US2240601A - Automatic gain control system - Google Patents

Description

May 6, 194l- A. R. APPLEGARTH, -JR 2,240,601

AUTOMATIC GAIN CONTROL SYSTEM Filed May 24, 1939 2 Sheets-Sheet l 10050 NPL /F/EP IL ZHIT.

May @1941- l A. R. APPLEGARTH, JR 2,240,601

AUTOMATIC GAIN CONTROL SYSTEM 2 Sheets-.Sheet 2 Filed May 24, 1939 Patented May 6, 194i UNITED STATES PATENT FFCE i A 2,240,601- i i AUTOMATIC GAiN CONTROL SYSTEEM Alexander R. Applegarth, Jr., Philadelphia, Pa.,

a corporation of Delaware Application May 2.4, 1939, seran No. 275,525A 5 claims. (ol. 17a-7.3)

y This invention relates to a method of andV apparatus for automatically controllingthe gain of an electrical signaling system. This application is a continuation in part of copending application Serial N0. 219,643, led Julygl, 1938. ,a

For reasons which will appear presently, the present invention is particularly adapted for use in a television receiver, though it is by no means restricted to this use and presents advantages Which are desirable in automatic gain control systems in general.

One object of the invention is to provide an improved system for obtaining delayed automatic gain control in an electrical signaling system.

Another object of the invention is to provide anl automaticgain control system in which an exceptionally large change in control voltage is obtained for a given change in signal amplitude, whereby a very flat control characteristic results.

A further object of the invention is to provide a novel automatic gain control system which functions to maintain constant the peak valut,` rather than the average value of a modulated carrier signal. y

, A still further object is to provide, in a television system of the type employing a signal comprising video and synchronizing components established on opposite sides of a predetermined blanking level, an automatic gain control system which tends to maintain the blanking level and the amplitudes of the synchronizing pulses constant, and which yields an unusually large variation in control voltage for a given change in signal strength. I y

In order to understand fully the operation of a system in accordance with the invention, reference may be had to the following description and the accompanying drawings in which:

Fig. l shows a specific embodiment of the invention applied to a television receiver; and

Figs. 2 and 3 are diagrams which explain the operation of the system according to the invention.

In Fig. 1, the incoming modulated carrier signal is picked up by the antenna I and converted, in a well-known manner, to an intermediate frequency in the rst detector 2. The intermediate frequency signal is transferred through an'intermediate frequency amplifier 3 to a second detector 4. The resulting demodulated signal contains the video component which is amplified in the Video amplier 5, whence it passes directly to the grid of a picture tube 6. Suitable deecting signals are produced in the deflecting circuit I in response to the synchronizing component in the demodulated signal, which component may be derived from some point in the video amplifier. The deecting signals may be used to actuate suitable deflecting means,'such as the coil 8, `The .Cil

portion of the' system described thus far and shown schematically is of conventional form and requires no further description. vThe remainder of the system shown in detail and now to bedescribed -constitutes the novel gain control means provided in accordance with the invention.

The signal from which the control is developed may be derived from the last stage of the intermediate frequency amplier though it will, of course, be understood that this is not a restriction upon the invention. For example, it would also be, possible to use the detected signal obtained from the output of the second detector or from some point later in the sys-tem, the principal consideration being that the signal utilized should have its D. C. component present, as is the case when it is derived from the intermediate frequency amplifier in the manner shown. If the D. C. component'has been removed it may, of course, .be restored by the use of leveling means in the manner set forth in my copending application Serial No. 219,643 of which this application is in part a continuation. This signal may be applied to the grid of the tube 9 across a suitable leak resistor I0, the lower end of which may be connected to a source II of negative bias voltage. The cathode of the tube is maintained at a slightly less negative voltage by connecting it to the center tap of a voltage divider comprising the resistors I2 and I3. The resistor I3, being in the cathode circuit of the tube 9,V is suitably bypassed by the condenser III'. Plate voltage may be supplied to the tube from a suitable source represented by the battery I5 through a largev plate load resistor I6, which is by-passed by the condenser I1. The tube is so biased that it becomes conducting only when the synchronizing peaks of the received signal exceed a predetermined amplitude level, and hence it functions as a detector for the signal peaks which exceed this amplitude level. Additional smoothing of the voltage developed across the resistor I6 may be provided by means of the resistor I8 and the condenser I9. The condenser is shunted by a diode 20 connected in the sense shown. The filter comprising the resistor I8 and the condenser I9 is, of course, in thenature of a refinement and may be omitted without essentially modifying the operation of the device. The control voltage derived from the device maybe supplied to the intermediate frequencyamplifier stages or to some earlier point in the channel by means of the connection 2| and may be applied to control the gain of the said stages according to any of the conventional methods.

A preliminary consideration of the operation of the system Will show that no positive voltage can be built up across the condenser I9 by reason of the presence of the diode 20 which acts as a short-circuit to any such voltage. Furthermore,

until a signal is applied to the grid oi the tube 9 which is sufficiently large to cause the plate current to increase to a point at which the plate goes negative with respect to ground, there will be no negative volt-age developed across the condenser IQ.' vBut When the plate of the tube 9 becomes negative the diode ceases to conduct Vand ythe condenser I9 will be charged through the resistor i8 until equilibrium has beenY restored. As the plate potential again drops and the tube 9 becomes less conducting, `the condenser I9 will be discharged at a much slower rate than that at whichit was charged through the resistors YIt and I8, until another peak of ,signal .occurs-which is suicient to again drive vthe plate negative.v

The' unique advantages inherent in thesystem just described will be'better understood by refer- 'ence to Fig. 2, and for this purpose itrWill be convenient to disregard the resistor I8 and assume that the diode 20 is connected directly across the plate load resistor I6 and the battery I5. Fig. 2 shows the plate current-platevoltage characteristics for the tube 9 of Fig. 1 functioning as a detector for various peak values of modulated intermediate frequency signal. It

should be noted at this point that strictly speaking'the'se" curves cannot be used to describe the operation under these circumstances since actually'they represent an ordinary plate family; what is really needed is a detection characteristic.v However, as a iirstapproxirnation made for the sake of simplicity, they serve to show qualitatively the operation of the system as shown and would give an accurate picture of ifthe operation in the case where the detected signal'is applied to theA grid of tube 3. In the absence oi the "diode shunting the Vload resistor,

and with a smaller resistor in place of IB, the tube might operate on aload line such as that represented by the line ab in Fig. 2. From this ity may be readily seen' that a change in the peak input signal from the value 4 to the value 5 will 'cause' theV plate Ycurrent to increase an amount corresponding to thelength of the line dei The value of load impedance then being equal to lne/de, vthe slope of the line ab, it follows that the change in voltage across the lload resistor is equalV to the product toxin/deletek which is designated E1.

f As is well known in the art, and subject to certain practical limitations, the change in control voltage fora given change lin signal applied tothe grid may be increased by increasing the load impedance. This appears when we consider Atheload line ac in Fig. 2. YFor a change in peak input signal from value 4 to'value 5 the plate current will change by the amount corresponding to, the length of the line fg. The slope of the load line being gh/ fg it follows that thev change finco'ntr'ol voltage is (measured by gh which is line being Zic/bf it vvill'be` seen that the change` in control voltage is represented by the length of the line bc which is "designated byrEa. This change in the control voltage will immediately' ibe seen to be much larger than can possibly be obtained by simply changing the magnitude of the plate load resistor alone.V Furthermore, the

change in control voltage, which it is possible to effect for a given change in the magnitude of the grid signal by applying the method of the invention, is limited only by practical considerations in the choice of the plate load resistor.

Of course, it is now apparent that the greater the change in control voltage developed across the resistor I6 for a given change in grid signal in the device of Fig. l, the more rapid will be the charging ofthe condenser I9, and hence the more quickly the gain vof the intermediate frequency amplier will be changed to compensate for variations in received signal strength. However, it will be desirable to refer to the diagram oi-Fig. 3 toexplain more clearly the operation of the device of Fig. `l when a standard television signal is impressedl on the grid of the tube 9.

This figure sho-ws a conventionalv plot of plate current versus grid voltage for a single value of plate voltage, the characteristic being represented by the dotted and solid line kim. 'Ihis is-'modied in the device of the invention, by the introduction'of'the diode, to correspond to the heavy line kim. Thus, for all values of grid signal not exceeding the value n, no plate current will be drawn. When the signal exceeds this value, plate current increases very rapidly along the form shown at 't'.

the line m' to vthe point iwhere it aga-in follows the tube characteristic. Ify a television signal whose envelope is represented by p be applied to the grid of the tube, the synchronizing peaks q and small portions of the signal belowthe blanking level T will tend to exceed the level designated n-s corresponding to cut-ofi in the tube. When this condition obtains, the impulses of plate current which areA produced will ybe of 'Ihe magnitude of the control signal as compared with the magnitude of the input signal will be immediately apparent.

It will be well, at this point, toV mention the delay in Ythe operation of the system which obtains by the use'of the diode in the output of the automatic gain Icontrol tube. Since no control voltage is developed across the diode until the'plate oi the gainY control tube goes negative, the point at which the control begins to operate may be denitely iixed by suitably biasing the diode, or by `varying the other voltages and circuit elements. VIn Fig. 1 the cathode of the diode is grounded, but it will be clear that a battery or other convenient voltage source may be insertedto maintain the diode lcathode at any desired potential whereby the threshold of control is iixed. The exceptionally large change in control voltage which obtains` when the threshold is reached may be regarded as a result of achieving the delay according to the method of the invention. By the cooperation of these two features of delay and exceptionally large control, it is possible toobtain what is referred toV in the art as avery nat gain control. Y

A further important detail relating to the mode of operation of the system will also be apparent from the/diagram of Fig. 3. The amount of control is dependent upon the areas of the plate current pulses which are designated tand by far the greater part of this ai'ea corresponds to the portion of the input signal below theV blanking level r as com-pared with that which corresponds to the synchronizing component of the input signal. The portion of the signal below the blanking level used to form the control signal must necessarily `be keptvsiall in` order that the background component be not removed lfrom .the

video signal. This is fully explained in applicants above-mentioned copending application. In order to make the best possible use of this small portion below the blanking level, it is desirable that the average amounts by which the blanking level r exceeds the cut-olf level ns should correspond to the change in grid voltage along the characteristic corresponding to the steep portion of the characteristic m' in Fig, 3. This permits of the greatest degree of control with the utilization of a minimum of the blanking signal below the level, the resultant inclusion of the least possible amount of video component, and hence the least tendency to destroy the D. C. or average brightness component contained in the video signal. 'I'he amount of signal utilized is dependent fundamentally upon the size of the plate load resistor i6 of Fig. 1 and the bias, if any, applied to the diode 20. This will be clear by reference to Fig. 3 where it will be seen that the desired operation obtains when the average value of the blanking level corresponds to a value of grid voltage on the part of the characteristic designated n--i or slightly greater (i. e. less negative) but preferably not less.

It will be understood that the present disclosure is illustrative only and is not intended to impose any limitation upon the invention, the scope of which is defined by the appended claims.

I claim:

1. In a television system of the type employing a composite signal `having video and synchronizing components established on opposite sides of a predetermined blanking level, a signal channel, means for deriving a signal from said channel, a gain control tube operable by said derived signal, means for causing the eifective plate current of said tube to decrease rapidly and abruptly from a substantial value to cut-oli, means for adjusting the operation of said tube in predetermined relation to its cut-off point such that the blanking level normally exceeds the cut-oir point of said tube but not sufciently to permit the tube output to include an appreciable portion of the video component, whereby the tube output is substantially independent of variations in the video component, and means for applying the output signal from said tube to said channel to control the gain thereof.

2. In a television system of the type employing a composite signal having video and synchronizing components established on opposite sides of a predetermined blanking level, a signal channel, means for deriving a signal from said channel, a gain control tube operable by said derived signal, means for causing the effective plate current of said tube to decrease rapidly and abruptly from a substantial value to cut-oir, means for adjusting the operation of said tube in predetermined relation to its cut-off point such that the average value of the blanking level exceeds the cut-ofi point of said tube but not sutiiciently to permit the tube output to include an appreciable portion of the video component, whereby the tube output is substantially independent of variations in the video component, and means for applying the output signal from said tube to said channel to control the gain thereof.

3. In a television system of the type employing a composite signal having video and synchronizing components established on opposite sides of a predetermined blanking level, said synchronizing component comprising time-spaced pulse signals occurring during intervals of duration greater than said pulse signals and in which said video signal component is established at said blanking level, a signal channel, means for deriving a signal from said channel, a gain control tube operable by said derived signal, means for causing the eiective plate current of said tube to decrease rapidly and abruptly from a substantial value to cut-oil, means for adjusting the operation of said tube in predetermined relation to its cut-01T point such that the blanking level normally exceeds the cut-ofi point of said tube but not suliiciently to permit the tube output to include an appreciable portion of the video component, whereby the tube output is substantially independent of variations in the video component, and means for applying the output signal from said tube to said channel to control the gain thereof.

4. In a television system of the type employing a composite signal having video and synchronizing components established on opposite sides of a predetermined blanking level, a signal channel, means for deriving a signal from said channel, a gain control tube having at least an anode, a cathode and a control grid, means for supplying said derived signal to the control grid of said tube, a load impedance for said tube characterized in that it decreases suddenly in magnitude when the plate current in the tube falls below a predetermined value, thereby causing the plate current to decrease rapidly for a small change in grid voltage within a certain range, means for so operating said tube that the average value of the blanking level of the applied composite signal exceeds the grid voltage corresponding to cut-oli of the tube but not suiciently to permit the tube output to include an appreciable portion of the video component, whereby the tube output is substantially independent of variations in the video component, and means for applying the output signal from said tube to said channel to control the gain thereof.

5. In a television system employing a composite signal having video and synchronizing components established on opposite sides of a predetermined blanking level, a signal channel, means for deriving a-signal from said channel, a gain control tube having at least an anode, a

y cathode, and a control grid, means for supplying said derived signal to the control grid of said tube, a load impedance for said tube, a connection including a diode between a point on said load impedance and a point of predetermined potential, for effectively causing said load impedance to decrease suddenly in magnitude when the plate current in the tube falls below a predetermined value, thereby causing the plate current to decrease rapidly for a small change in grid voltage within a certain range, means for so operating said tube that the average value of the blanking level of the applied composite signal exceeds the grid voltage corresponding to cut-off of the tube, but not suiiiciently to permit the tube output to include an appreciable portion of the video component, whereby the tube output is substantially independent of variations in the video component, and means for applying the output signal from said tube to said channel to control the gain thereof.

ALEXANDER R. APPLEGARTH, JR.

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