trend of the signal Vp is proportional to the difference between these edges.

The combined pulse sampling complex described above also advantageously provides a convenient mechanism for compensating for the different mutualmeatt conduction characteristics and correspondingly different cut-off voltages of the electron guns. of the kinescope due, for example, to the manufacturing tolerances of the kinescope. This aspect of the version described? been analyzed in detail, in the correlative US patent application (RCA 78.859), in-j titled "Automatic KEnKinescope Bias Control System Compensated for Kinescope Electron G-un Conductiai Dissimilarities", filed at the same time as this, this system is now described in brief in the course of this discussion.

When the electron guns of the kinescope are identical and when, consequently, they have the same conductive characteristics, they will conduct equal black-level currents to them while they will have equal cut-off voltages, represented, for example, by the voltages between the grid and the cathode. However, in practice, electron guns exhibit mutually different conducting characteristics. In this last KKExy case, the different currents conveyed by the electronic guns are considered as currents at the correct black level and, therefore, the MB system should remain in quiescent conditions and should not alter the kinescope polarization even if the guns electronic devices have different mutualment black current levels and the corresponding associated switching voltages, ????????????, mutually different.

This rs result is obtained for. bunch of the complex described, since? the amplitude of the x voltage variation V ^, developed at node B is linearly proportional to the component represented by the direct voltage which becomes available at the node A. This component, represented by the direct voltage, is proportional to the cathode interdiction voltage, according to what is manifested by the component, consisting of the direct voltage, present at the output of the pilot transistor 22 and corresponding to the cathode voltage, during the interval MB, neglecting the effect of the cathode current pulse of the induced output, developed in response to the? pola-V grid pilot pulse? rit? positive. . Therefore, if the three cannons Gi and the tt ren Iani ci of the EEH kinescope exhibit mutual currents. While different and cut-off voltages corresponding to the initial black level tuning conditions, the voltage variation V, respectively associated with the signal processors 14a, 14b and 14c will present, individually and a? different amplitude, even if each of them is emitted by a common signal V_ ir. The different amplitudes of the voltage variations represent a function of the different cut-off voltages, according to what is manifested by the direct current components, of different amplitude, developed at the nodes A. The different amplitudes of the voltage variations are such that, for the ? associated AKB control loop, the tension developed at the node. B will not vary? when the voltage variations are combined and, perpetually, each contact ring will remain? in the state of quiescence. The AKB control loops will remain in dhax Quiescent conditions as long as they are the currents of the electron guns, at the black level, established initially, will vary due to a variation in the operating parameters of the kinescope, due to the aging effects of the kinescope or the effects deriving from changes in temperature.

It should be noted that the impedance is relevant vate at the detection node A during the intervals AKB, varies as a function of the characteristic of the electron gun of the kinescope and corresponding to the cathodic cut-off voltage associated with a given correct level of the black current. The desired effect, whereby the amplitude of the voltage variations at node B? predictably proportional to the direct voltage component (DC) at node A and the cathodic interdiction voltage, it will not be obtained if the impedance variations at node A were not compensated. In systems which do not require the amplitude of the SXKX voltage variations to be proportional to the cathodic cut-off voltage, the impedance variations at node A would not, however, render the amplitude of the voltage variation undesirably unpredictable.

The node A pu? be considered, in a model, ... as a source of "1 voltage" in series with the impedance 2 previously mentioned; sensing resistor 30 dividing by a gain factor of the control loop, which, in turn, represents a function of the working or operating point of the transition? _2Q. The operating point, or d? work, _d | the transistor 20, during the intervals AKB is proportional to the cut-off voltage of the corresponding cathode KKE. In practice, it has been found that the impedance z can have a minimum value and a maximum value

mo of 30 ohms and 50 ohms, respectively, under conditions corresponding to a corrected black current. Therefore, the value of ZQ at point A can? vary by $ 67 from a minimum value.

The resistor 31 compensates for the change in impedance at node A, so that the change in impedance should not compromise the desired operation of the circuit that supplies the auxiliary pulses, including the source of the signals Vp, the diode 35 and the resistors 32, 34. In this example, the value of resistor 31, not critical,? of the order of 200 ohms. Therefore, the total impedance presented at node B, from node A, includes the resistor 31 and the impedance ZQ and the same varies from 230 ohms to 250 ohms, depending on the variations in the impedance at node A. Consequently, node B is presented with acceptably low impedance varielions below 10 $, under the conditions corresponding to a black current of correct level, which is significantly lower than the impedance variation of 67 $ present in the absence of resistor 31. In other words , the impedance presented at node B varies only by 4 $

compared to a nominal value of 40 ohms for impedance z o.

Resistor 31 also advantageously involves an increase in immunity. of the level fixing and sampling network 50, with regard to spurious signals which can distort or obscure the control voltage of the polarization finally developed at the ends of the storage capacitor 56. Periodic spurious signals such as, for example, are of particular interest for example, those generated locally and represented by interference signals, in alternating current at times, considered as "rings" of the frame. "These latter signals occur periodically at the horizontal line frequency whose value is approximately equal to 15,734 Hz, in accordance with US standards and including oscillatory pulsed signals, of the damped type, with an average value substantially equal to zero. These signals are generated by the deflection circuits of the receiver during the horizontal retrace intervals of the image, that is to say including the intervals during which the system operates AKB, such signals being able to be coupled to the AKB system through the power connections and through the processing circuits of the luminance and chrominance signals ?! Spurious signals are particularly annoying in an AKB system due to the fact that they have amplitudes which are significant with respect to the signals of low intensity, that is, of the order of a few millivolts which are processed by the AKB system. The impact of spurious signals can be reduced through the use of separate filtering and shielding techniques, although such solutions represent more complex and costly alternatives.

The voltage developed across the level 51 holding capacitor, whose value? equal to 0, 12 microfarads, during the interval of fixing the level, pu? be seriously influenced by the spurious signals represented, for example, by the? raster rings "which have a significant amplitude not equal to zero, such signals being able to occur at the end of the level fixing interval that is to say close, temporally, to the instant in which the feedback switch 54 opens. In the absence of the resistor 31, the capacitor 51 can charge to a voltage equal to 67% of the peak amplitude of the previously defined spurious signal, causing the presence of a serious error in the level fixing reference voltage developed across the capacitor 51. This error is significantly reduced by the presence of the resistor 31, in accordance with what will now be described in more detail .

During the level fixing interval, the signals including a direct component and the spurious signals, in alternating current, are fed to the positive terminal of the capacitor 51 through an impedance Z whose value? approximately equal to 240 ohms, corresponding to the series combination of the impedance ZQ at node A and resistor 31. The reference voltage? - ?? it is fed to the negative terminal of the capacitor 51 through a low value impedance Z, corresponding to the low output impedance of the amplifier 52 which operates as a voltage repeater during the level fixing interval. The impedance Z is mean-Zi

less than the impedance Z. The amplitude of a reactive impedance Z presented by the capacitor 51 in the presence of the spurious signals recurring at the horizontal frequency? equal, approximately 84 ohms. The alternating current component of the spurious signals through the capacitor 51 is attenuated in. significant measure, from the ratio between the impedance Z and the sum of the impedances Z, Z and Z in C

so that the capacitor 51 can charge to a voltage equal to only about 25% of the peak amplitude of the spurious signal (raster ring). Consequently, the fixing capacitor of the level 51 responds more closely to the average value of the signals coming from the node A / while the amplitude peaks of the spurious signals present a much less significant plate on the level fixing reference voltage developed by the capacitor 51? .

The system described produces, automatically at the output current of the amplifier, of value parX at zero, for the storage condenser 56 when the amplitude not Rare at 0 of the voltage variation K v.j corresponds to the correct level of the kero current. Consequently, the use of / controll? pre-set biasing manuals, to offset the response as far as the sampling amplifier ccmduction is concerned, to produce an amplifier output current flow of zero value for the storage, when the sampled signal presents

Claims (1)

. . . 1._ Jn ... a system for processing a video signal, including a playback device. . of the images, sensitive to video signals fed to an intensity control electrode,. apparatus for the automatic control of the polarization, comprising: means of deriving a signal rap-? presentatives) of the level of the current of the black image, conducted by the aforementioned electrode for the control of the intensity? during bias control intervals, the aforementioned shunt means exhibiting a variable output impedance in relation to the bias of said intensity control electrode; . sampling amplifier means with an input to an output, for providing an output bias correction signal to the aforementioned imaging device for maintaining a correct black current level, in response to the applied signals. at the aforementioned entrance; means for generating an auxiliary control signal, having an amplify and a predetermined sense; - a capacitor, for coupling said representative, derivative signal and said signal. auxiliary to the aforesaid input of the aforesaid, sampling amplifier means;] e. . - an impedance, to couple the aforesaid representative signal derived, from the aforesaid output of the aforesaid branching means, to the aforesaid capacitor, the aforesaid impedance being large in relation to the aforesaid variable output impedance, in order to significantly reduce the impedance variations presented to the aforementioned generation means, by the aforementioned output of the aforesaid derivation means 2. Apparatus according to claim 1, characterized in that: . the aforesaid derivative signal is developed at a first circuit point, corresponding to the aforesaid output of the aforesaid derivation means; the aforesaid auxiliary signal coming from the aforesaid generation means is coupled to the aforesaid capacitor, at a second cirucital point; IP? said impedance is coupled; from the aforementioned primary circuit point, to the aforementioned second circuit point; means of ..generation include . . a source of timing signals, having first and second states; - a controllable impedance network coupled to the aforesaid second circuit point and sensitive to the aforesaid timing signals, to provide a first and a second impedance. 3. Apparatus according to Claim 2, catacted by the fact that: ... the aforesaid.jrete with variable impedance? coupled to x ?? said second circuit point and to a reference potential point; the aforesaid first impedance present from the aforesaid controllable impedance network ,? induces a first current between the aforesaid first. and the aforementioned second circuit point, as a function of the aforementioned coupling impedance. and the aforementioned first impedance presented by said controllable impedance network; the aforesaid second impedance taken from the aforesaid impedance controlled network induces a second current wave between the aforesaid first and. the aforesaid second point, circuit, different from the aforesaid first induced current, primary. as a function of the above coupling impedance and the above presented second impedance of the above controllable impedance network; and the aforesaid auxiliary signal is proportional in amplitude to the difference between the aforesaid first and the aforesaid second induced factor. 4. Apparatus in accordance with claim 1, characterized in that the aforementioned means of derivation include: amplifier means having a first terminal for receiving the video signals to be amplified, a second terminal coupled to the. the aforementioned el? ttrode of control of the i tensit? of the aforesaid device for reproducing images, for feeding amplified video signals thereto, and a third terminal coupled to a reference potential and with the aforesaid second terminal defining a path for conducting the current nrincipal6 of the aforesaid amplification means; And feedback means coupled from said second terminal to said first terminal of said amplification means, to cause. ita conduction variation of the aforesaid HfiHQ course for the conduction of the main current of the aforesaid amplification means, in accordance with the representative current variations. of the variations of the black current manifested in correspondence with the aforesaid second terminal of the aforesaid amplification means; and characterized by the fact that the aforesaid signal, represented as being derived from the aforesaid path for conducting the main current of the aforesaid amplification means. . . 5. Apparatus according to claim 4 and further comprising:. a detection resistance incl in the aforesaid path for conducting the current e. of the aforesaid amplification means for developing a voltage across them corresponding to the aforesaid derived signal. 6 * Apparatus according to claim 1, characterized in that the aforesaid branching means comprise: a first transistor with a first electrode for receiving video signals to be amplified, a second electrode and an electrode coupled to a reference and reference power; . _. . a second transistor with a first electrode coupled to a bias voltage, a second electrode coupled to an operating power supply voltage through a load impedance and coupled to said intensity control electrode to power it an amplified video signal, and a third electrode coupled to said second and aforementioned first transistor; a detection resistance coupled between said second electrode of said first transistor and said third electrode of said second transistor; and feedback means coupled from said second electrode of said second transistor to said first south electrode, said first transistor, to cause the conduction variation of said first transistor, in accordance with the current variations representative of the black current variations manifested in correspondence with the aforesaid second terminal of the aforesaid second transistor; and characterized by the fact that _ _. the aforesaid representative signal is derived from the aforesaid second electrode of the aforesaid first transistor. 7. Apparatus according to claim 1 and further comprising: - - a reference-voltage-source; ? - -. a source of switching signals which. occur during the nolarization control intervals; And . .. means sensitive to the aforesaid switching signals, for applying the aforesaid? reference voltage at a low ima at the foot of the aforesaid capacitor, in correspondence with the aforesaid fat of the aforesaid amplification means. 8. Apparatus according to claim 1 and further comprising: . a reference voltage source PC, coupled to the aforementioned amplification means, a charge storage device; a source of switching signals, occurring during a first and a second interval, within a given bias control interval; switching means, coupled to the aforesaid output of the aforesaid amplifying means. to said capacitor and to said charge storage device and sensitized to said switching signals during said first interval to (1) apply the said reference voltage at a low impedance to said capacitor, in correspondence with said input of said amplifier means and (2) decoupling of said amplifier output from said charge storage device; the aforesaid switching means being sensitive to the aforesaid mutation signals during the aforesaid second interval. by (3) decoupling EK? to said source of reference voltage from said capacitor, and (4) ... to coupling said charge storage device to said output of said amplification means. 9. Apparatus according to claim 8, characterized in that: the aforesaid sampling amplifier means comprise an operational amplifier with an input for inversion signals, coupled to the aforesaid capacitor and a non-inversion input coupled to the aforesaid source. reference voltage. EXTRACT In an automatic kinescope bias (AKB) system, a signal having an amplitude representative of the level of the black current conducted by the kinescope of a television receiver viane? derived from a point of revelation. The derived signal is coupled, through a signal path, to a processing network including input fixing and sampling networks, to develop a bias correction voltage. A source of an auxiliary control signal, coupled to the signal path, before the level fixing network,? subject to undesirably being XH ?? KZHX; influenced by changes in impedance manifested by the detection point. An impedance included in the deb path. senglai, between the detection run and the auxiliary signal source, compensates for the variations of ???????? OF A COMPENSATED KINESCOPE FOR THE OFFICE OF A DETECTION POINT described and claimed therein; we declare that we do not know and do not believe that this invention was ever known or used in the United States of America prior to our invention in urit or patented or described in any publication published in any country prior to our invention in this regard 0 more than one year prior to this application, or used in public or for sale in the United States of America one year prior to this application; we declare that the present invention is not? been patented or did not constitute the subject of any certificate of invention issued prior to the date of this application in any foreign country outside the United States ?? ifc? EK3C of America on an application filed by nod or our legal representatives or transferees more than twelve months prior to this application; we ask that we are aware of our duty that Khxarxa imposes on us; clarify any information in our possession that may be important to the? purposes of examining the present invention, and that no patent application or certificate of invention relating to the present invention? st ^ ta? was filed by us or our representatives or assigns in any foreign country other than the United States of America prior to application. And with the first note I name, individually and collectively: Name Registration number Eugene K. Whitacre 17.462 Paul J. Rasmussen 24 - 872 P. eter KL Emanuel 26. 542 Ronald H. Kurdyla 26.932 my agents, whose address? RCA Corporation, Patent Operations, P. O. Box 432, Princeton, New Jersey 08540, to pursue this application and deal with all transactions thereto? the US Patent and Marks Office connected. Please send all communications to: Eug6ne Ti. Whitacre RCA Corporation Patent Operations P. O. Box 432 Princeton, New Jersey 08540 Therefore we request that we be granted certain trustworthy patents for the invention or discovery described, and claimed in the foregoing description and claims, and we hereby subscribe our name to the description and claims that they precede, to the declaration, the whole of the assignment and to the present request. ? - The undersigned applicants also declare that all the declarations issued in this form, of their knowledge, are true and that all the declarations made on information and beliefs are believed to be true, and furthermore these declarations have been issued without knowledge that intentional false statements and the like so? released are punib with a fine or with prison, or with both, the confomit? with Section 1001 of article 18 a. . of the United States Code and that such intentional false declarations may affect the validity? of the question, as well as? the granting of any patent in this regard Inventor (l) (sign with your full name, not just ini. _ Anyone, in some way within. the jurisdiction of any department or organization of the United States intentionally and deliberately. or plotting an important fact, or issuing any declaration, or false, fictitious or fraudulent testimony, or drawing up or using any document or document, knowing that it contains any <1> that declares or writing false, fictitious or fraudulent, will be He is fined _with a fine not exceeding // 10. | 000., .. or he will be imprisoned for a period exceeding five years, or will suffer both | punishments, as established on June 25, 1948. c. .645, 62 Stat. 749. SUMMARY A system for the automatic control of the the kinescope polarization (AKB -automatic kine- scope bias) includes a sampling amplifier (jf?) which provides an output current for the charging and discharging of a storage capacitor action {#?) in response to the amplitude of a pulse de- rivato (? ^) representative of the current level of the black conveyed by the kinescope of a receiver television. The amplitude of the derived impulse results greater than zero, when the current level of black is correct and is applied to the amplifier through an input signal coupling path. During the sampling intervals, an auxiliary pulse is applied to the input signal path with an amplitude and direction that negates the amplitude of the derived pulse at the amplifier input when the amplitude of the derived pulse corresponds to the correct black current level. Consequently, the conduction of the amplifier and the voltage remain unchanged DESCRIPTION OF THE INVENTION The present invention relates to a complex signal processor used in a system to automatically control the level of a current representative of a black image, conveyed by a device for displaying images associated with video signals and consisting, for example, of the kinescope of a television receiver. of a point of revelation dad. What is derived a signal representative of the black Xfel current in such a way? that / the variations of inm? den? a cannot negatively alter the operation of the successive / control circuits to which the detection point is connected. Television receivers sometimes use an automatic kinescope bias (AKB) system to automatically establish the appropriate representative black image current levels for each kinescope electron gun. As a result of this operation, the images reproduced by the kinescope are prevented from being negatively influenced by the variations that may occur in the operating parameters of the kinescope and caused, for example, by aging phenomena and by effects associated with temperature variations. A type of AKB system? been described in U.S. Pat. 4. 263. 622 to Werner Hinn, entitled "Automatic Kinescope Biasing System." An AKB system typically operates during image suppression intervals, i.e. when the kinescope carries a small representative black level suppression current, in response to a reference voltage representative of the black video signal information. This current is monitored by the AKB system in order to generate a kinescope bias correction voltage representing the difference between the detected black current level and a desired black current level. The correction voltage is applied to the kinescope, for example through the video signal processing circuits preceding the kinescope with a sense that reduces the difference. Typically, the correction voltage is applied to a bias control input of a DC coupled kinescope driver amplifier which feeds the output video signals, having an appropriate level, in order to directly drive a cathode electrode of control of the intensity? of the kinescope. The correction voltage modifies the output bias voltage of the driver amplifier, so as to modify the cathode bias voltage, in order to obtain the desired level of the black cathode current. In an AKB system, of the type described in the previously indicated Hinn patent, the control circuits respond to a periodically developed voltage pulse, with an amplitude representative of the level, of the black cathode current. L ? The voltage pulse has a prescribed level different from zero, when the black current level is correct and different levels (for example, more or less positive) when the black current level is too high or too low. The voltage pulse is sampled by a carmoioning amplifier which charges or discharges a storage capacitor, in accordance with the level of the voltage pulse. The kinescope bias correction voltage is derived from the charge stored by the kinescope. storage capacitor, which increases or decreases, as required, to maintain a correct level of the black current. The polarization correction voltage, derived from the storage capacitor, should remain unchanged when the level of the voltage pulse represents a correct level of the black current. This requires the denser c storage tank is not loaded n? discharged by the output current from the sampling amplifier, when the level of the voltage pulse represents a correct level of the black current. More specifically, in an AKB system of the type described in the Hinn patent, this requires that the sampling amplifier must not supply any current to the storage capacitor when a current at the correct level of the kinescope black is indicated, according to what indicated by a representative voltage pulse having a predetermined amplitude other than zero. This result can? be obtained by staggering the bias of the sampling amplifier, for example by means of a manually adjustable pre-settable potentiometer connected to an appropriate bias control point of the amplifier. Is it a rule? Pre-set manual actions are undesirable in an otherwise automatic signal control system. Furthermore, these manual adjustments are undesirably time-consuming while the associated potentiometers result in increased system costs. It should also be noted that the signal processing techniques adopted by some AKB systems can produce an offset or "offset" error if the cut-off voltages and signal gains of the individual kinescope electron guns are not identical due to tolerances of manufacture of the kinescope, by way of illustrative example. In a case of this type, the level of the black current, established by the AKB system, can? present an error, which can? be compensated by means of preset potentiometers, manually adjustable. The system described advantageously facilitates the design of signal processing circuits for the automatic polarization of the kinescope (AKB), this complex not requiring the use of manually adjustable controls, for the compensation of such staggering or "offset" errors. . The present invention relates to a video signal processing system, in which a derived signal, representative of the black current level conveyed by an image display device, has a given amplitude other than zero when the black current level is corrected. - straight. The representative derived signal is applied, through a coupling path of the input signals, to a sampling amplifier which supplies an output current to charge and discharge a charge accumulation device, in accordance with the amplitude of the derived signal. In accordance with a principle of the present invention, an auxiliary signal is fed to the coupling path of the input signals, with an amplitude and a direction such as to cancel the amplitude of the signal derived at the input of the amplifier, when the amplitude of the derived signal? representative of a correct level of the black current. Consequently, the conduction of the sampling amplifier remains unchanged when the amplitude of the derived pulse corresponds with the correct level of the black current and when the voltage on the storage device remains unchanged. In accordance with a characteristic of the present invention, the amplitude of an auxiliary signal is proportional to the amplitude of the interdiction potential of the cathode of the kinescope, developed during the AKB interval. In accordance with another characteristic of the invention, the input of the sampling amplifier is fixed at a reference voltage, during a fixing interval of the level preceding the sampling interval of the signals. The derived signal, representative of the black level, is developed during the level fixing interval, so that the reference voltage at which the amplifier input is "fixed", during the level fixing interval , represents a function of the amplitude of the derived signal, while the auxiliary signal is developed during the next sampling interval. amplitude of the derived signal corresponds with the correct level of the black current. In accordance with a further feature of the invention, the amplifier input is "fixed" to a reference voltage during the level fixing interval, while the derivative signal and the auxiliary signal are both developed during the sampling interval. In accordance with the principles of free control, FIG. 1 illustrates a portion of a color television receiver including an AKB system and associated signal sampling network, embodying the characteristic principles of the present invention; Figure 2 illustrates the waveforms of the signals which are associated with the operation of the system shown in Figure 1; Figure 3 illustrates an alternate version of the signal waveforms shown in Figure 2; Figure 4 illustrates the circuit details of the sampling network schematized in Figure 1; and Figure 5 illustrates the circuit details of a timing signal generator associated with the system shown in Figure 1. In accordance with as shown in Figure 1, the television signal processing circuits 10 supply the separate luminance (Y) and chrominance (c) components of a color composite television signal to a signal processing network 12 of luminance-chrominance. The processor 12 includes circuits for controlling the luminance and chrominance gains, circuits for setting the direct current level (including, for example, manipulated circuits for fixing the black level), color demodulators for the development of the difference signals of color of red, green and blue r-y, g-y b-y, respectively, and matrix amplifiers to combine these latter signals with the processed luminance signals, in such a way as to allow the generation of the associated low-level color image representative signals to the color red, the color green and the color blue, r, g and b, respectively. These signals are amplified and otherwise processed by means of circuits contained in the processing networks of the output video signals, distinguished by the references 14a, 14b and 14c, respectively, which feed signals representative of the color images, suitably amplified and of a high level, associated with red, green and blue, R, G and B, respectively ^, to the respective cathode electrodes for controlling the intensity? 16a / and 16c of a color kinescope 15? The networks 14a, 14b and 14c also perform functions related to the AKB operation according to what will happen. described below. The kinescope 15? of the in-line, self-converging electron gun type, having a grid 18, excited in common, associated with each of the electron guns comprising the cathode electrodes 16a 16b and 16c, Since? in this practical embodiment according to the invention, the processors 14a, 14b and 14? of the output signals are similar, the description that will come? now reported concerning the operation of the processor 14a will result? also valid for computers 14b and 14? . The processor 14a includes a kinescope driver stage comprising a common emitter input transistor 20, which receives the video signal r from the processor 12, through an input resistor 21 and an output transistor 22, with common base, to high voltage, which, together with the transistor 20, forms an ascode video driver amplifier. "The high level video signal R, suitable for driving the cathode 16a of the kinescope, is developed at the ends of a load resistor 24 present in the output collector circuit of the transistor 22. An operating supply voltage for the amplifier 20, 22 via a DC voltage source of high value B + whose value is equal to, for example, 230 volts. negative feedback, in direct current, for the driver stage comprising the transistors 20, 22 is obtained by means of a resistor 25. The gain, as far as the signal is concerned, which characterizes the cascode amplifier 20, 22,? determined mainly by the ratio between the value of the feedback resistor 25 and the value of the input resistor 21, The feedback network allows to obtain an output impedance from the amplifier, of an appropriately low value and allows to favor the stabilization of the operating level in direct current, at the output of the amplifier. A detection resistor 30 connected, in direct current, in series with the collector-emitter paths of the transistors 20 and 22, serves to develop a voltage at node A, of relatively low value, representing the level of the black cathode current of the kinescope which is conveyed during the kinescope erasing intervals. Resistor 30 operates in conjunction with the receiver's AKB system, depending on what will happen. now described in more detail. A timing signal generator 40, containing suitable control logic circuits, responds to the periodic signals recurring at the horizontal synchronization frequency (H) and to the periodic signals occurring at the synchronization frequency. vertical action (V), these signals being derived from the deflection circuits of the receiver, in order to generate the timing signals V_, V_, V ^, Vp and V capable of controlling the operation of the circuit AKB, during the periodic intervals AKB . Each of the AKB intervals begins just after the end of the vertical retrace interval within the vertical blanking interval and encompasses several horizontal line intervals, which are also within the vertical blanking interval and during which the representative information of? image associated with the video signal? These timing signals were represented by the respective waveforms in Figure 2. Referring specifically to figure 2 for the moment, can you? be detected that the timing signal Vp, constituting a video suppression signal comprises a positive pulse, generated just after the end of the vertical retrace interval at the instant f as indicated by the reference to the waveform of the signal V The blanking signal V exists for the duration of the AKB interval and the same is applied to a blanking control input terminal of the luminance-chrominance processor 12, such that the outputs r, g and b of the processor 12 can present a reference level in direct current representative of a black image, corresponding to the absence of video signals, This can? be obtained by reducing the gain, as regards the signal of the processor 12, to a value substantially equal to zero, through the gain control circuits of the processor 12, in response to the signal V, and by means of a modification of the direct current level of the video signal processing path, through the direct current level control circuits of the processor 12, so as to produce a reference level representative of the black image at the outputs, for the signals of the processor 12. The timing signal V, constituting a grid pilot pulse, of polarity? positive, spans three horizontal line intervals within the vertical blanking interval. The timing signal V controls the operation of a level fixing circuit associated with the signal sampling function of the AKB system. The timing signal V, constituting a sampling control signal, occurs after the V signal and the same is used to terrify, or synchronize, the operation of a sampling and holding or storage circuit which develops a control signal of the direct current polarization, to control the cathode current level of the black of the kinescope. The Vg signal embraces a sampling interval, the beginning of which is slightly delayed with respect to the end of the level fixing interval embraced by the V signal, and the C. whose term substantially coincides with the term of the AKB interval. An auxiliary pulse Vp, with a negative trend, whose function will come? hereinafter described in more detail ^ coincides with the sampling interval. The delays in the timing of the signals, indicated in Vp, according to what is represented in Figure 2, are of the order of 200 nanoseconds. Referring again to Figure 1, it must be noted that during the interval AKB, the positive pulse V l_ ?, whose value? of the order of +10 volts polarizes, in a direct direction, the grid 18 of the kinescope, causing an increase in the conduction of the electron gun comprising the cathode 16a and the grid 18. At different time intervals (? the intervals AKB, the signal V provides the normal polarization, less positive, for the kinescope grid 18. In response to the positive impulse of grid V, a positive current impulse, of similar phase, is evident at the cathode 16a, during the interval corresponding to the Grid pulse The amplitude of the output deficient cathode pulse thus developed is proportional to the conduction level of the black current of the cathode, the value of which is typically equal to a few microamps. The induced positive output cathode pulse occurs at the collector of the transistor 22 and is fed to the base input terminal of the transistor 20, through the resistor 25, causing a proportional increase in the degree of conduction of the current of the transistor 20. , while the cathode pulse is present. The greater current conveyed by the transistor 20 causes the development of a voltage across the sensing resistor 30. This voltage takes the form of a voltage variation, with a negative trend, which becomes available at node A and which is proportional, as amplitude, to the amplitude of the cathode output pulse representative of the black current. The amplitude of the voltage variation at node A is determined by the product of the value of the resistor 30 by the amplitude of the current flowing through the resistor 30, The voltage variation at node A is transferred, through a resistor having a low ohmic value, indicated in 3 <*> 1, to a node B at which a voltage variation V, j is developed, corresponding essentially, the voltage variation at node A. The node B? connected to a bias control voltage processing network 50. The network 50 includes an input coupling capacitor 51, a clamping and sampling input operational amplifier 52 consisting, for example, of a trans-conductance operational amplifier, with an associated feedback switch 54. sensitive to the level fixing V timing signal and also includes a charge storage capacitor 56 with an associated switch 55 responsive to the V sampling reaction times signal. The voltage developed across the capacitor 56 is used to feed a kinescope bias correction signal through a network 58 and a network of resistors 60, 62, 64, to the kinescope pilot, through the bias control input at of the base of the transistor 20. The network 58, includes signal translation circuits and separation circuits, for supplying the bias control voltage, at an appropriate level and at a low level impedance, in accordance with the requirements of the ? bias control input of transistor 20. Will come now described the operating principle of the system schematized in figure 1, with specific reference to the wave forms shown in figure 2. The auxiliary signal is applied to the circuit node B, shown in figure 1, through a diode 35 and an impedance network translation, including resistors! 32 and 34 presenting, for example, values of 220 kilohm and 270 kilohm, respectively. The signal Vp has a positive level, in direct current, equal, approximately, to. + 8,0 volts in all conditions, except during the sampling interval AKB, at the time of keeping the diode 35 in the conducting state , in such a way as to develop, at node B, a normal direct current bias voltage. When? present the positive direct component of the signal Vp, the junction point between the resistors 32 and 34 is "fixed" at a voltage equal to the positive direct component of the signal Vp, decreased by the voltage drop across the diode 35. The signal Vp shows a impulsive component of fixed amplitude, less positive, with negative trend, during the AKB sampling interval. The diode 35 is switched to the non-conducting state in response to the negative pulse Vp, so that both resistors 32 and 34 are coupled between node B and the circuit ground. The resistor 31 causes an insignificant attenuation of the voltage variation developed at node A with respect to the corresponding voltage variation (V1) developed at node B, since? the value of resistor 31 (of the order of 200/1)? small compared to the resistor values! 32 and 34. Before the level fixing interval but during the time interval A KB the pre-existing nominal direct voltage present at the node Bf charges the positive terminal of the capacitor 51. During the level fixing interval, when the grid pilot pulse VQ is developed, the voltage at node A decreases in response to the pulse V_ by an amount? representative of the black current level. This causes a decrease in the voltage at node B up to a level substantially equal to V -. Furthermore, during the level fixing interval, the timing signal V causes the closing, i.e. the switching to the conducting state of the level fixing switch 54 and, therefore, the inversion input (-) of the signal, belonging to the amplifier 52, is coupled to the output of the considered amplifier, thus obtaining an amplifier 52 operating as a repeater amplifier, of the combined gain type of fixed reference voltage, in direct current V in? capable of supplying a voltage equal to, for example, 5 volts, applied to a non-inversion (+) input terminal of the amplifier 52, is coupled, by means of the feedback action, to the signal inversion input terminal of the amplifier 52, through the output of the amplifier 52 and by means of the switch 54 to the conducting state. Thus, during the level fixing interval, the voltage across capacitor 51 represents a function of a set-up voltage determined by the voltage V at the negative terminal of capacitor 51 and a function of a voltage at the positive terminal of capacitor 51, corresponding to the difference between the nominal direct current level (V ?? w) pre-existing at node B and the voltage variation developed in node B during the level fixing interval. Therefore, the voltage across the capacitor 51, during the level fixing reference interval, represents a function of the level of the voltage variation V,!, Representative of the black current, the value of which can? to vary. The tension can come expressed as (VN0M? vi)? VRIF? During the immediately following sampling interval, the grid pilot impulse is absent. positive VG causing a positive increase of the voltage on node B, up to the pre-existing nominal level in direct current V-T_, present before the sampling interval. Simultaneously, a pulse Vp with a negative trend is highlighted, with consequent polarization, in the reverse direction, of the diode 35 and perturbing, i.e. by momentarily varying the normal translation and coupling action of the voltage carried out by the resistors 32 and 34 and, therefore , the voltage on node B is reduced by a quantity? , as indicated in Figure 2. At the same time, the level fixing switch / $ 4 is switched to the non-conducting state, while the sampling switch 55 is closed, i.e., switched to the conducting state, in response to the signal V and, therefore, the charge storage capacitor 56 is coupled to the output of the amplifier 52. Therefore, during the sampling interval, the input voltage applied to the inversion input for the signals, i.e. to the (-) input of amplifier 52,? equal to the difference between the voltage at node B and the voltage across the input capacitor 51. The input voltage applied to the amplifier 52 represents a function of the amplitude of the voltage variation V1, which can? vary according to the variations that occur in the level of the black current of the kinescope. The voltage on the output storage capacitor 56 remains unchanged during the sampling interval, when the amplitude of the voltage variation developed during the level fixing interval is equal to the amplitude of the voltage variation V2 developed during the sampling interval, indicating a correct level of the kinescope black current. This arises from the fact that during the sampling interval, the voltage change at node B increases in the positive direction (from the level-fixing set-up reference level), when the pilot pulse of grid, while the voltage variation causes a simultaneous voltage perturbation, with a negative trend, at node B. When is the kinescope polarization? correct, the voltage variation, with a positive trend and the voltage variation, with a negative trend V? have equal amplitudes and, therefore, will result? evident that these voltage variations cancel each other out during the sampling interval, leaving unchanged the voltage value at node B, When the amplitude of the voltage variation is less than the amplitude of the voltage variation, the amplifier 52 proportionally charges the storage capacitor 56 in a direction such as to cause an increase in conduction of the black cathode current. Conversely, the amplifier 52 proportionally discharges the charge storage capacitor 56 to cause a decrease in the conduction of the black cathode current when the amplitude of the voltage change? greater than the amplitude of the voltage variation V ^? In accordance with as represented, more specifically by the waveforms shown in Figure 2, the amplitude "A" of the voltage variation is assumed to be equal, approximately to three millivolts, when the level of the black cathode current is correct, while it varies within a range of a few millivolts (_?) when the level of the black cathode current increases and decreases from the correct level, as the operating or working parameters of the kinescope vary. Therefore, the set-up reference voltage of the level fixing interval, present across the capacitor 51? it varies with variations in voltage amplitude as the level of the black cathode current varies. The voltage variation at node B has an "A" amplitude of about 3 millivolts, corresponding to the "A" amplitude associated with the voltage variation when the black current level is correct. As indicated by the waveform V shown in Figure 2, the voltage present at the inversion input (-) of the amplifier 52 remains unchanged during the sampling interval, when the voltages V2 both have an amplitude " However, as indicated by the waveform V, the input voltage of the amplifier 52 increases by an amount when the voltage variation V has an amplitude "A", corresponding to a high level of the current In this case, the amplifier 52 discharges the output storage capacitor 56 and, therefore, the bias control voltage, applied to the base of the transistor 20, causes an increase in the collector voltage of the transistor 22 and, therefore the cathode current of the black decreases towards the correct level. to quantity? ? during the sampling interval, when the voltage variation has an amplitude "A -?", corresponding to a low level of the black current. In this case, the amplifier 52 charges the output storage capacitor 56, with a consequent decrease in the voltage of the collector of the transistor 22 and, consequently, this will occur. an increase in the black cathode current, towards the correct level. In both cases, several sampling intervals may be required to obtain the correct black current level. The voltage developed at node B during the level fixing and sampling intervals AKB represents a function of the values of resistors 31, 32 and 34 and of the value of an output impedance Zq, present on node A. When the signal Vp manifests the positive level, in direct current (+ 8 volts) during the fixing interval of the level, the junction point between the resistors 32 and 34 is "fixed" as voltage, while a current conveyed by the resistor 31 , from node A to node B, represents a function of the values of Z, of the value of resistor 31 and of the value of resistor 34. During the next sampling interval, when the component of the signal Vp is present, represented by a trend pulse negative, the diode 35 is in the non-conducting state, while the junction point of the resistors 32 and 34 does not remain "fixed". In this time, a current of different value is conveyed by the resistor 31 from node A to node B, as a function of the value of the resistor 32, in addition to the values of the impedance Z o and of the resistors 31, 34. The variation voltage? ^, developed at node B in response to the impulsive component, a negative trend of the signal V is proportional to the difference between these currents. The impedance ZQ at node A can? undesirably vary as a function of the cathode bias level of the kinescope i.e. as a function of the cathode cut-off voltage level associated with the correct predicted level of the black cathode current? The resistor 31 compensates for the variations found in the value of the impedance Zq, and is also used to increase immunity. of the level fixing and sampling circuits, of the ete 50 towards the spurious signals generated locally and representing, for example, the recurrent interferences at the horizontal frequency? / the node A pu? be considered, in a model, as a voltage source, in series with the impedance ZQ mentioned above, the value of impedance ZQ represents a function of the value of the sensing resistor 30 divided by a factor of gain of the control loop, which, in turn, ^ z ^ <represents a function of the operating or working point of transistor 20, the point opei & ivo, o ?? work, of the transistor 20, during the injections AKB is more proportional to the cut-off voltage of the corresponding cathode. Practically, ? it was found that the impedance To provide a minimum value and a maximum value These functional aspects of the resistor 31 have been illustrated in a simultaneously filed US patent application having reference number (RCA 78.456) in the name of J. T. T II, et al. , titled "Automatic Kinescope Bias Control System Compensated for Sense Point Imped Variations". The described system automatically produces an output current of the value amplifier equal to zero, for the storage capacitor 56 when the amplitude not equal to 0 of the voltage variation V- corresponds to the correct level of the current. of black. Consequently, the use of manual pre-set bias controls is not required to offset the response with respect to the sampling amplifier output, producing a zero-value amplifier output current flow for the storage capacitor, when the ca to signal is present a non-zero amplitude for the correct polarization conditions. The assembly for coupling the input signals of the sampling amplifier described above, using an auxiliary pulse Vp, is advantageous in a system in which the operating amplifier 52 comprises a differential input amplifier, consisting, for example, of the differential amplifier to coupled emitters, according to when? hereinafter described with reference to Figure 4. A differential amplifier of this type has a symmetrical signal transfer response between input and output, which is non-linear in character within most of its operating range. The otherwise symmetrical operating range of the differential amplifier can? be made asymmetrical if the amplifier bias is offset by means of a pre-set manually adjustable bias control, by way of illustrative example. In this case, ? more likely that the amplifier produces an output that is contaminated by the effects of noise and similar spurious signals, since? the asymmetrical and staggered response of the amplifier pu? lead to a straightening of the noise in the non-linear operating region of the amplifier. Consequently, the sample of the output signal and the corresponding voltage developed on the output charge storage device will be distorted or obscured due to the effects of the rectified noise * The combined pulse sampling assembly described above also advantageously provides a convenient mechanism for compensating for mutually different conduction (gain) characteristics and correspondingly different turn-off voltages of the kinescope electron guns due to example, to the manufacturing tolerances of the kinescope. This aspect of the version described? been analyzed in detail, in the United States Patent Application No. 4 *? 3 ?? 3? 0, entitled "Automatic Kinescope Bias Control System Compensated for Kinescope Electron Gun Conduction Dissimi1arities", filed 1A October 1-9frS - <^ this system being now described briefly in the course of this discussion. When the electron guns of the kinescope are identical and when, consequently, they have the same conduction characteristics, they will conduct equal currents at the black level while they will have equal cut-off voltages, represented, for example, by the voltages between the grid and the cathode. However, in practice, electron guns exhibit mutually different conduction characteristics. In the latter case, the different currents conveyed by the electron guns are considered as currents at the correct black level and, therefore, the AKB system should remain in a quiescent condition and should not alter the kinescope polarization even if the electron guns have levels of the mutually different black current and the corresponding associated mutually different cut-off voltages. This result is obtained by means of the complex described, since? the amplitude of the voltage variation V2, developed at node B, is linearly proportional to the component represented by the direct voltage that becomes available at node A. This component, represented by the direct voltage, is proportional to the cathode interdiction voltage, according to what is manifested by the component, constituted by the direct voltage, present at the output of the pilot transistor 22 and corresponding to the cathode voltage, during the? AKB interval, neglecting the effect of the cathodic current pulse of the induced output, developed in response to the grid pilot pulse VQ of polarity? positive. Therefore, if the three electron guns of the kinescope exhibit mutually different currents and cut-off voltages corresponding to the initial black level tuning conditions, the voltage variation respectively associated with the processors of the signals 14a, 14b and I 4c will present, individually a different amplitude, even if each of them is derived from a common signal V ^? The different amplitudes of the voltage variations V? represent a function of the different interdiction voltages, second? what is manifested by the direct current components, of different amplitude, developed at the nodes A. The different amplitudes of the voltage variations V2 are such that, for the associated control loop AKB, the voltage developed at the node B will not vary ? when are the voltage variations V, j and V combined? and, therefore, each AKB control loop rhyme: r? in the state of quiescence. The AKB control loops will remain in quiescent conditions as long as? the electron gun currents, at the black level, established initially, will vary due to a variation in the operating parameters of the kinescope, due to the aging effects of the kinescope or the effects resulting from changes in temperature. In some systems AKB can? be desirable to develop a voltage variation representative of the black current, during the sampling interval, instead of? during the previous level fixing interval, as previously described. In an alternative system of this type, the grid pilot pulse V is timed in such a way that it can occur during the sampling interval, while the relations between the timings of the signals of the type represented by the forms d can be adopted. The wave shown in Figure 3. In a system of this type, the timing of the signals V, H, V, vs and Vc remains unchanged. The waveforms for the alternate system have been placed back in Figure 3. A grid V pilot pulse? of polarity? positive and an auxiliary pulse of polarity? positive, they are coincident during the sampling interval. During the initial level fixing interval, the "tuning reference level" is a function of the DC voltage then present at nodes A and B. During the next sampling interval, the voltage variation V, j has an amplitude "A" when the black current level is correct, an amplitude A? when the black current level? low and an amplitude A -? when the black current level is high. The voltage variation V.j is summed, during the sampling interval, with the amplitude voltage variation "A". Therefore, when the black current level is correct, the voltage variation V? it is canceled with the variation of voltage V ^, since? both have the same amplitude "A", but polarity? opposite. The voltage which is then applied to the fixing capacitor of level 51, by node B, is therefore corresponding to the reference level applied by node B during the previous sampling interval so that the input voltage of the amplifier 52 does not vary during the sampling interval, according to when indicated by the waveform V of the signal for the condition corresponding to a corresponding to a correct current. Therefore, the storage capacitor 56 is not charged n? discharged from amplifier output current 52. For this alternative system,? It is possible to develop a voltage variation at node B by selectively transferring a split version, as voltage, of the positive pulse to node B during the sampling intervals. The level fixing reference level, developed during the level fixing interval, for the conditions corresponding to a low level and high level black current, is similar to the level fixing reference level developed when the level b of the black current is correct.However, in the case of a high value of the black current, the voltage variations V ^ 'e are not completely canceled during the sampling interval, and the input voltage of the amplifier 52 increases by a quantity? ? during the sampling interval (waveform V). Conversely, a black current condition results in incomplete cancellation and, therefore, the input voltage supplied to the amplifier 52 decreases by an amount. ? during the sampling interval (waveform Figure 4 illustrates the circuit details of the signal level fixing and sampling network 50 schematized in Figure 1, while in this figure, the various corresponding elements have been identified by the same reference numbers previously adopted. In accordance with as shown in Figure 4, the amplifier 52 comprises a transconductance operational amplifier, in which an output current is produced as a function of the product of the input voltage of the amplifier and the transconductance (g). The amplifier 52 includes the coupled emitter transistors 66, 68, arranged in a differential input amplifier configuration and a current repeating network ("current mirror") including a transistor 71, connected to a diode and a transistor 74 arranged in the collector circuit of transistor 68, in accordance with as indicated. A first constant current source including a forward biased transistor 69 and a resistor R provides an operating or working current I for transistors 66 and 68. A second constant current source including a biased transistor 75 in the direct direction, and a resistor 2R provides an opaque current l / 2 for the transistor 74. The source of the reference direct voltage V_TC1 is connected to the non-inversion input of the amplifier 52, at the base of the transistor 68. The The input signal to be sampled, according to what is derived from node B shown in Figure 1, is applied, through the input capacitor 51, to the inversion input terminal of the amplifier 52 at the base of the transistor 66. During the fixing interval of the AKB level, the collector of transistor 68? coupled to the input capacitor 51 through the diode-connected transistor 71, the transistor 74 and the switch 54 in the conducting state, so as to form a negative feedback current path. During this time, the charge accumulation capacitor 56 is decoupled from the amplifier 52, through the switch 55 which results in the non-conducting state. The input capacitor 51 is charged through the currents conveyed by the transistors 68, 71 and 74, as a function of the reference voltage V ^ and of the potential then applied to the input of the capacitor 51 from node B of figure Q ^ est ^ cai? - <5 <m ti? Ua finch? the base voltages of the transistors 66 and 68 are substantially the same, that is to say until? the differential input voltage of the amplifier 52 is substantially equal to zero. The current I supplied by the transistor 69 is therefore divided equally between the transistors 66 and 68 and, therefore, the collector currents of the transistors 68 and 74 are equal to the collector current (1/2) conveyed by the transistor 75. Consequently, all of the collector current carried by transistor 74 flows as collector current in transistor 75. The current feedback path stabilizes at a zero current condition before the end of the level fixing interval and, in turn. these conditions, the transistor 75 "absorbs" all the collector current of the transistor 74, while the feedback current, having a value equal to zero, flows towards the base of the input transistor 66. During the subsequent AKB sampling interval the switch 54 is switched to the non-conducting state, while the switch 55 passes to the conducting state, in such a way as to connect the charge storage capacitor 56 to the output of the amplifier 52. The pre-existing charge on the capacitor 56 remains unchanged as long as? the input signal applied to the capacitor 51 is sufficient to alter the balanced base bias of the transistors 66 and 68, as established during the previous level fixing interval. Therefore, when the voltage variation has an amplitude "A" corresponding to a current condition at the black level, the voltage input to the transistor 66 will remain of a correct value. unchanged, as indicated by the waveform V shown in Figure 2. Consequently, the balanced input bias of the transistors 66, 68 and the charge on the output storage capacitor 56 remain unchanged. When the black current level is wrong so that the input voltage fed to the transistor 66 is increased, as represented by the waveform V shown in Figure 2, it will occur? a decrease in the currents carried by transistors 68, 71 and 74. The charge storage capacitor 56 is discharged through the transistor 75 by a quantity of proportional to the lower conduction of transistor 74, in response to the higher input voltage. In this case, the transistor 75 operates as a current absorber with respect to the charge storage capacitor 56 being discharged. Similarly, a decrease in the input voltage applied to transistor 66, as indicated by the waveform VL shown in Figure 2, causes a corresponding increase in the collector current of the output transistor 74. The charge storage capacitor 56 charges through transistor 74, in response to this increased current conduction and, therefore, will occur? an increase in the voltage across capacitor 56. In this case, transistor 54 acts as a current source for charging capacitor 56. Figure 5 illustrates a block diagram of a logic assembly for the timing signal generator 40 shown in Figure 1. A binary counter 90 includes CLOCK and RESET inputs sensitive, respectively, to the horizontal signal H and to the vertical signal V, a DISABLE input and the binary outputs -Q ^. Counter 90 is reset in response to the positive pulse portion of the V j? Vt ?? as? - & ^ signal which occurs during the vertical retrace interval. Therefore, the outputs Q1 - all have a low logic level (0000 ^ while the RESET input is positive during the vertical retrace interval. During this time, counter 90 does not respond to clock pulses H recurring at the horizontal frequency. A combination logic array 92 including, for example, a plurality of of logic gates controls the binary states of the outputs Q, j - of the counter 90, through the inputs A -D. At the end of the vertical retrace interval, the correspondence of the instant, the counter 90 is enabled for operation. The logic states of the outputs of the counter 90 vary in order to indicate a binary number corresponding to the number of clock pulses H occurring from the end of the vertical retrace interval. A logic output F, derived from array 92, produces a high logic level ("1") during the interval embracing the second-eighth clock pulse H, by detecting the expected condition of the counter outputs Q ^ - Q ^ during this time interval. This signal is delayed by means of a delay network 93, so as to obtain a time delay ?? in the production of the timing signal AKB which is developed at the output of the delay network 93. The delay introduced by the delay network 93 can? be produced, for example, by means of a plurality? of logic gates coupled in series, each capable of introducing a given time delay. The timing signal V is developed at the output G of the array of logic gates 92, during the time interval including the third to fifth clock pulse H from the end of the vertical retrace interval. This signal is delayed by an amount? through a net 94 and the layer is moved by means of a net 95, in such a way as to produce the grid pilot pulse VQ. The level displacement network 95 consisting, for example, of a voltage translator is used to produce the VQ signal with an amplitude suitable for driving the electrode. 1) Automatic polarization control apparatus in a video signal processing system including an image reproduction device sensitive to video signals fed to an intensity control electrode. of the same, characterized in that it comprises: means for deriving a periodic signal representative of the level of the black current conveyed by said intensity control electrode, during the intervals of suppression of the images of the video signal, said derived signal presenting an amplitude non-zero, when the black current level is correct; means of storing information? amplifier means with an input for the signals and an output coupled to said storage means, for modifying the information content of said storage means, in accordance with the conduction condition of said amplifier means, in response to applied input signals; input signal coupling means for coupling said derived signal to said amplifier input; means Js ^) for supplying a periodic auxiliary signal to said means for coupling the input signals, with an amplitude and a sense such as to substantially cancel the response of said amplifier towards the amplitude of said derived signal when the amplitude of said derivative signal? representative of a correct level of the black current; And means for supplying a bias correction voltage derived from said storage means to said imaging device for maintaining a correct level of black current, 2) Apparatus according to the claim 1, characterized by the fact that: impedance means ??? 2 ', ??) are coupled to said input signal coupling means, to establish a bias for said input signal coupling means, in the presence of said derivative signal and in which; said auxiliary signal is applied to said impedance means, to modify said established bias, with a sense such as to produce said negative response of the amplifier. 3) Apparatus according to claim 1, characterized in that; said auxiliary signal cancels said derivative signal in said path for the input signals, in order to produce said negative response of the amplifier. 4) Apparatus according to claim 3, characterized in that said derivative signal) and said auxiliary signal (?? ^) comprise coincident polarity pulses? mutually opposite and of substantially equal amplitude, when said derivative signal? representative of a correct level of the black current. 5) Apparatus according to claim 1, characterized in that: said auxiliary signal has an amplitude proportional to a component represented by a direct voltage, manifested by said intensity control electrode. during said blanking intervals. 6) Apparatus according to claim 1, characterized in that it comprises: level fixing means coupled to said input signal coupling means at said amplifier input; switching means coupled to said amplifier output, to said level fixing means and to said storage means; means (J && T for operating said switching means during an initial level fixing interval for: (1) fixing said amplifier input to a reference voltage in response to a reference source coupled to said amplifier input during said level fixing interval and (2) decoupling said amplifier output from said storage means and for operating said switching means during a subsequent sampling interval to (3) unlock said amplifier input and (4) couple said output of the amplifier to said storage means; said signal representative of the black current is developed during said fixing interval dt ?. level and coupled to said level fixing means, so that said reference voltage to which said amplifier input is fixed, also represents a function of the amplitude of said derived signal; And said auxiliary signal (V ^) is developed during said subsequent sampling interval 7. Apparatus according to claim 6, characterized in that: the aforementioned level fixing means comprise a capacitor for coupling the signals, coming from the aforementioned input coupling means, to the aforementioned input of the amplifier, and the voltage developed across said capacitor during the aforementioned level fixing interval represents a function of the amplitude of the aforementioned derived signal; And the aforesaid auxiliary signal has an amplitude for maintaining the voltage, at the aforesaid input of the amplifier, substantially unchanged, when the amplitude of the aforesaid derived signal? representative of a correct level of the black current. 8. Apparatus according to claim 7, characterized in that: the aforementioned image reprozuion device? consisting of a kinescope, including an electron gun, comprising a grid electrode and an associated intensity control electrode. cathodic; . the aforesaid apparatus for automatic bias control also includes means for biasing the bias. said kinescope electron gun during said level fixing interval, for inducing an output cathode signal having a proportional amplitude at the black cathode current level; And the aforesaid shunting means derive the aforesaid periodic representative signal from the aforementioned induced output cathode signal. 9. Apparatus according to claim 1 and further comprising: fixing means, coupled to said input signal coupling means, in correspondence with said amplifier input; switching means, coupled to said amplifier output, to said level fixing means and to said storage means; means for operating the aforementioned switching means during an initial level fixing interval for (1) fixing said amplifier input to a reference voltage, in response to a reference source coupled to said amplifier input filer. during the aforementioned interval. fixing the level and, (2) uncoupling it on said, outlet of the a & plffi catar. by the aforementioned storage means; and to make the aforementioned switching means operative during a following sampling interval for (3) freeing the ruddett input of the amplifier, and (4.) coupling the on said output of the amplifier to the aforesaid storage means and characterized from the fact that the aforesaid signal derivative, representative of the black current. and the aforementioned auxiliary signal come. .both developed during the aforementioned wide -10 interval. Apparatus according to claim 9, characterized in that the aforementioned. level fixing means comprise a capacitor for coupling signals from said input coupling means to said amplifier input; The aforesaid auxiliary signal has an amplitude and a sense for maintaining the voltage at the aforesaid input of the amplifier, substantially unchanged, when the amplitude of the aforesaid derived signal? representative of .un. correct-to-black-current-level .__ 11. Apparatus according to claim 1? ,. characterized by. fact that: The aforesaid device stands for image replaying consisting of a kinescope, including an electron gun, along with a grid electrode and an associated electrode. from. XHK control .dell Vint.ens.it? ca.todica; the aforesaid apparatus for the automatic control of the polarization, further includes means for polarizing the aforesaid electron gun of the kinescope during the aforesaid sampling interval, to induce an output cathode signal, having an amplitude proportional to the level of the black cathode current; and the aforesaid derivation means derive from the aforesaid. periodic signal representative of the aforementioned signal ia. output cathode. 12.. Apparatus according to claim 1, characterized in that: the aforesaid amplifier means comprise a differential input amplifier. 13. In a system for processing video signals, including? a device for the reproduction of the jjnmaginjei, sensitive to video signals, powered, to an intensity control electrode, an apparatus for the automatic control of the polarization, comprising: means for deriving a signal period representative of the level of the current of the ne conducted by the aforesaid control electrode lasts the intervals of suppression of image of the video itself, the aforesaid derived signal presenting an amplitude different from zero, when the aforesaid live of the current some black? correct; a capacitor; means for coupling said derivative segment to said capacitor, to alter its charge; means for providing a periodic auxiliary signal to the aforesaid capacitor, for altering its parity; ..scilappata.in response to the above derivative seg, when the amplitude of the above derivative signal? representative of the correct black current level; And means to feed a -GE voltage? biasing the aforementioned imaging device to maintain a correct level of black current in response to the charge on the said capacitor. 14. Apparatus according to claim. 13, characterized by ... the fact that; . _the_signal.auxiliary.presen an amplitude as a function., d? a component. DG voltage manifested by the aforementioned control electrode of the intensity? during the above blanking intervals,. . I declare that I have read the description and the claims above and that I believe absolutely d? to be the original, first and only inventor of the invention relating to: _ SIGNAL PROCESSING NETWORK FOR AN AUTOT, LITIC CONTROL SYSTEM OF THE. POLARIZATION OF A KINESCOPE described and claimed therein; I declare d? not to know and not to believe that the present invention was never known or used in the United States of America prior to my prior invention, or patented or described in any publication escaped in any country prior to my invention. p pi? one year prior to this application; or used in public or for sale in the United States of America pi? one year prior to the previous application; I declare that the present invention does not? has been patented or has not been the subject of any previously issued certificate of invention! of the date of this application in any foreign country outside the United States of America on an application filed by me or my legal representatives or multiple assignees. twelve months prior to this application; I declare that I am aware of my duty which requires me to clarify any information in my possession that may be important for the purposes of examining this application; and that no patent application or certificate of invention relating to the present invention? been filed by me or by the MKs? x my representatives or assigns in any country outside the United States of America, in accordance with the present invention ..... _ _ And with the presents io_nomino. individually and with .1 actively:. Name Registration NumB. Fugene H. Wfc.it; acre 17,462 Paul J. Ha.sm s.sen. .. 24-872 Peter M. Emanuel 26.542 Ronald H. Kurdyla 26.932 my .. agents, whose address? RCA Corporation, Patent Operations, P.O. Box 432, Princeton, New Jersey 08.5.40 ,. ~. ' to continue with this application and to deal with all transactions connected with it with the United States Patent and Trademark Office. Please send all communications to: Eugene tl. Whitacre "Signal processing network for an automatic kinescope polarization control system" SUMMARY A system for automatic kinescope bias control (AKB) includes a sampling amplifier (52) which provides an output current for charging and discharging a storage capacitor (56) in response to the? amplitude of a derivative pulse (V1) representative of the level of the black current conveyed by the kinescope of a television receiver. The amplitude of the derived pulse is greater than zero when the black current level is correct and is applied to the amplifier through a coupling path of the input signals. During the sampling intervals, an auxiliary pulse (V) is applied to the path of the input signals with an amplitude and a direction such as to negate the amplitude of the pulse derived at the input of the amplifier when the amplitude of the The derivative pulse corresponds to the correct level of the black current. Consequently, the conduction of the amplifier and the voltage on the storage capacitor remain unchanged. An impedance included in the signal path compensates for changes in impedance at a detection point from which the signal representative of the black current is derived. DESCRIPTION OF THE INVENTION The present invention relates to a complex signal processor used in a system to automatically control the level of a current representative of a black image, conveyed by a device for displaying images associated with video signals and consisting, for example, of the kinescope of a television receiver. Furthermore, the invention relates to a complex for compensating for the various impedance actions at a detection point from which a signal representative of the black current is derived so that the impedance variations cannot negatively alter the operation of the subsequent circuits. to which the detection point is coupled. Television receivers sometimes use an automatic kinescope bias (AKB) system to automatically establish the appropriate representative black image current levels for each kinescope electron gun. As a result of this operation, the images reproduced by the kinescope are prevented from being negatively influenced by the variations that may occur in the operating parameters of the kinescope and caused, for example, by aging phenomena and by effects associated with temperature variations. A type of AKB system? has been described in U.S. Patent No. 4,263,622 to Werner Hinn, entitled "Automatic Kinescope Biasing System." An AKB system typically operates during image suppression intervals, i.e. when the kinescope carries a small suppression current represented by the black level, in response to a reference voltage representative of the black video signal information. This current is monitored by the AKB system, in order to generate a kinescope bias correction voltage representing the difference between the detected black current level and a desired black current level. The correction voltage is applied to the kinescope, for example through the video signal processing circuits preceding the kinescope with a sense that reduces the difference. Typically, the correction voltage is applied to a bias control input of a DC coupled kinescope driver amplifier which feeds the output video signals, having an appropriate level, in order to directly drive a cathode electrode of intensity control? of the kinescope. The correction voltage modifies the output bias voltage of the driver amplifier, so as to modify the cathode bias voltage, in order to obtain the desired level of the black cathode current. In an AKB system, of the type described in the previously indicated Hinn patent, the control circuits respond to a periodically derived signal, with an amplitude representative of the level of the black cathode current. The derived signal has a prescribed level different from zero, when the black current level is correct and different levels (for example more or less positive) when the black current level is too high or too low. The derived signal is developed at a derivation point which is connected to control circuits including level fixing and sampling networks, for the development of a kinescope polarization correction signal, in accordance with the signal amplitude derivative. For example, the derived signal can? be sampled by means of a sampling amplifier which determines the charge or discharge of a storage capacitor in accordance with the level of the derived signal. The bias correction signal increases, or decreases, accordingly. as required, to maintain a correct black current level. In the course of the present discussion, it is recognized that the control circuits to which the detection point is coupled can be negatively affected when the detection point from which the signal representative of the black current is derived exhibits variations in impedance. as a function of the polarization level of the kinescope pilot. Consequently, an apparatus is disclosed for substantially canceling the effect of these impedance variations on the control circuits. The described apparatus also advantageously increases immunity? of a level fixing network associated with the control circuits, with respect to spurious signals including locally generated interference which could otherwise distort or obscure the bias correction signal. When the bias correction signal is derived from a storage capacitor, it takes the form of a bias correction voltage. The bias correction voltage, derived from the storage capacitor, should remain unchanged when the level of the derived signal, in the form of a voltage pulse, represents a correct level of the black current. This requires that the storage capacitor not be charged n? discharged by the output current from the sampling amplifier, when the voltage pulse level represents a correct level of the black current. More specifically, in an AKB system of the type described in the Hinn patent, this requires that the sampling amplifier must not supply any current to the storage capacitor when a current at the correct level of the kinescope black is indicated, according to what indicated by a representative voltage pulse having a predetermined amplitude other than zero. This result can? be obtained by staggering the bias of the sampling amplifier, for example by means of a manually adjustable preset potentiometer connected to an appropriate amplifier bias control point. It is also recognized that these pre-set manual adjustments are undesirable in an otherwise automatic signal control system. Furthermore, these manual adjustments are undesirably lengthy / while the associated potentiometers result in increased system costs. It should also be noted that the signal processing techniques adopted by some AKB systems can produce an offset or "offset" error if the cut-off voltages and signal gains of the individual kinescope electron guns are not identical due to tolerances of manufacture of the kinescope, by way of illustrative example. In a case of this type, the level of the black current, established by the AKB system, can? present an error, which can? be compensated by means of preset potentiometers, manually adjustable. The described system advantageously facilitates the design of signal processing circuits for the automatic polarization of the kinescope (AKB), this complex not requiring the use of manually adjustable controls, for the compensation of such staggering or "offset" errors. . The present invention relates to a video signal processing system in which a derived signal, representative of the black current level conveyed by an image display device, has a given amplitude other than zero when the black current level is correct. . The representative derived signal is applied, through a coupling path of the input signals, to a sampling amplifier which supplies an output current to charge and discharge a charge storage device, in accordance with the amplitude of the derived signal. . In accordance with a principle of the present invention, an auxiliary signal is fed to the coupling path of the input signals, with an amplitude and a direction such as to cancel the amplitude of the signal derived at the input of the amplifier, when the amplitude of the derived signal? representative of a correct level of the black current. Consequently, the conduction of the sampling amplifier remains unchanged, when the amplitude of the derived pulse corresponds with the correct level of the black current and when the voltage on the storage device remains unchanged. In accordance with a characteristic of the present invention, the amplitude of an auxiliary signal is proportional to the amplitude of the kinescope cathode interdiction potential, developed during the AKB interval. According to another characteristic of the invention, the input of the sampling amplifier is fixed at a reference voltage, during a fixing interval of the level which precedes the sampling interval of the signals. The derived signal, representative of the black level, is developed during the level fixing interval, so that the reference voltage at which the amplifier input is "fixed", during the level fixing interval , represents a function of the amplitude of the derived signal, while the auxiliary signal is developed during the next sampling interval. Does the auxiliary signal have a? amplitude and a sense such as to maintain substantially unchanged the input voltage of the amplifier when the amplitude of the derived signal corresponds with the correct level of the black current. In accordance with a further feature of the invention, the amplifier input is "fixed" to a reference voltage during the level fixing interval, while the derived signal and the auxiliary signal are both developed during the level fixing interval. sampling. Furthermore, in accordance with With the principles of the present invention, the automatic kinescope bias control apparatus includes a capacitor for coupling a derivative signal, representative of the kinescope black current level and an auxiliary signal having a predetermined amplitude and direction, at all. input of a sampling amplifier. The source of the derived signal has a variable output impedance, which is proportional to the magnitude of the kinescope polarization level. The derived signal, representative of the kinescope black current level, is supplied from the variable impedance output of the derived signal source to the capacitor via a coupling impedance. The coupling impedance is high with respect to the variable output impedance, so as to significantly reduce the impedance variations presented to the source of the auxiliary signal by the output of the source of the derived signal. In accordance with a further feature of the invention, the coupling capacitor? included in a level fixing network, the coupling impedance also increases the immunity? of the response of the level fixing network towards spurious signals. The present invention will result? more? evident from? analysis of the following detailed description which must be considered in conjunction with the attached drawings in which: Figure 1 illustrates a portion of a color television receiver including an AKB system and a * associ et? signal sampling network, incorporating the characteristic principles of the present invention; Figure 2 illustrates the waveforms of the signals which are associated with the operation of the system shown in Figure 1; Figure 3 illustrates an alternative version of the signal waveforms shown in Figure 2; Figure 4 illustrates the circuit details of the sampling network schematized in Figure 1; and Figure 5 illustrates the circuit details of a timing signal generator associated with the system shown in Figure 1. In accordance with As shown in FIG. 1, the television signal processing circuits 10 supply the separate luminance (Y) and chrominance (C) components of a color composite television signal to a L1 signal processing network 12. minanza-chrominance. The processor 12 includes circuits for controlling luminance and chrominance gains, direct current level setting circuits (including, for example, manipulated black level fixing circuits), color demodulators for developing the difference signals. color of red, green and blue r-y, g-y b-y, respectively, and matrix amplifiers to combine these latter signals with the processed luminance signals, in such a way as to allow the generation of the associated low-level color image representative signals to the color red, the color green and the color blue, r, g and b, respectively. These signals are amplified and otherwise processed by means of circuits contained in the processing networks of the output video signals, distinguished by the references 14a, 14b and 14?, Respectively, which feed signals representative of the color images, suitably amplified and of a high level. , associated with red, green and blue, R, G and B, respectively ^! respective cathode electrodes for intensity control? 16a / and 16c of a color kinescope 15? The networks 14a, 14b and 14c also perform functions related to the AKB operation according to what will happen. described below. The kinescope 15? of the in-line, self-converging electron gun type, having a grid 18, excited in common, associated with each of the electron guns comprising the cathode electrodes 16a, 16b and 16c; Since? in this practical form, in accordance with the invention, the processors 14a, Hb and 14? of the output signals are similar, the description that will come? now reported concerning the operation of the computer 14a will result? also valid for processors 14b and 14c? The processor Ma includes a kinescope pilot stage comprising a common emitter input transistor 20, which receives the video signal r from the processor 12, through an input resistor 21 and an output transistor 22, with common base, to high voltage, which, together with the transistor 20, forms an ascode video driver amplifier. "The high level video signal R, suitable for driving the cathode 16a of the kinescope, is developed at the ends of a load resistor 24 present in the circuit of the output collector of the transistor 22. An operating supply voltage for the amplifier 20, 22 is supplied by means of a direct voltage source of high value B + whose value is equal to, for example, 230 volts. , in direct current, for The pilot stage comprising the transistors 20, 22 is obtained by means of a resistor 25. The gain, as far as the signal is concerned, which characterizes the cascode amplifier 20, 22,? determined mainly by the ratio between the value of the feedback resistor 25 and the value of the input resistor 21, The feedback network allows to obtain an output impedance from the amplifier, of an appropriately low value and allows to favor the stabilization of the operating level in direct current, at the output of the amplifier. A detection resistor 30 connected, in direct current, in series with the collector-emitter paths of the transistors 20 and 22, serves to develop a voltage at node A, of relatively low value, representing the level of the black cathode current of the kinescope that is conveyed during the kinescope cancellation intervals. Resistor 30 operates in conjunction with the receiver's AKB system, depending on what will happen. now described in more detail. A timing signal generator 40 containing suitable control logic circuits, responds to the periodic signals recurring at the horizontal synchronization frequency (H) and to the periodic signals recurring at the vertical synchronization frequency (v), these signals being derived from the deflection circuits of the receiver, in order to generate the creation time signals ??, V? f V_ ,? V_ and V can control the operation of the AKB circuit, during the AKB periodic intervals. Each of the AKB intervals begins just after the end of the vertical retrace interval within the vertical blanking interval and encompasses several horizontal line intervals, also falling within the vertical blanking interval and during which the representative information of the image associated with the video signal. These timing signals were represented by the respective waveforms in Figure 2. Referring specifically to figure 2 for the moment, can you? it can be detected that the timing signal V ^, constituting a video suppression signal, comprises a positive pulse, generated just after the end of the vertical retrace interval at the instant f as indicated by the reference to the waveform of the signal V. The suppression signal V exists for the duration of the AKB interval and the same is applied to a suppression control input terminal of the luminance-chrominance processor 12, so that the outputs r, g and b of the processor 12 may present a direct current reference level representative of a black image, corresponding to the absence of video signals. be obtained by reducing the gain, as regards the signal of the processor 12, to a value substantially equal to zero, through the gain control circuits of the processor 12, in response to the signal V?, and by means of a modification of the direct current level of the video signal processing path, through the direct current level control circuits of the processor 12, in such a way as to produce a reference level representative of the black image at the outputs, for the signals, of the processor 12, the timing signal V, constituting a grid pilot pulse, of polarity? positive, spans three horizontal line intervals within the vertical blanking interval. The test signal V controls the operation of a level fixing circuit associated with the signal sampling function of the AKB system. The timing signal V, constituting a sampling control signal, occurs after the signal V and the same is used to timing, or synchronize, the operation of a sampling and holding or storage circuit which develops a control signal of the direct current polarization, to control the cathode current level of the black of the kinescope. The signal V embraces a sampling interval, the beginning of which is slightly delayed with respect to the end of the level fixing interval embraced by the signal V, and whose term substantially coincides with the end of the AKB interval. An auxiliary pulse Vp, with a negative trend, whose function will come? hereinafter described in more detail, it coincides with the sampling interval. The delays in the timing of the signals, indicated in Vp, according to what is represented in Figure 2, are of the order of 200 nanoseconds. Referring again to Figure 1, it must be noted that during the interval AKB, the positive pulse V ?, whose value? of the order of +10 volts polarizes, in a direct direction, the grid 18 of the kinescope, causing an increase in the conduction of the electron gun comprising the cathode 16a and the grid 18. At different time intervals there are the intervals AKB, the signal V provides the normal, less positive, polarization for the kinescope grid 18. In response to the positive grid impulse V_, a positive current impulse, of analogous phase, is evident at the cathode 16a, during the interval corresponding to the grid impulse. The amplitude of the output deficient cathode pulse thus developed is proportional to the conduction level of the black current of the cathode, whose value? typically equal to a few microamps. The induced positive output cathode pulse is highlighted at the collector of transistor 22 and is fed to the base input terminal of transistor 20, through resistor 25, causing a proportional increase in the degree of conduction of the current of transistor 20, while the cathode pulse is present. The greater current conveyed by the transistor 20 causes the development of a voltage across the sensing resistor 30. This voltage is presented in the form of a voltage variation, with a negative trend, which becomes available at node A and which is proportional , as amplitude, to the amplitude of the cathode output pulse representative of the black current. The amplitude of the voltage variation at node A is determined by the product of the value of the resistor 30 by the amplitude of the current flowing through the resistor 30. The voltage variation at node A is transferred, through a resistor having a low ohmic value, indicated at 31, to a node B at which a voltage variation is developed, essentially corresponding to the voltage variation at node A. The node B? connected to a bias control voltage processing network 50. The network 50 includes an input coupling capacitor 51, a clamping and sampling input operational amplifier 52 consisting, for example, of a trans-conductance operational amplifier, with an associated feedback switch 54 sensitive to the timing signal V , level fixing and also includes a storage capacitor. charge 56 with an associated switch 55 responsive to the sampling timing signal V. The voltage developed across capacitor 56 is used to feed a kinescope bias correction signal through a network 58 and a resistor network 60, 62, 64, to the kinescope pilot, through the bias control input at of the base of the transistor 20. The network 58, includes signal translation circuits and separation circuits, for supplying the bias control voltage, at an appropriate level and at a low level impedance, in accordance with the requirements of the bias control input of transistor 20. Will come now described the operating principle of the system schematized in figure 1, with specific reference to the wave forms shown in figure 2. The auxiliary signal Vp is applied to the circuit node B, shown in figure 1, through a diode 35 and a voltage shift impedance, including resistors! 32 and 34 presenting, for example, values of 220 kilohm and 270 kilohm, respectively. The signal Vp has a positive level, in direct current, equal, approximately, to 8.0 volts in all conditions, except during the sampling interval AKB, in order to keep the diode 35 in the conducting state, so that to develop, at node B, a normal direct current bias voltage. When ? present the positive direct component of the signal Vp the junction point between the resistors! 32 and 34 is "fixed" at a voltage equal to the positive direct component of the signal Vp, decreased by the voltage drop across the diode 35? The signal Vp manifests a pulse component of fixed amplitude, less positive, with negative trend, during the sampling interval AKB, The diode 35 is switched to the non-conduction state in response to the negative pulse Vpl so that both resistors 32 and 34 are coupled between node B and the circuit ground. The resistor 31 causes an insignificant attenuation of the voltage variation developed at node A with respect to the corresponding voltage variation (V ^) developed at node B, since? the value of resistor 31 (of the order of 200J1)? small compared to the values of resistors 32 and 34. Before the level fixing interval but during the time interval AKB the pre-existing nominal DC voltage present at node Bf charges the positive terminal of the capacitor 51. During the level fixing interval, when the pulse is developed grid pilot VQ, the voltage at node A decreases in response to the pulse V_ by a quantity? representative of the black current level. This causes a decrease in the voltage at node B up to a level substantially equal to V? _W - V ?? Furthermore. during the level fixing interval, the timing signal V causes the closing, i.e. the switching to the conducting state of the level fixing switch 54 and, therefore, the inversion input (-) of the signal, belonging to the amplifier 52, is coupled to the output of the considered amplifier, thus obtaining an amplifier 52 operating as a repeater amplifier, of the unity gain type. In this time, the charge storage capacitor 56 is decoupled from the amplifier 52, through the switch 55 in the non-conducting state. Consequently, a source of fixed reference voltage, in direct current V capable of supplying a voltage RXF equal, for example, to 5 volts, applied to a non-inversion (+) input terminal of the amplifier 52, is coupled, by means of the feedback action, to the signal inversion input terminal of the amplifier 52, through the output of the amplifier 52 and by means of the switch 54 to the conduction state. Thus, during the level fixing interval, the voltage across capacitor 51 represents a function of a set-up voltage determined by the voltage V at the negative terminal of capacitor 51 and a function of a voltage at the positive terminal of the capacitor 51> corresponding to the difference between the nominal direct current level (v) pre-existing at node B and the voltage variation developed in node B during the level fixing interval. Therefore, the voltage? V across capacitor 51? during the reference interval of fixing the level represents a function of the level of the voltage variation, representative of the black current, whose value can? to vary. The tension can be expressed as (VN0M? v- |) ~ VRIF * During the immediately following sampling interval, the grid pilot impulse is absent. positive VQ causing a positive increase of the voltage on node B, up to the pre-existing direct current nominal level V, present before the sampling interval. Simultaneously, a negative trend pulse Vp is highlighted, with consequent polarization, in the reverse direction, of the diode 35 and perturbing, that is to say momentarily varying the normal translation and coupling action of the voltage carried out by the resistors! 32 and 34 and, therefore, the voltage on node B is reduced by a quantity? V? fruitful as indicated in Figure 2. At the same time, the level fixing switch / 64 is switched to the non-conducting state, while the sampling switch 55 is closed, i.e., switched to the conducting state, in response to the signal V and, therefore, the charge storage capacitor 56 is coupled to the output of the amplifier 52. Therefore, during the sampling interval, the input voltage applied to the inversion input for the signals, that is, to the (-) input of amplifier 52,? equal to the difference between the voltage at node B and the voltage across the input capacitor 51. The input voltage applied to the amplifier 52 represents a function of the amplitude of the voltage variation, which can? vary according to the variations that occur in the level of the black current of the kinescope. The voltage on the output storage capacitor 56 remains unchanged during the sampling interval, when the amplitude of the voltage variation developed during the level fixing interval is equal to the amplitude of the voltage variation V2 developed during the sampling interval, indicating a correct level of the kinescope black current. This arises from the fact that during the sampling interval, the voltage change at node B increases in the positive direction (from the level fixing setup reference level), when the grid pilot pulse is eliminated, while the voltage variation V causes a simultaneous voltage perturbation, with a negative trend, at node B. When is the kinescope polarization? correct, the voltage variation, with a positive trend and the voltage variation, with a negative trend, have equal amplitudes and, therefore, will result? evident that these voltage variations cancel each other out during the? sampling interval, leaving unchanged the voltage value at node B. When there amplitude of the voltage variation is less than? amplitude of the voltage variation V ^, the amplifier 52 proportionally charges the storage capacitor 56 in a direction such as to cause an increase in conduction of the black cathode current. Conversely, the amplifier 52 proportionally discharges the charge storage capacitor 56 to cause a decrease in the conduction of the black cathode current when the amplitude of the voltage change? greater than the amplitude of the voltage variation V2? In accordance with as represented, more specifically by the waveforms shown in Figure 2, the amplitude "A" of the voltage variation is assumed to be equal, approximately to three millivolts, when the level of the black cathode current is correct, while varies within a range of a few millivolts (*?) as the black cathode current level increases and decreases from the correct level, as the operating or working parameters of the kinescope vary. Therefore, the set-up reference voltage of the fixing interval of the level, present at the ends of the capacitor 51, varies according to the variations in the amplitude of the voltage when the level of the black cathode current varies. The variation of voltage at node B has an amplitude "A" of about 3 millivolts, corresponding to the amplitude "A" associated with the voltage variation when the black current level is correct. As indicated by the waveform V R shown in Figure 2, the voltage present at the inversion input (-) of the amplifier 52 remains unchanged during the sampling interval, when the voltages V2 both present an amplitude. However, as indicated by the waveform V, the input voltage of amplifier 52 increases by an amount when the voltage variation has an amplitude "A?", Corresponding to a high level of current. In this case, the amplifier 52 discharges the output storage capacitor 56 and, therefore, the bias control voltage, applied to the base of the transistor 20, causes an increase in the collector voltage of the transistor 22 and, therefore the black cathode current decreases towards the correct level.In contrast, and as indicated by the waveform V, the input voltage of the amplifier 52 decreases d i a quantity? ? during the sampling interval, when the voltage variation has an amplitude "A -?", corresponding to a low level of the black current. In this case, the amplifier 52 charges the output storage capacitor 56, with a consequent decrease in the collector voltage of the transistor 22 and, therefore, will occur. an increase in the black cathode current, towards the correct level. In both cases, several sampling intervals may be required to obtain the correct black current level. The voltage developed at node B during the level fixing and sampling intervals AKB represents a function of the values of resistors 31, 32 and 34 and of the value of an output impedance Zq, present on node A. When the signal Vp manifests the positive level, in direct current (+ 8 volts) during the level fixing interval, the junction point between the resistors 32 and 34 is "fixed" as voltage, while a current carried by the resistor 31 , from node A to node B, represents a function of the values of Z, of the value of resistor 31 and of the value of resistor 34. During the next sampling interval, when the component of the signal Vp is present, represented by a trend pulse negative, the diode 35 is in the non-conducting state, while the junction point of the resistors 32 and 34 does not remain "fixed". In this time, a current of different value is conveyed by the resistor 31 from node A to node B, as a function of the value of the resistor 32, in addition to the values of the impedance Z and of the resistors 31, 34. The variation of voltage Vg, developed at node B in response to the impulsive component, a negative trend of the signal Vp is proportional to the difference between these currents. The impedance ZQ at node A pu? undesirably vary as a function of the cathodic polarization level of the kinescope, i.e. as a function of the cathode cut-off voltage level, associated with the correct predicted level of the black cathode current. The resistor 31 compensates for the variations found in the value of the impedance Zq, and is also used to increase immunity? of the level fixing and sampling circuits, of the ether 50 towards the spurious signals generated locally and representing, for example, the recurring interferences at the horizontal frequency. The node A pu? be considered, in a model, as a voltage source, in series with the impedance ZQ mentioned above. The value of the impedance ZQ represents a function of the value of the sense resistor 30 divided by a gain factor of the control loop, which, in its volt a, represents a function of the operating or working point of the transistor 20. The operational or working point of transistor 20, during the intervals AKB is proportional to the cut-off voltage of the corresponding cathode. Practically, ? it was found that the impedance z0 Pu? have a minimum and a maximum value of 30 ohms and 50 ohms, respectively, the conditions corresponding to a corrected black current. Therefore, the value of ZQ at point A can? vary by 67% from a minimum value. The resistor 31 compensates for the change in impedance at node A, so that the change in impedance - does not have to compromise the intended operation of the circuit providing the auxiliary pulses, including the source of the signals Vp, the diode 35 and the resistors! 32, 34. In this example, the value of resistor 31, not critical,? of the order of 200 ohms. Therefore, the total impedance presented to node B, from node A, includes resistor 31 and impedance ZQ and the same varies from 230 ohms to 250 ohms, depending on the variations in impedance ZQ at node A. Consequently , node B is presented with acceptably contained impedance variations of less than 10%, under the conditions corresponding to a black current of correct level, which is significantly lower than the 67% impedance variation present in the absence of resistor 31. In other words, the impedance presented at node B varies by only 4% compared to a nominal value of 40 ohms for impedance z o. Resistor 31 also advantageously involves an increase in immunity. of the level fixing and sampling network 50, with regard to the disappeared signals which can distort or obscure the control voltage of the polarization finally developed at the ends of the storage capacitor 56. Spurious signals of a periodic nature are of particular interest, such as, for example, for example, those generated locally and represented by interference signals, in alternating current, sometimes considered, as "rings" of the frame. "These latter signals occur periodically at the horizontal line frequency whose value is equal, approximated to 15- 734 Hz, according to US standards and including pulsed oscillatory signals, of the damped type, with an average value substantially equal to zero. These signals are generated by the deflection circuits of the receiver during the horizontal retrace intervals of the image, i.e. including the intervals during which the AKB system operates, such signals being able to be coupled to the AKB system through the power connections and through the processing circuits of the luminance and chrominance signals. spurious signals are particularly annoying in a system ??? due to the fact that they have amplitudes that are significant with respect to the low intensity signals, that is to say of the order of a few millivolts that are processed by the AKB system. The impact of spurious signals can? be reduced by the use of separate filtering and shielding techniques, although such solutions represent more alternatives. complex and expensive. The voltage developed across the level 51 holding capacitor, whose value? equal to 0.12 microfarad, during the interval of fixing the level, pu? be seriously influenced by the spurious signals represented, for example, by the? raster rings? which have a significant? amplitude not equal to zero, such signals being able to occur at the end of the level fixing interval say close, temporally, to the instant in which the feedback switch 54 opens. In the absence of the resistor 31, the capacitor 51 can charge to a voltage equal to 67% of the peak amplitude of the previously defined spurious signal, causing the presence of a serious error in the level fixing reference voltage developed across the capacitor 51. This error is significantly reduced by the presence of the resistor 31, in accordance with what will now be described in more detail . During the level fixing interval, the signals including a direct component and the spurious signals, in alternating current, are fed to the positive terminal of the capacitor 51 through an impedance Z whose value? equal, approximately, to 240 ohms, corresponding to the series combination of the impedance ZQ at node A and resistor 31? The reference voltage V_Tl-I is fed to the negative terminal of the capacitor 51 through a low value impedance Z? corresponding to the low output impedance of the amplifier 52 which operates as a voltage repeater during the level fixing interval. The Z impedance is significantly lower than the Z impedance. The amplitude of a reactive impedance Z presented by the capacitor 51 in the presence of the spurious signals recurring at the horizontal frequency? equal, approximately 84 ohms. The component, in alternating current, of the spurious signals, through the capacitor 51 is significantly attenuated by the ratio between the impedance Z and the sum of the impedances Z, Z and Z so that the capacitor 51 can be charged to a voltage equal only to the 25j? about the peak amplitude of the spurious signal (raster ring). Consequently, the fixing capacitor of the level 51 responds more closely to the average value of the signals coming from node A / while the amplitude peaks of the spurious signals present a much less significant plate on the fixing reference voltage d? L level developed by the capacitor 51. The system described automatically produces an output current of the amplifier, of value equal to zero, for the storage capacitor 56 when the amplitude not equal to 0 of the voltage variation corresponds to the correct level of the black current. Consequently, the use of pre-set manual bias controls is not required, to offset the response as regards the conduction of the sampling amplifier, to produce an output current flow of the amplifier of equal value. to zero for the storage capacitor, when the sampled signal has an amplitude other than zero for the correct bias conditions. The assembly for coupling the input signals of the sampling amplifier previously described, using an auxiliary pulse Vp, is advantageous in a system in which the operating amplifier 52 comprises a differential input amplifier, consisting, for example, of the differential amplifier to coupled emitters, according to when it will come? hereinafter described with reference to Figure 4. A differential amplifier of this type has a symmetrical signal transfer response between input and output, which is non-linear in character within most of its operating range. The otherwise symmetrical operating range of the differential amplifier can? be made asymmetrical if the amplifier bias is offset by means of a pre-set manually adjustable bias control, by way of illustrative example. In this case, ? more likely that the amplifier produces an output that is contaminated by the effects of noise and similar spurious signals, since? the asymmetrical and staggered response of the amplifier pu? lead to a straightening of the noise in the non-linear operating region of the amplifier. Consequently, the sample of the output signal and the corresponding voltage developed on the output charge storage device will be distorted or obscured due to the effects of the rectified noise. The combined pulse sampling assembly described above also advantageously provides a convenient mechanism for compensating for mutually different conduction (gain) characteristics and correspondingly different cut-off voltages of the kinescope electron guns due, for example, to , to the manufacturing tolerances of the kinescope. This aspect of the version described? been analyzed in detail, in the US patent application No. 4,434,328, entitled "Automatic Kinescope Bias Control System Compensated for Kinescope Electron Gun Conduction Dissimilarities", filed on October 14, 1982, this system being now briefly described in the present discussion . When the electron guns of the kinescope are identical and when, consequently, they have the same conduction characteristics, they will conduct equal currents at the black level while they will have equal cut-off voltages, represented, for example, by the voltages between the grid and the cathode. However, in practice, electron guns exhibit mutually different conduction characteristics. In the latter case, the different currents conveyed by the electron guns are considered as currents at the correct black level and, therefore, the AKB system should remain in a quiescent condition and should not alter the kinescope polarization even if the electron guns have levels of the mutually different black current and the corresponding associated mutually different cut-off voltages. This result is obtained by means of the complex described, since? the amplitude of the voltage variation V ^, developed at node B is linearly proportional to the component represented by the direct voltage which becomes available at node A. This component, represented by the direct voltage, is proportional to the cathode interdiction voltage, as shown by the component, consisting of the direct voltage, present at the output of the pilot transistor 22 and corresponding to the cathode voltage, during the AKB interval, neglecting the effect of the cathode current pulse of the induced output, developed in response to the pilot pulse of polarity grid? positive. Therefore, if the three electron guns of the kinescope exhibit mutually different currents and cut-off voltages corresponding to the initial conditions for setting the black level, the voltage variation respectively associated with the processors of the signals 14a, 14b and 14c. will present individually a different amplitude, even if each of them is derived from a common signal Vp. The different amplitudes of the voltage variations represent a function of the different cut-off voltages, second? what is manifested by the direct current components, of different amplitude, developed at the nodes A. Are the different amplitudes of the voltage variations such that, for the associated control loop AKB, the voltage developed at the node B will not vary? when are the variations in voltage and V combined? and, therefore, each ring of control AKB rhyme, r? in the state of quiescence. The AKB control loops will remain in quiescent conditions as long as? the electron gun currents, at the black level, established initially, will vary due to a variation in the operating parameters of the kinescope, due to the aging effects of the kinescope or the effects resulting from changes in temperature. In some systems A KB can? be desirable to develop a voltage variation V.j representative of the black current, during the sampling interval, instead of? during the previous level fixing interval, as previously described. In an alternative system of this type, the grid pilot pulse V is G timed in such a way that the same can occur during the sampling interval, while the relationships between the timing of the signals of the type represented by the waveforms shown in Figure 3 can be adopted. In a system of this type, the time fi cation of signals V, H, V, B V and V remain unchanged, The waveforms for the alternate system have been placed back in Figure 3. A grid pilot pulse V 'of polarity? positive and an auxiliary pulse of polarity? positive, they are coincident during the sampling interval. During the initial level fixing interval, the "set-up reference level" is a function of the direct voltage then present at nodes A and B. During the next sampling interval, the voltage variation V.j presents an amplitude "A" when the black current level is correct, an? amplitude A? when the black current level? low and an amplitude A -? when the black current level is high. The voltage variation Vjj is summed, during the sampling interval, with the amplitude voltage variation "A". Therefore, when the black current level is correct, the voltage variation V * is canceled out with the voltage variation V ^, since? both have the same amplitude "A", but polarity? opposite. The voltage which is then applied to the fixing capacitor of level 51, by node B, is therefore corresponding to the reference level applied by node B during the previous sampling interval so that the input voltage of the amplifier 52 does not vary during the sampling interval, according to when indicated by the waveform V of the signal for the condition corresponding to a corresponding to a correct current. Therefore, the storage capacitor 56 is not charged n? discharged from amplifier output current 52. For this alternative system,? It is possible to develop a voltage variation at node B by selectively transferring a split version, as voltage, of the positive pulse to node B during the sampling intervals. The level fixing reference level, developed during the level fixing interval, for the conditions corresponding to a low level and high level black current, is similar to the level fixing reference level developed when the level b of the black current is correct; however, in the case of a high value of the black current, the voltage variations V ^ 'e are not completely canceled during the sampling interval, and the input voltage of the amplifier 52 increases by a quantity? ? during the sampling interval (waveform V). Conversely, a black current condition results in incomplete cancellation and, therefore, the input voltage supplied to the amplifier 52 decreases by an amount. ? during the? sampling interval (waveform V? Figure 4 illustrates the circuit details of the signal level fixing and sampling network 50 schematized in Figure 1 * while in this figure, the various corresponding elements have been identified by the same reference numbers previously adopted. In accordance with as shown in Figure 4, the amplifier 52 comprises a transconductance operational amplifier, in which an output current is produced as a result of the product of the input voltage of the amplifier and of the transconductance (g). The amplifier 52 includes the coupled emitter transistors 66, 68, arranged in a differential input amplifier configuration and a current repeating network ("current mirror") including a transistor 71, connected to a diode and a transistor 74. disposed in the collector circuit of transistor 68, in accordance with as indicated. A first constant current source including a forward biased transistor 69 and a resistor R provides an operating or working current I for transistors 66 and 68. A second constant current source including a biased transistor 75 in a direct sense, and a resistor 2R provides an operating current l / 2 for transistor 74? The reference direct voltage source V is connected to the non-inversion input of the amplifier 52, at the base of the transistor 68. The input signal to be sampled, according to what is derived from the node B shown in Figure 1, is applied, through the input capacitor 51, to the inversion input terminal of the amplifier 52 at the base of the transistor 66. During the fixing interval of the AKB level, the collector of transistor 68? coupled to the input capacitor 51 through the diode-connected transistor 71, the transistor 74 and the switch 54 in the conducting state, so as to form a negative feedback current path. During this time, the charge accumulation capacitor 56 is decoupled from the amplifier 52, through the switch 55 which results in the non-conducting state. The input capacitor 51 is charged through the currents conveyed by the transistors 68, 71 and 74, as a function of the reference voltage V then applied to the input of capacitor 51 from node B of FIG. 1. This charge continues until? the base voltages of the transistors 66 and 68 are substantially the same, that is to say until? the differential input voltage of the amplifier 52 is substantially equal to zero. The current I supplied by the transistor 69 is therefore divided equally between the transistors 66 and 68 and, therefore, the collector currents of the transistors 68 and 74 are equal to the collector current (1/2) conveyed by the transistor 75. Consequently, all the collector current conveyed by the transistor 74 flows as a collector current in the transistor 75. The current feedback path stabilizes at a zero current condition before the end of the level fixing interval and, in these conditions, the transistor 75 "absorbs" all the collector current of the transistor 74, while the feedback current, having a value equal to zero, flows towards the base of the input transistor 66. During the subsequent AKB sampling interval the switch 54 is switched to the non-conducting state, while the switch 55 passes to the conducting state, in such a way as to connect the charge storage capacitor 56 to the output of the? amplifier 52? The pre-existing charge on the capacitor 56 remains unchanged as long as? the input signal applied to the capacitor 51 is sufficient to alter the balanced base bias of the transistors 66 and 68, as established during the previous level fixing interval. Therefore, when the voltage variation has an amplitude "A" corresponding to a current condition at the black level, the voltage input to the transistor 66 will remain of a correct value. unchanged, as indicated by the waveform shown in Figure 2? Consequently, the balanced input bias of transistors 66, 68 and the charge on the output storage capacitor 56 remain unchanged. When the black current level is wrong so that the input voltage fed to the transistor 66 is increased, as represented by the waveform shown in Figure 2, it will occur? a decrease in the currents carried by transistors 68, 71 and 74. The charge storage capacitor 56 is discharged through the transistor 75 by a quantity of proportional to the lower conduction of transistor 74, in response to the higher input voltage. In this case, the transistor 75 operates as a current absorber with respect to the charge storage capacitor 56 being discharged. Similarly, a decrease in the input voltage applied to transistor 66, as indicated by the waveform shown in FIG. 2, causes a corresponding increase in the collector current of the output transistor 74. The charge storage capacitor 56 charges through the transistor 74, in response to this increased current conduction and, therefore, will occur? an increase in the voltage across capacitor 56. In this case, transistor 54 acts as a current source for charging capacitor 56. Figure 5 illustrates a block diagram of a logic assembly for the timing signal generator 40 depicted in Figure 1. A binary counter 90 includes the CLOCK and RESET inputs sensitive to the horizontal signal H and the vertical signal V respectively, a DISABLE input and the binary outputs Q? Counter 90 is reset in response to the positive pulse portion of the V signal (see FIG. 2) which occurs during the vertical retrace interval. Therefore, the outputs all have a low logic level (0000) ^ while the RESET input is positive during the vertical retrace interval. During this time, counter 90 does not respond to clock pulses H recurring at the horizontal frequency. A combination logic array 92 including, for example, a plurality of of logic gates controls the binary states of the outputs Q, - Q, of the counter 90, through the inputs A -D. At the end of the vertical retrace interval, at the instant, counter 90 is enabled for operation. The logic states of the outputs of the counter 90 vary in order to indicate a binary number corresponding to the number of clock pulses H occurring from the end of the vertical retrace interval. A logic output F, derived from array 92, produces a high logic level ("1") during the interval spanning the second to eighth clock pulse H by detecting the expected condition of the counter outputs during this time interval. This signal is delayed by means of a delay network 93, so as to obtain a time delay in the production of the timing signal AKB V which is developed at the output of the delay network 93. The delay introduced by the delay network 93 can? be produced, for example, by means of a plurality? of logic gates coupled in series, each capable of introducing a given time delay. The timing signal V is developed at the output G of the logic gate array 92, during the time interval including the third to fifth clock pulse H from the end of the vertical retrace interval. This signal is delayed by an amount? through a network 94 and the level is moved by means of a network 95, so as to produce the grid pilot pulse V_. The level displacement network 95 consisting, for example, of a voltage translator is used to produce the signal with a suitable amplitude to drive the electrode CLAIMS 1) Automatic polarization control apparatus in a video signal processing system including an image reproduction device sensitive to video signals fed to an intensity control electrode. of the same, characterized in that it comprises: means (30) for deriving a periodic signal representative of the level of the black current conveyed by said intensity control electrode, during the intervals of suppression of the images of the video signal, said derived signal presenting an amplitude other than zero, when the black current level is correct; information storage means (56); amplifier means (52) with an input for the signals and an output coupled to said storage means, for modifying the information content of said storage means, in accordance with the conduction condition of said amplifier means, in response to input signals applied; input signal coupling means (31) for coupling said derived signal to said amplifier input; means (99) for supplying a periodic auxiliary signal (?? ^ 2) to means for coupling the input signals, with an amplitude and a sense such as to substantially cancel the response of said amplifier towards the amplitude of said derived signal when the amplitude of said derived signal? representative of a correct level of the black current; And means (58) for supplying a bias correction voltage derived from said storage means, to said imaging device, for maintaining a correct level of the black current, 2) Apparatus according to the claim 1, characterized by the fact that: impedance means (32, 34) are coupled to said input signal coupling means, to establish a bias for said input signal coupling means, in the presence of said derivative signal and in which; said auxiliary signal (Vp, V2) is applied to said impedance means, to modify said established bias, with a sense such as to produce said negative response of the amplifier. 3) Apparatus according to claim 1, characterized in that: said auxiliary signal (V, V) cancels said derived signal in said path for the input signals, in order to produce said negative response of the amplifier. 4) Apparatus according to claim 3, characterized in that said derivative signal (V ^) and said auxiliary signal (V ^) comprise coincident polarity pulses? mutually opposite and of substantially equal amplitude, when said derivative signal? representative of a correct level of the black current. 5) Apparatus according to claim 1, characterized in that: said auxiliary signal (V?) has an amplitude proportional to a component represented by a direct voltage, manifested by said intensity control electrode? during said blanking intervals. 6) Apparatus according to claim 1, characterized in that it comprises: level fixing means (51) coupled to said input signal coupling means at said amplifier input; switching means (54,55) coupled to said amplifier output, to said level fixing means and to said storage means; means (92) for operating said switching means during an initial level fixing interval for: (1) fixing said amplifier input to a reference voltage in response to a reference source coupled to said amplifier input during said level fixing interval and (2) decoupling said amplifier output from said storage means and for operating said switching means during a subsequent sampling interval to (3) unlock said amplifier input and (4) couple said output of the amplifier to said storage means; said signal representative of the black derivative current is developed during said level fixing interval and coupled to said level fixing means, so that said reference voltage to which said amplifier input is fixed, also represents a function of the amplitude of said derivative signal said auxiliary signal (Vp) is developed during said subsequent sampling interval 7) Apparatus according to claim 6, characterized in that: said image playback device? consisting of a kinescope (15) including an electron gun comprising a grid electrode (18) and an associated intensity control cathode electrode (16a); said automatic bias control apparatus also includes means (40) for biasing said kinescope electron gun during said level fixing interval, to induce an output cathode signal with an amplitude proportional to the black cathode current level; And said shunt means (30) derive said representative periodic signal from said induced output cathode signal. 8) Apparatus according to claim 1, characterized in that it comprises: level fixing means (51) coupled to said means for coupling said input signal at said amplifier input; switching means (54, 55) coupled to said output of the amplifier, to said fixing means and to said storage means; means (92) for operating said switching means during an initial level fixing interval for: (1) fixing said amplifier input to a reference voltage in response to a reference source coupled to said input of the amplifier. amplifier, during said level fixing interval and (2) decoupling said amplifier output from said storage means and for operating said switching means during a subsequent sampling interval to (3) unlock said amplifier input and ( 4) coupling said output from the amplifier to said storage means; and in which: said derivative signal, representative of the black current and said auxiliary signal are both developed during said sampling interval. 9) Apparatus according to claim 6, or claim 8, characterized in that: said level fixing means comprises a capacitor (51) for coupling the signals from said input coupling means to said amplifier input; And said auxiliary signal (V, ??) has a? amplitude and a sense such as to maintain substantially unchanged the voltage at said input of the amplifier, when the amplitude of said derived signal? representative of a correct level of the black current, 10) Apparatus according to claim 9, characterized in that: said image playback device? consisting of a kinescope (15) including an electron gun comprising a grid electrode and an associated intensity control cathode electrode; said automatic bias control apparatus also includes means (40) for biasing said kinescope electron gun, during said sampling interval, to induce an output cathode signal with an amplitude proportional to the black cathode current level; and said shunt means (30) derive said representative periodic signal from said cathode output signal which? been induced. 11) Apparatus according to any one of the preceding claims, characterized in that: said amplifier means (52) comprise a differential input amplifier. 12) Apparatus according to claim 1, characterized in that an alternating current coupling capacitor (51)? coupled to the input of the amplifier means and wherein said input signal coupling means couples said derivative signal to said capacitor to alter its charge; And said auxiliary signal, of a periodic nature? coupled to said capacitor, in such a way as to alter its charge; said auxiliary signal having an amplitude and a sense for substantially negating the altered charge of said capacitor, developed in response to said derivative signal when the amplitude of said derivative signal? representative of a correct level of the black current. 13. Apparatus according to claim 12, characterized in that :. said auxiliary signal has an amplitude as a function of a component, consisting of a direct voltage, manifested by said intensity control electrode. during said cancellation or suppression intervals * 14) Apparatus according to claim 1, characterized by the fact that: said means for deriving said periodic signal have a variable output impedance correlated to the bias of said intensity control electrode; said input signal coupling means including an impedance (31) for coupling said representative derived signal, from said output of said branching means to said information storage means, said impedance being high relative to said variable output impedance, in order to significantly reduce the impedance variations presented to said means which supply said auxiliary periodic signal from said output of said bypass means. 15) Apparatus according to claim 14, characterized in that: said derivative signal is developed at a first circuit point (A) which corresponds with said output of said derivation means; said auxiliary signal, of a periodic character, is coupled to said capacitor at a second circuit point (B); said impedance (31)? connected from said first circuit point (A) to said second circuit point (B). said amplifier means (52) comprise a differential input amplifier. 12) Apparatus according to claim 1, characterized by the fact that: said information storage means consist of a capacitor (51), said input signal coupling means couple said derivative signal to said capacitor, in order to alter its charge; And said auxiliary signal, of a periodic nature? coupled to said capacitor, in such a way as to alter its auxiliary signal by presenting an amplitude and a sense to substantially negate the altered charge of said capacitor, developed in response to said derivative signal when the amplitude of said derivative signal? representative of a correct level of the black current. 13) Apparatus according to claim 12, characterized in that: said auxiliary signal has an amplitude as a function of a component, consisting of a direct voltage, manifested by said intensity control electrode. during said cancellation or suppression intervals * said amplifier means (52) comprise a differential input amplifier. 12) Apparatus according to claim 1, characterized by the fact that: said information storage means consist of a capacitor (51), said input signal coupling means coupling the derived signal cP to said capacitor, in order to alter its charge; And said auxiliary signal, of a periodic nature? coupled to said capacitor, so as to alter its charge, said auxiliary signal having an amplitude and a sense for substantially negating the altered charge of said capacitor, developed in response to said derived signal when the amplitude / of said derived signal? representative of ^ a ^ correct level of the black current. 13) Apparatus according to claim 12, characterized in that: said auxiliary signal has an amplitude as a function of a component, consisting of a direct voltage, manifested by said intensity control electrode. during said cancellation or suppression intervals * said amplifier means (52) comprise a differential input amplifier. 12) Apparatus according to claim information are constituted by a capacitor (51) said means for coupling the if? ric; And said auxiliary sign periodic? coupled to said capacitor, in such a way as to alter its Cari The altered charge of said capacitor developed in response to said derivative signal when the amplitude of said derivative signal? representative of a correct / level of the black current. 13) Apparatus according to claim 12, characterized in that: said auxiliary signal has an amplitude as a function of a component, consisting of a direct voltage, manifested by said intensity control electrode. during said cancellation or suppression intervals *