US2307249A

US2307249A - Television device

Info

Publication number: US2307249A
Application number: US376827A
Authority: US
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1933-04-13
Filing date: 1941-01-31
Publication date: 1943-01-05
Anticipated expiration: 1960-01-05
Also published as: GB422906A; DE939510C; FR771789A; BE402568A

Description

Jan. 5, W43. P. W. WILLANS 2,307,249

TELEVISION DEVICE Original Filed April l2, 1934 3 Sheets-Sheet l Hg?, /Zf Ml* u u ffy. 4.

Jan. 5, 1943. P w. wlLLANs 2,307,249

TELEVISION DEVICE Original Filed April l2, 1934 3 Sheets-Sheet 2 I/v VENTO/e Pear lv/l//lam /1////a/75, Deceased 2y, War/zn .me M//a/Ls, fica/797x TELEVIS ION DEVICE Original Filed April l2, 1934 3 Sheets-Sheet 3 /Wa x/mam P/c fa re Bla cK L e ve/ .f Z @KM Patented `lan. 5, 1943 UNITED OFFICE TELEVISION DEVICE Original application April 12, 1934, Serial No. 720,205. Divided and this application January 31, 1941, Serial No. 376,827. In Great Britain April 13, 1933 6 Claims.

This application is a division cf the United States Patent 2,252,746, issued en August 19, 1941. Said patent was filed as United States patent application Serial No. '720,205 on April 12, 1934.

The present invention relates to signalling systems, such for example as television or picture transmitting systems, in which signalling is effected with the aid of signals of the kind which may comprise oscillations of any frequency between a predetermined maximum value and zero or at least a lower limiting value which presents dimculties in transmission and reception.

In television systems, for example, an object to be transmitted is usually scanned in contiguous strips and electrical variations are derived from the changes in brightness vof the elemental areas of the object. When the object has been completely scanned, the operation is repeated. This complete scanning may take place 24 times per second for example. Since the absolute level of brightness has significance to the eye it is necessary to reproduce in the final picture all changes in brightness of the object, however slowly these changes may occur, if the final result is to be an undistorted representation of the object. It will be evident that in order to convey always the correct impression of absolute brightness it is necessary to transmit and receive frequencies down to and including zero frequency. The use of ampliers capable of handling direct currents as well as alternating currents covering a wide frequency band is usually inconvenient and costly.

Usually, the amplifiers have a low frequency cut-off, for example they may be capable of amplifying from about 10 cycles per second to 40,000 or more cycles per second, and below 10 cycles per second the amplication falls rapidly. Such amplifiers are almost always resistance-capacity coupled and include as coupling between stages, a condenser arranged in series between the anode of one valve and the grid of the next valve and a resistance between the grid of the second valve and the common cathode lead. The low cut-off frequency of the amplifier is then determined by the time constant of the condenser and resistance. If the anode circuit of the first mentioned valve contains oscillations having components ol' frequency lower than the cut-olf frequency of the coupling, only the higher frequency components which are passed by the coupling will aect the grid circuit of the second valve. Once the steady state has been reached, the potential of the grid of the second valve will vary in accordance with the wave form of the higher components about an electrical zero in such a manner that the area of the grid potential-time curve above the electrical zero line is equal to the area below that line. The electrical zero is a potential diderlng from the cathode potential by the amount of the grid bias and is wholly independent of the D. C. and low frequency components ln the signal, which cannot pass through the coupling.

In some known television systems, the picture scanning is interrupted for a short time between the scanning of successive lines and during this short time the picture signal assumes a value corresponding to complete black or a value even further removed from white than this. The pulse is often used for synchronising purposes and will be called the synchronising pulse.

In scanning, relative motion is usually produced between an image of the object to be transmitted and a scanning aperture arranged in front of a photo-electric cell. Where, for example, the object is a transparency such as a motion picture lm, the signal generated in the cell will have its maximum possible amplitude in the white direction (hereinafter referred to as full white) when the part of the image of the film opposite the scanning aperture is that corresponding to the most transparent part of the iilm'. Similarly with any artificially illuminated object, the value of the maximum possible signal amplitude can be determined.

As an example we may assume that referred to black, full white gives a signal voltage of -10 volts and that the synchronising pulse has an amplitude of `-10 volts and lasts for a time equal to one tenth of the time taken to scan one line. The explanation will be made clearer by reference to the illustrative diagrams in Figs. 1 to 5 of the accompanying drawings. If first the picture signal be assumed to be in the form representative of a black dot, of length equal to onetenth of a line, upon a white ground, occupying the remainder of the line, such as would be obtained by scanning a strip shown in Fig. 2, the picture signal voltage V representative of this strip, referred to black as zero voltage and plotted against time t as abscissa may be in the form represented in Fig. 3 of two square topped waves a and b of -10 volts amplitude each extending over nine-twentieths of the line and between them a return to zero extending over one-tenth of the line. In addition there is a synchronising pulse c of +10 volts lasting for one-tenth of the line scanning time. This signal is passed repeatedly through a device having a low frequency cut-od. such for example as that asoaaea of transmitting signals of the kind referred to shown in Fig. l comprising a series condenser `"--rwherein the signal amplitude is caused to attain and a shunt resistance t, the signal being applied to input terminals 9 and taken od from output terminals d. When the steady state has been reached, the oscillations in the output of.' the device will take place, as explained above, about an electrical zero (represented by the dotted line d ln Fig. 3) such that areas of the potential Wave above and below this zero are equal to one another. In the particular case mentioned the amplitudes about this electrical .zero will be found to be about [-17 volts for the synchronising pulse and -3 volts for the white background. The electrical zero is then 7 volts negative with respect to complete black.

If the picture signal then has to represent a white dot on a black ground as shown in Fig. (l, it will be found as shown in Fig. 5 that the electrical zero is coincident with the potential marked o corresponding to complete black and the amplitudes relative to the electrical zero are :10 volts.

It will therefore be seen that because the D. C. component has been removed, the valves of an amplier require to be biased in such a way that they can handle not only the apparent maximum amplitude, in the case mentioned 10 volts positive and negative, but also a swing from -3 volts to +17 volts. In the particular example given, therefore, the bias would have to be such that the swing could extend between -10 volts and +17 volts Without distortion.

If account is taken of all circumstances, it will be found that the swing to be accommodated is liable to be as high as twice that which would be necessary if the D. C. component were present. The same applies where the missing component is an alternating current of any frequency below the cut-off frequency of the ampliner.

The result is that, in practice, it has not been possible to take full advantage of the fact that the maximum picture signal amplitude, unlike acoustic signal amplitude for exampleI is a known quantity and to use the amplifiers handling the picture signals emciently.

Similarly, in the case of modulation in a carrier wave transmitter, the carrier amplitude in the absence of a picture signal may have to be be made double what it would require to be were the modulation effected with the D. C. components present.

It is an object of the present invention to overcome or at least greatly reduce the above mentioned dimculties whilst permitting the use of amplifiers which themselves are incapable of amplifying over a relatively wide band of low frequencies including zero frequency.

A transmission system for signals of the kind referred to wherein the signal amplitude periodically, but not necessarily regularly, attains a fixed maximum or minimum value, characterised in that the direct and low frequency signal components below a minimum transmission frequency which is less than the minimum frequency of the recurrent maximum or minimum amplitude, are not transmitted through the transmission system but are re-created at a desired point by means of the operation of the recurrent maximum or minimum signal amplitude on a device which supplies the direct and low frequency components with reference to the amplitude of said recurrent maximum or minimum signal amplitude.

The present invention also provides a mdthod 75 periodically, but not necessarily regularly. a predetermined maximum value in one direction and wherein low frequency signal components are removed from the signal and re-inserted at a desired point, the re-insertion being edected under the influence of the periodic maxima. The upper frequency limit of the removed and reinserted components is lower than the frequency of the periodic maxima or than the minimum frequency of these maxima where they are irregular. The reinsertion is accomplished by arranging that the maxima either themselves constitute the zero of the signal oscillation or are displaced by a fixed amount from that zero.

In electrical carrier wave signalling the present invention provides a method of transmitting signals of the kind referred to wherein the signal amplitude is given a form in which it attains periodically, but not necessarily regularly, a maximum fixed value in one direction, the signal being used in this form to modulate a carrier wave, so that the fixed value is represented by a fixed carrier amplitude.

The present invention also provides a method of transmitting signals of the kind referred to according to which frequencies between zero and a predetermined intermediate value are removed from the signal and are re-inserted into the signal at a desired point at the transmitter or at the receiver with the aid of means responsive to the peak value of a recurrent maximum signal amplitude in one direction. The re-inserting means are capable of providing, in response to the said peak value, all frequencies from zero to a limiting value which lies between the frequency of recurrence of the peak value (or the minimum frequency of recurrence where this is irregular) and the said intermediate value. The removal of the lower frequencies is usually accomplished by the inherent properties of the transmission channel, including ampliers, through which the signals are passed. 'Ihe reinserting means should be capable of providing all frequencies below the lowest which .the transmission channel is capable of transmitting without appreciable distortion.

Other features of the invention will appear from the following description and the appended claims.

The invention will be described by way of example with reference to Figs. 1 to 13 of the accompanying drawings showing various circuits according to this invention, and Fig. 14 shows schematically a television transmitter according to the invention.

A vtelevision transmitter as shown schematically in Fig. 14 comprises means 3B of any known or suitable type for generating picture signals and means 39 for generating. between groups of picture signals representative of adjacent lines of an object, a synchronising pulse in the black direction. The amplitude of the pulse will for convenience be assumed to be +V volts, with reference to complete black as zero, and the maximum picture signal amplitude will be assumed to be -V volt, corresponding to complete white. 'Ihese signals are generated in two photo-electric cells, one generating the picture signals p and the other the synchronising pulses s. The outputs of the two cells are mixed together in a mixer d0 and are fed to the input of a resistancecapacity coupled amplifier tl. The nature of the signals existing at various parts of the circuit of Fig. 14 are indicated, the picture signals being represented at p and the synchronising signals at s.

The time constants of the couplings of the resistance-capacity coupled amplifier di may be made about one hundredth of a second for example. The frequency of recurrence of the synchronising pulses s may be 3000 per second. In the output of the amplier di the oscillations across the output impedance z will take pla'ce about an electrical zero line e, the anode end of this impedance varying in potential from positive to negative with respect to the electrical zero, which may be assumed to be at a xed potential relative to earth, in such a way that the average area of the anode potential-time curve is zero. lf at some instant the conditions are such that the amplitude of the synchronising pulse is +nV volts relative to the electrical zero where 11. is the overall magnification of the amplifier, then it will be evident that 4at that instant the electrical zero corresponds to complete black and the picture signal is truly representative of the brightness of the object. If now the average brightness of the object increases, the amplitude of the synchronising pulse increases to say HMV-tv) volts. In order now to represent this new brightness accurately the amplitude of the picture signal requires to be increased (in a negative direction) by nv volts, or in other words the true vzero iine should be at +121) volts relative to to electrical Zero. This can be done by arranging that the true zero is made 'nV volts less positive than the peak of the synchronising pulse. In this way the missing D. C. component deiining the absolute brightness of the object can in effect be ire-inserted.

It has already been explained that in order to enable modulation of a carrier wave to be carrie-d out efficiently, the D. C. and low frequency components of the signal should be included in the signal used to modulate the carrier. The signal has been deprived of the D. C. and low frequency components by the ampliiier and these components may therefore be re-inserted by means of the device t2 one example of which is shown in Fig, 6.

Referring to Fig. 6, the earthed terminal of the amplifier output is connected to the cathode 5 of a modulator valve and the other terminal of i the output is connected through a condenser 'i to the grid of the valve t, the grid being connected to the cathode through a grid leak resistance 8. In the anode circuit of the modulator is an impedance 9 associated with an oscillation generator (not shown) in known manner such that the amplitude of the carrier oscillation generated is controlled by the voltage developed across the impedance 9. The time constant of the resistance 8 and condenser l is made shorter than that of the amplier couplings, and in fact shorter than that of any circuit through which the signals have passed since they contained the low frequency components. Furthermore, the time constant of the resistance a and condenser 'l must be con' than the cathode 5. Assuming that at first the grid potential is equal to that of the cathode, the first pulse makes the grid positive and causes grid current to flow. The condenser 'l thus becomes charged in such a way as to make the grid more negative than the cathode when the pulse is over. The condenser l commences to discharge through the grid leak B but, owing to the relatively long time constant, only a little of the charge has leaked away before the next pulse arrives. This again makes the grid positive, but less so than before, and some grid current again flows, thus still further increasing the charge on the condenser. This process goes on until the grid potential in the absence of a pulse is such that the peak of the pulse just causes grid currentto flow.

If now a picture signal is superimposed upon the signal already considered, it will make the grid more negative and will therefore not cause grid current to ow and will not aect apperciably the average charge on the condenser l. It will be observed, however, that the voltage of the grid is caused by the signal to vary between that at which grid current just flows and a xed value t which is, in 4the example considered above, 211V volts more negative than this. Thus whatever value the electrical zero may assume relative to complete black, the signals upon the grid of the modulator 6 always cause voltage excursions extending from the point at which grid current just flows, this point corresponding to the peak of the synchronising signal and therefore to a predetermined diierence from complete black.

In the arrangement described the synchronising pulses constitute regularly recurring maxima of predetermined value, namely a predetermined voltage diierence from that representing complete black. Because the time constant of the condenser 'l and resistance acting as the reinserting means, is short compared with that of all circuits through which the signal has passed since it contained the low frequency components, this condenser and resistance will determine the position of the signal datum with substantially no interference from preceding circuits. Also because the time constant of the reinserting means is long compared with the time period of the synchronising pulses, the datum determined at the commencement of a train of picture signals representing one line of the object will not vary greatly during the time elapsing before the arrival of the next pulse. Clearly the arrangement will also operate if the recurring maxima are not regularly spaced in time. In this case the time constant of the re-inserting means should be long compared with the maximum interval between recurring maxima. Further the arrangement will serve to insert not only the D. C. component but also alternating components of frequency not exceeding some value which is less than the minimum frequency of recurrence of the maxima and which (because the time constant of the re-inserting means is shorter than that of preceding circuits) is greater than the cut-off frequency of the transmission channel through which the signals have passed.

The transmitted carrier will thus, for maximum picture signal amplitude, be modulated between fixed amplitude limits, one limit being that corresponding to the modulator grid voltage at which grid current just ows and the other limit being displaced from the first limit by the sum of the amplitude of the synchronising pulse and the maximum picture amplitude, due allowance being of course made for amplification occurring in the amplifier and modulator.

At the receiver the carrier is detected and the picture signals so obtained are amplified in a resistance-capacity coupled amplifier. These signals will as before be deprived of low frequency components. In order to re-insert these components in the signals applied to the reproducing device, which will in this case be assumed to be a cathode ray tube, a procedure similar to that adopted at the modulator valve may be employed. This is illustrated in Fig. r1. The signals are applied to the grid circuit of a valve IIl which in this case is the last amplifier valve and not a modulator. Otherwise the circuit arrangement in the grid circuit is the same as before. The anode of the valve III is connected directly to the control grid of the cathode ray tube Ii and through a resistance I2 and a source of high tension supply I3 to the cathode of the valve. The positive terminal of the source is connected to a variable tapping point on a potentiometer It `arranged in parallel with a voltage source I5, a

point on this potentiometer being connected to the cathode of the cathode ray tube II.

The potentiometer tapping point is adjusted so that, in the absence of a picture signal, when the synchronising pulses have reduced the grid potential of the amplifier valve to a substantially steady value at which grid current just flows, the potential on the control grid of the cathode ray tube relative to the cathode thereof has a value corresponding to black. Thus the grid of the cathode ray tube is held by the pulses at a normal potential corresponding to black and any picture signals which arrive will make this grid more positive by amounts truly representative of the original brightness of the object.

An alternative arrangement to that shown in Fig. 7, whereby the necessity for the balancing battery l5 is removed, is shown in Fig. 8. The condenser 1 and resistance Il in the grid circuit of valve 36, which is shown as a screen grid valve, perform the function already described. The anode of the valve is connected through resistance I2 to a source of current of such a nature that the voltage is maintained substantially constant over the operating current range. One such arrangement is known as a neon stabiliser comprising an enclosed envelope containing neon at reduced pressure. This discharge device has two electrodes arranged in series with a resistance across a suitable voltage source and connections may be taken from the same or other electrodes to the anode circuit of the valve 36,

The screen grid of the valve may be supplied from the same source. Across the source is also connected a preferably variable resistance 31 shunted by a condenser 38 in series with a resistance 39, these resistances constituting a potential divider. The cathode of the tube II is connected to the junction point of the

resistances

31 and 39. The value of the resistance 31 is then adjusted in the way above described in connection with the adjustment of the potentiometer It of Fig. '7.

It will be observed that both in the case of the modulator valve and also that of the cathode ray tube, correct settings are dependent upon a knowledge of the amplification which the signals have undergone. As this may sometimes be inconvenient, the synchronising pulse may be given a peak value corresponding to black instead of a peak value displaced from black in a direclin accanto tion opposite to white as in the arrangements described. In this way all settings will be independent cf amplification.

It has already been mentioned that the pulses in the black direction transmitted between the scanning of lines of the object may be used at the receiver for synchronising purposes. For example the pulses can -be used to control the generation of currents of sawtooth wave form which serve to deflect the cathode ray across the screen. The separation of these pulses from the picture signal offers difficulty in known systems because of the Wandering zero. Due to this the generation of the sawtooth currents may be interfered with by picture signals and, if the zero wander be in the opposite direction, the amplitude of the pulses may be insuflicient to control the generator. A circuit similar to those already described may be used to overcome this difficulty in the following way:

The signals are fed to an amplifier valve I6 shown in Fig. 9 having a condenser 1 and resistance B arranged in its grid circuit as already described. The phase of the signals is such that synchronising pulses momentarily tend to make the grid more positive. The anode circuit of the valve I6 is resistance-capacity coupled to a second valve I1 serving to reverse the phase of the signals. The output of the second valve is provided with selective means I8 adapted to separate the pulses of line scanning frequency (usually of relatively short duration) from the socalled frame pulses, that is pulses, usually of longer duration than the line pulses, which are transmitted between each complete scanning of the object. The line pulses may then be taken from terminals I9 and the frame pulses from terminals 20. The rst valve I6 is of high magnification factor and has such a characteristic that its anode current falls substantially to zero with a. fairly low negative voltage on its grid. The operating conditions of the valve I6 are made such that the operative range of grid voltage (that is the range between the point at which grid current just flows and the value at which anode current ceases) is slightly less than the amplitude of the synchronising pulses to be applied to the grid compared with black. As before the synchronising signals cause grid current to flow until the condenser 1 is charged to such a negative voltage that grid current only just flows on the peaks of these signals. Clearly, since the picture signals all tend to make the grid more negative, they will have no effect upon the current in the anode circuit whilst the amplitude of the synchronising pulses developed in the anode circuit will be constant and dependent upon the operative range of grid voltage. This latter can be adjusted, by adjustment of the voltage in the anode circuit of the valve I6, so that the amplitude of the pulses developed in the anode circuit is just sufcient to control the saWtooth generator with certainty.

In all the above examples it has been arranged that the recurrent maxima were in the form of positive voltages. An alternative circuit according to this invention, wherein a diode rectier is employed, is shown in Fig. 10. It is arranged that the recurrent maxima are in the form of negative voltages. The circuit will be described as applied to the circuit of a cathode ray tube at a receiver. The last valve of the receiver amplifier has its cathodel connected to the cathode of the cathode ray tube II and its anode connected through a coupling condenser 'I to the control grid of the tube il. To this control grid is connected the cathode of a diode rectifier 2l, the anode or which is connected to a tapping point on a potentiometer 22 and through a condenser 2i to the cathode of the cathode ray tube il. The potentiometer is arranged in parallel with a source of voltage 2d and the positive terminal of the source is connected to the cathode of the tube ll. A grid leak 8 is connected across the terminals of the diode. The coupling condenser ll and the resistance 8 of the grid leak are given a time constant, as before, which is long compared with the frequency of thesignal maxima and short compared with that di preceding circuits. The potentiometer 22 isfadjust-y ed so that the normal bias on the grid of the cathode ray tube l l, in the absence of any signal, is more negative than that corresponding to black by an amount equal to the amplitude of the maxima relative to black. The signals are fed to the circuit in such phase that the maxima, constituted by the synchronising pulses for example, tend momentarily to make the control grid more negative.

In operation, first assuming that only the synchronising pulses are arriving and that initially the coupling condenser l is uncharged, the rst pulse tends to make the grid more negative and current flows through the diode 2l. When the pulse has ceased, the grid will be more positive than before and the condenser ll will commence to discharge through the leak resistance d. The succeeding pulses act similarly until current just ilows through the diode on the peaks of the pulses. Under these conditions the grid will have a potential corresponding to black and picture signals will be accurately reproduced. If the average brightness of the object increases, more current will flow through the rectifier and the average grid potential will become more positive thus increasing the average brightness of the reproduced image.

lt should be noted that both in the arrangement just described and also in those involving the flow of grid current, there is used as means for re-inserting the absent low frequency components the combination of a condenser and a unidirectionally conducting device.

Other arrangements of circuit are possible based on the same general principle of re-inserting the low frequencies by reference to a limiting peak amplitude. For example, instead of the above described arrangements wherein a low impedance amplifier output feeds a series condenser with shunt leak and shunt rectifier, a high impedance amplier output may be used working into a shunt inductance across which is shunterl a rectiner and a low resistance in series. The resistance of the rectifier (when passing current) together with the resistance in series therewith must give with the inductance a time constant lying between the period of the lowest frequency transmitted by the preceding amplier and the period of the recurrent maxima. Suppose that the direction of amplifier output current which will pass through the rectifier be called positive, then the recurrent maxima must occur in the negative sense. The device will operate to give a current in the resistance in series with the rectifier which is zero for the recurrent negative maxima. Negative pulses arriving from the amplier will not pass through the rectifier but will charge the inductance, that is to say initiate current flow in the inductance. This inductance discharging through the rectifier and resistance will supply the necessary currents representing the low frequency components. A circuit capable of operating in this way is shown in Fig. il in which the signals are applied across an inductance 28 in such a sense that the maxima .make the upper end of the inductance 26 more negative., A diode 21 in series with a resistance 28 is arranged in parallel with the inductance 2t and the potential dierences across resistance 28 are applied to the grid circuit of a valve 2E. The output containing the re-inserted D. C. component is taken from across the resistance 29. The above is given merely by way of example to show how a widely different circuit may be used to accomplish the same result. Many other circuits are possible.

As unidirectionally conducting device in the forms of re-inserting meansabove described, other forms of rectiiier than thermionic rectiers may be used. For example in some cases a contact type rectiiler such as a copper-copper-oxide rectifier may be employed.

The re-insertion may take place as often as desired in a transmission system. It is preferably done immediately before any point where overloading may occur.

In all cases described, at any point where reinsertion has been eected, the steady output in absence of any signal (including the recurrent maxima) has a value representing that normally due to the recurrent maxima. This is not a necessary condition of operation but is in many respects advantageous.

In re-inserting the D. C. components some losses are inevitable. For example, it is a necessary condition of operation of the various re-inserting devices described that some current should ow through a unidirectionally conducting device and a drop of applied voltage thus occurs. The effect of this is that the Whole of the missing D. C. component is not re-inserted. In some cases this may not be material. Where it is desired to obtain more complete re-insertion means may be provided at some suitable point for favouring the D. C. and low frequency signal components relatively to the remainder of the signal frequency band. One arrangement of this kind is shown in Fig. l2.

Here the signal in which the D. C. component is to be inserted is applied to terminals 30 having a condenser l and a resistance t functioning in the manner already described. A diode 3l is connected across the resistance B as shown. A tappingr point on the resistance 3, which may for example be half way along it, is connected through a resistance 32 to the grid of an amplifier Valve 33. Across the grid-cathode circuit of the valve 33 is connected a circuit tending to bypass alternating components. In this case this circuit comprises a resistance 3i in series with a condenser 35 and the values of these components are so chosen that signal frequencies from about `10 cycles per second upwards are substantially uniformly attenuated whilst frequencies below l0 cycles per second are less and less attenuated until at zero frequency there is no attenuation. The output is taken across the resistance 36.

Assuming for example that owing to the action of the condenser l, the resistance 8 and the diode 3l. the voltage across the resistance 8 has the D. C. component present to the extent of 90%, regarding the higher frequency components as present to the vextent of 100%. A fraction, in this case about one-half, of this voltage is applied to the grid of valve 33 through resistance 32. Thus in the absence of the circuit 3d, dii, fifty per cent of the voltage at all frequencies across resistance t will appear across the grid circuit of valve t3, that is to say 45% of the correct amount of D: C. component and 50% of the higher frequency A. C. components. Owing to the circuit M, 35, however, although about 45% of the correct D. C. component for l the signal across resistance still appears across the grid circuit of valve t3, the higher frequency components are attenuated to the extent of about so that only about 45% of the voltage across resistance d at these frequencies also appears across the grid circuit of the valve d3.

An alternative way of achieving the same result is shown in Fig. 13. Here the circuit is similar to that shown in Fig. 6 except that a circuit comprising for example resistance 3@ and condenser 35 is shunted across the output resistance Ei. This circuit M, 35, as before, favours the D. C. component.

Althoughv the invention has been described as applied to television systems, it is not applicable only to television and picture transmitting systems but may be applied to any system Where it is required to transmit signals covering a range of frequencies extending below the lowest frequency which the transmission channel can handle without distortion.

What is claimed is:

1. A television receiver comprisinga cathode ray tube having an electron gun structure including a cathode and an anode element within the tube for developing an electron beam, a target area responsive to impact by the electron beam to develop luminous effects, control electrode means for controlling the intensity of the developed beam upon the target, a source of signal energy, means including an electrical storage element for applying said signals between the cathode and control electrode to control the intensity of the beam developed within the cathode ray tube, a series combination comprising a unidirectional conductor and a resistor connected between the cathode and control electrode of said cathode ray tube for developing a bias voltage for the control electrode of the cathode ray tube on the said electrical storage element in accordance with the current flowing through the unidirectional conductor, and means for applying the bias voltage to the control electrode cathode ray tube.

2. A television receiver comprising a cathode ray tube having an electron gun structure including a cathode and an anode element within the tube for developing an electron beam, a target area responsive to impact by the electron beam to develop luminous effects, control electrode means for controlling the intensity of the developed beam upon the target, a. source of signal energy, means including an electrical storage element for applying said signals between the control electrode and cathode of the cathode ray tube to control the intensity of the beam developed within the cathode ray tube, a series circuit comprising a diode and a resistor element connected between the cathode and control electrode of said cathode ray tube for developing a bias voltage for the control electrode of the cathode ray tube on the said electrical storage element in accordance with the current ilowing through said diode and means for applying the bias voltage to the cathode ray tube to control the response level thereof to received signals.

3. In a television receiver wherein video and synchronizing signals are supplied and the synchronizing signals are in the direction of one polarity of the video signals and of greater amplitude in that direction than any of the video signals, a cathode ray tube having means comprising an electron gun for developing a cathode ray beam and a target area for producing pictorial indications under the impacting inuence of the developed electron beam, means to receive thf video and synchronizing signals, means to supply the received signals to control the intensity oi the electron beam developed within the cathode ray tube, an electrical storage means connected in the path of the signals supplied to control the intensity of the cathode ray beam, an energy discharge path for said storage element comprising a series connected diode and a resistance element` means to produce a bias voltage on said electrical storage means from the current flowing in the discharge path of said storage element and means to apply said produced bias voltage to the cathode ray tube.

4. A television receiver comprising a cathode ray tube having an electron gun structure including a cathode and an anode element within the tube for developing an electron beam, a target area responsive to impact by the electron beam to develop luminous effects, control electrode means for controlling the intensity of the developed beam upon the target. a source of signal energy, means including an electron storage element for applying the received signals between the control electrode and cathode of the cathode ray tube to control the intensity of the developed electron beam, a series combination comprising a unidirectional conductor and a resistance element connected between the control electrode and the cathode of said cathode ray tube, said series circuit comprising a unilaterally conducting path for developing a predetermined normal bias Voltage on said electron storage element whereby said bias voltage may be impressed upon the cathode ray tube such that in the absence of signals the applied bias maintains between the control electrode and the cathode a potential more negative than that corresponding to black in a picture signal by a predetermined amount, and means for providing a conducting path for gradually permitting said bias voltage to diminish in intensity.

5. A television receiver comprising a cathode ray tube having an electron gun structure including a cathode and an anode to develop an electron beam, a target area to develop luminous eects in response to impact of the electron beam, control electrode means to control the intensity of the cathode ray beam as it impacts the target area, a source of signal energy, means including a condenser for applying signals from said source to the control electrode of the cathode ray tube to control the intensity of the developed cathode ray beam, a unidirectional conductor having its cathode connected to the control electrode of the cathode ray tube, a voltage source, a potentiometer connected in parallel with said voltage source, a connection from the anode of said unidirectional conductor to the cathode of said cathode ray tube including a portion of said potentiometer, whereby the normal bias of the control electrode of the cathode ray tube may be controlled by the potentiometer andan operating bias potential may be automatically produced on said condenser by current conduction through the unidirectional conductor on peak signals, and a grid-leak element conthe cathode and anode of said tonal conductor, for gradually reducing aainatically produced operating hias potvhereloy the average potential applied e control electrode of the cathode ray tube a dctermined in accordance `with the intgv the signals from said source.

television receiver comprising a cathode 'tatie having an electron gun structure inng a cathode and an anode to develop an i ctron. beam, a target area responsive to impact oit the electron beam to develop luminous eilt-iets, control electrode means to control the intensity of the cathode ray beam as it impacts the taraet area, a source of signal energy, means including a condenser for applying signals from :said source to the control electrode of the cathode ray tuhe to control the intensity of the developed cathode ray beam, a diode having its cathode connected to the control electrode of the cathode ray tune, a voltage source, a potentiometer resistance element connected in parallel with said voltage source, a connection from the anode of said diode to the cathode of said cathode ray tube, including a portion of said potentiometer resistance, whereby the nxed bias on the control electrode of the cathode ray tube may be controlled by the potentiometer and an operatlng bias potential may be automatically produced on said condenser by current conduction through the diode on peak signals, and a grid-leak element connected between the cathode and anode of said diode for gradually reducing the automatically produced operating bias potential whereby the average potential applied to the control electrode oi the cathode ray tube may be determined in accordance with the peak intensity of the signals from said source.

MARIAN JANE W'IILANS, Executri of the Estate of Peter William Willens,

Deceased.