US2481045A - Automatic volume control and sync separator for television receivers - Google Patents

Description

EN W4- A. c. SCHROEDER 21,4539@ y AUTOMATIC VOLUME CONTROL AND SYNC SEPRATOR FOR TELEVISION RECEIVERS Filed MaTCh l2, 1945 I our/Dur /4 TTOR/VEV atenter Sept. 6, 317949 2,481,045 AUTOMATIC VLUME CONTROL AND SYNC SEPARATOR FOR Radio Corporation o of Delaware TELEVISION RECEIVERS Alfred C. Schroeder, F

easterville, Pa., assignor to f America, a corporation Application March 12, 1945, Serial No. 582,386

11 Claims. l

The present invention relates to a control system for television signal receivers, and more particularly to an improved form of automatic gain control or AVC system which additionally includes means for separating the synchronizing information in the television signal from the video portion thereof.

In television systems in which the D.C. component of the video signal is transmitted, variations in this D.C. component result in variations in the average amplitude of the transmitted carrier. Consequently, gain control or AVC systems based on average carrier amplitude are not satisfactory, and other arrangements, such as those utilizing a voltage derived from the peak amplitude of the carrier, that is, its amplitude during the synchronizing portion of the composite television signal, have been proposed. One of these latter arrangements is disclosed in Patent No. 2,246,939, granted to R. S. Holmes on June 24, 1941. This patent also describes means whereby the synchronizing or sync" and video components which appear at relatively different amplitude levels in the television signal may be separated by the same circuit from which the AVC voltage is derived.

A common feature of automatic gain control systems, as incorporated in television receivers of the above class, lies in the provision of me-ans for peak-rectication of the synchronizing component of the received signal, together with additional means for utilizing this rectied component in inverse degree to contr-ol the ampliiication of the I. F. stages of the receiver. To obtain a relatively smooth rectified control voltage in such systems, it has heretofore been customary to provide filter elements in the rectifier circuit having a time constant which is long with respect to the period of the synchronizing impulses. Such systems, however, have the disadvantage of not responding quickly enough at certain times to eliminate certain types of interference, as for example that produced by aircraft in the vicinity of the receiver. The present invention discloses an AVC system which :provides a relatively smooth rectified voltage having a value which :is a measure of .the height of the sync pulses and therefore a measure of the amplitude of the incoming signal, and which, in addition, is extremely quick-acting. This voltage, when fed back :to the I. F. stages, improves the response of the receiver in a manner not possible with systems of conventional design. Furthermore, by the employment in the system of the present invention of direct capacitive coupling between the rectifier circuit and the control elements of the receiver I. F. stages, the time required for application of this AVC voltage is still further lessened in comparison with standard arrangements.

Accord-ing to a further feature of the present invention, the AVC voltage is derived from the system at a point following the video ampller, instead of at a point following the detector and before the video amplier as in the aforementioned Holmes patent. Not only does this result in a higher AVC potential, but it permits the D.-C. information in the television signal to be applied directly to :the kinescope without the necessity 'of employing separate D.C. reinserting elements.

One object of the present invention, therefore, Iis the provision of a quick-acting automatic gain control or AVC system for television receivers.

Another object of the present invention is the :provision of a circuit for receiving a television signal which includes the D.-C. component thereof, the circuit being so designed that this D.C. component is not lost in passing through the circuit, but is instead retained for application directly to the image-producing device without the necessity of employing separate D.C. rein-l sorting elements.

A still further object of the invention is the provision' of Vsimple and eiective means for limiting the amplitude of any random noise which maybe included in the television signal to an amplitude comparable to the amplitude of the synchronizing signal.

An additional object of the invention is the provision of means for utilizing a single threeelement electron discharge tube as an AVC voltage rectier and also to separate the synchronizing or sync pulses from the video signals. The cathode and control electrode of the tube act as a diodeV to peak-rectify the synchronizing pulses, while the cathode, control electrode, and anode of the same tube act as a separator to provide only sync pulses for application as control potentials to the deflection generators.

Other` objects and advantages will be apparent from the following description and from the drawing, the single figure of which is a circuit diagram of a :preferred form of the invention.

1n the drawing the invention is shown applied to a television receiver of the superheterodyne type. The receiver comprises a converter or first detector IIJ, a tunable oscillator l2, and an intermediate-frequency amplifier lll which usually includes a plurality of stages. The output of the 3 intermediate-frequency amplifier I4 is applied to a suitable demodulator or detector unit. One form of connection of this type is shown by the drawing where the amplifier output is supplied through a tuned I. F. .transformer I 6 to a second detector I8 which maybe of the push-pull type, as represented by the twin diode 20, which may be a GHG type, for instance.

The two plates or anodes of the twin diode 2U are respectively connected to opposite ends of .the secondary winding 22 of transformer I6. The midpoint of this secondary winding 22 is connected to the terminal 24 to which a voltage source (not shown) has its negative terminal connected. Through this connection a negative voltage of the yorder of 3 volts is supplied through a relatively low lor small resistor 26. While values which have been found suitable in practice are given herein for certain of the components' of the circuit herein described, as well 'as for various potentials applied to these components, it will be clearly understood .that these values are being given rnerely as examples, and that other values may be substituted for any or all of those given as may prove desirable or necessary.

The joined oathodes of diode 20 are connected to the terminal 30 whereat the negative terminal of a suitable voltage source (not shown) is connected. This source connected at terminal 30 may be arranged to supply to the cathodes of tube 20 a negative potential of the order of 1'7 Volts through a resistor element 32. The signal output of detector I8 is applied to the cathode of .a limiter tube 34, the anode of which is connected to the control electr-ode of a video amplier 36, the latter preferably being ya tube of the general type of a BAC?.

A grid resistor 38 is provided betweenthe conrtrol electrode of amplifier tube 36 and ground. It will be noted that the D.C. voltages on source terminals 24, 30 and the grounded end of resistor 38 are such in the example given that while the anodes of detector tube 20 are operated at fthe above assumed D.C. potential of 3 Volts negative to ground, these electrodes are relatively positive with respect to the cathodes of .the same tube to which an assumed D.C. potential of -17 volts relative to ground is applied. Furthermore, this latter D.C. voltage also appears on the cathode of the limiter ltube 34 the anode .of which is approximately at D.C. ground poten-tial. The relative polarity of the D.C. voltages on the various electrodes of tubes 20 and 34 is, therefore, correct for operation of these tubes.

The screen electrode of `the video amplifier tube 36 is connected to the positive terminal 40 of a relatively high voltage `source (not shown) through resistor element 42 forming part of a filter circuit which also includes a condenser 44. The anode of the video amplifier tube 36 -is connected through an output resistor 46 to one end of a resistor element 48 which in combination with a condenser D constitutes the plate filter circuit of tube 36. The opposite end of resistor 48 is connected to the positive terminal 52 of a suitable voltage source (not shown). Bias for tube 36 is provided by a cathode biasing 4resistor 54.

The anode of tube 36 is connected directly to the control electrode 56 of an image-producing cathode-ray tube 58 which is of conventional type and, therefore, not illustrated in detail. The upper end (in the drawing) of output resistor 46 is connected through an isolating or decoupling resistor |60 to the cathode 62 of -a .triode 64 which may be half of a tube of th general type of a 6SN7. The lower end of output resistor 46 is joined to the grid 66 of triode 64 through a grid resistor 6 8.

Grid |66 is connected to ground through two series condensers 'l0 and 12. A voltage regulator glow tube 'I4 such as one of the 991 type is connected between grid 66 and one end of a resistor 16 the other end of which is connected to the negative terminal 1-8 of a suitable voltage source. A resistor element `8i) connects the upper plate (in the drawing) oi condenser 'i2 to the upper end of resistor 16, while ythe charge on condenser 'I2 is applied :to the I. F. amplifier I4 over a conductorZ.

The triode 64 performs the function of an AVC and sync separator tube in a manner to be hereinafter described in connection with a description of the operation of the circuit. The anode 84 of triode 64 is provided with a positive D.C. voltage of suitable value from source terminal 86 through two resistors 88 and 90. The junction point of these resistors 88 'and 9o is grounded through a bypass condenser 62.

Anode 84 is coupled to the grid of a second triode 93 (which may be the other half of the 6SN'7 above mentioned) through a coupling condenser 94, the grid of triode 93 being biased by a resistor 96. The output of triode 93 appears across a load resistor 98 connected between the anode of the tube and the positive terminal |60 of a suitable voltage source.

The operation of .the circuit is as follows:

The television signal output of the I. F. amplifier I4, which includes the D.C. component of the video signal as well as periodically recurring horizontal and Vertical synchronizing pulses, is detected 'by the twin diode 29 in the usual manner. The output of twin diode 20 is applied to .the control electrode of video amplier tube 35 by a D.C. coupling circuit which includes the limiter tube 34, the latter acting to restrict any undesired noise pulses which may be present in the television signal to an `amplitude comparable to that of the synchronizing pulses, and thus reduce the deleterious effect of such noise pulses on thedeiiecting circuits.

By a proper choice of constants for the video ampliiier screen filter circuit 42, 44 and plate iilter circuit 48, 56, such, for example, as those values hereinafter are given, :the gain of the amplifier 3E can be made substantially constant for a wide band of frequencies down to D.C. Furthermore, since the distributed capacity to ground of the system is low, due in part to the omission of the usual coupling condensers, the high frequency response is materially improved inA comparison with conventional circuits.

The signal on the plate of the video amplier 3,6, which retains the D.C. information in the signal due to the D.C. input coupling to the video amplifier through limiter tube 34, is applied directly to the cathode-ray tube control electrode 56. The output of tube 36, which appears as a varying voltage with the polarity indicated in the drawing across the plate resistor 46, is also applied between the cathode 62 and grid 66 of triode 64, these two electrodes acting in effect as a diode rectifier.

Thecurrent flow through the rectier circuit, including cathode 62, grid 66, plate resistor 46 and grid resistor 68, causes voltage variations to appear across resistor 68 .in response to varia,- ticns in the output of tube 36 as developed across resistor 46. These voltage variations across resister 68 cause the potential at point |0| to liuctuate in value, and also cause uctuations in the positive charge built up on grid condenser from point |0| on the voltage-divider network consisting of resistors 48, 68 and 16 connected between terminals 92 and 18. The fluctuating positive charge on condenser 10 is smoothed out by the R.C. filter circuit 12, 80 to provide a positive D.C. voltage at point |04 the instantaneous value of which is a measure of the height of the sync pulses and, therefore, a measure of the amplitude of the incoming signal. The decoupling resistor 60 acts to prevent the triode 64 from loading the plate of the video amplifier 36 with extra capacity, and at the same time reduces the eiect of noise on the triode 64.

The polarity of the television signal at point |02 is such that the sync pulses extend in a negative direction. Consequently, a decrease in amplitude of the sync pulses as a result of fading or attenuation of the incoming signal will cause the voltage at point |02 to become less negative or,- in other words, will reduce'the potential drop across resistor 46. This will reduce the current ow in the rectifier circuit, which in turn decreases the voltage drop across resistor 6B. As the voltage on each end of resistor 68 approaches the same value, the positive charge on condenser 10 increases. This increase in the charge on condenser 10 is applied from point |04 over conductor 82 to the I. F. amplifier i4 to increase the gain thereof in any suitable manner.

In a circuit employing the values set forth, the D.-C. voltage on grid 66 i-s about 55 Volts above the required I. F. control voltage to produce a signal giving that voltage on grid 66. The latter voltage is, therefore, dropped through a 55 volt glow tube 14 so that the D.-C. voltage at point |04 is of proper value for application to the I. F. amplier I4.

It will be noted that since the I. F. amplifier I4 is directly connected to the grid 66 of triode 64 through the condenser 10, the system provides an unusually fast-acting AVC voltage in response to variations in current flow between cathode 62 and grid 66. Y

The composite television signal appearing on the cathode 62 of triode 64 includes both video and sync components. To separate the sync pulses from this composite signal, the plate 84 of triode 64 is connected to the terminal 86 whereat a source of positive voltage (not shown) is connected. To keep the voltage on plate 84 sufficiently low with respect to cathode 62 so that the triode 64 will act as a good sync signal separator, the voltage from terminal 86 is pas-sed through a relatively large dropping resistor 90. The sync pulses then appear across the plate resistor 88, but theV relative D.C. potentials on cathode 62 and plate 84 are such that the less negative video signals on cathode 62 are not passed by the tube. The sync pulses across resistor 98 are then applied through the coupling condenser 94 to the triode 93, where they are further amplified and clipped to appear across the output resistor 98.

The following values for the components of the system hereinabove described are given as being illustrative of those which have been found suitable in practice. However, it should be clearly understood that these values are merely examples, and that values which depart widely from those given below may be employed if desirable or necessary.

V Resistors 20:410. ohms s0= 10,000 ohms 32: 15,000 ohms 68:200,000 ohms 38: 3,900 ohms 16=330,000 ohms 42: 82,000 ohms 80:220,000 ohms 46: 3,300 ohms 88: 5,000 ohms 48: 12,000 ohms 90:5 megohms 54:15 ohms 96:330,000 ohms Condensers 44:1.5 mf. 12: 3,300 mmf. 50: 10 mf. 92: .1 mf. 10: 820 mmf. 94: 3,900 mmf.

Voliages (with respect to ground) At terminal 24:- 3 volts At terminal 30:- 17 Volts At terminal 40:-1-300 volts 'At-terminal 52:-1-135 volts At terminal 18:- 33 volts At terminal 86:-l-300 volts At terminal |00=l 15 volts While I have illustrated and described, and have pointed out in the annexed claims, certain novel features of my invention, it will be understood that various omissions, substitutions and changes in the form and details of the system illustrated may be made by those skilled in the art without departing from the spirit of the invention.

I claim:

1. In a television receiver which incudes a second detector preceded by an intermediate frequency amplifier, the combination of a video amplier having an impedance element in the output thereof across which a complete video signal is developed, a rectifier circuit, including a, further impedance element, for peak-rectifying the video signal, a resistance-condenser combination having an output time constant which is long with respect to the period between successive synchronizing impulses, said resistance-condenser combination including a pair of condensers connected in series, means for developing a normally substantially constant charge on both condensers oi' said pair, means for applying said peak-rectied signal in series with the series connected condensers of said resistance-condenser combination to vary the said normally constant charge on both condensers of said pair, and means for applying the output of said resistance-condenser combination aS it appears between said two condensers to said intermediate-frequency ampliiier to control the gain thereof.

2. The combination of claim 1, in which the time constant of one of the condensers of said pair as included in said resistance-condenser combination is greater than the time constant of the other of said condensers.

3. In a television receiver which includes a second detector preceded by an intermediate frequency amplifier, the combination of a video amplifier having an impedance element in the output thereof across which a complete video signal is developed, means for connecting the video amplifier plate end of said impedance element to the cathode of a grid-controlled electron diS- charge device, means for connecting the other end of said impedance element through a resistor to the grid of said electron discharge device, a pair of capacitors, means connecting the grid of said electron discharge device to ground through said pair of capacitors, means for developing a normally substantially constant direct current charge on each of said capacitors, means providing a discharge path for each of said Acapacitors, and a connection from a point intermediate the capacitors of said pair to said intermediate-frequency amplier.

4. The combination of claim 3 in which the time constant of one of said capacitors and its associated discharge path is greater than the time constant of the other capacitor and its associated discharge path.

5. The combination of claim 3, further comprising voltage-regulating means direct current connected from a point intermediate the two capacitors of said pair to the grid of said electron discharge device, whereby the directcurrent voltage at said point intermediate said two capacitors is lower by a substantially constant amount than the direct current voltage appearing on said grid.

6. In a television receiver adapted to receive a composite series of video signals, said receiver including a detector preceded by at least one inn termediate-frequency amplifier stage, and a video amplifier connected to said detector so that the video signal output of said video amplifier contains sync pulses extending in a generally negative direction, the combination of an impedance in the output of said video amplifier across which said video signals are developed, an lelectron discharge device having at least an anode, a cathode and a control electrode, means conductively connecting the cathode and control electrode of said electron discharge device in shunt with the output impedance of said video amplifier so that the cathode and control velectrode of said electron discharge device will act in eliect Vas adiode to rectify the sync pulse voltage output of `said video amplifier as developed across vsaid Youtput impedance, Aa voltage divider network, means for applying the rectied output of said electron discharge device to an intermediate point of positive polarity on said voltage divider network so that the positive potential of such point AWill vary in accordance with variations in the rectified output of said electron discharge device, means 'for applying said varying positive potential to -said intermediate-frequency amplier stage so as to control the gain thereof, and `means in the anode circuit of said electron discharge device for separating from the output of Asaidvideo 'amplifier the said sync pulses.

7. A `television receiver in accordance with claim-6, in which said-means for applying afvarying positive potential Ato said intermediate-*frequency amplifier stage includes a pair of capacitors /connected between ground and the point 'on said voltage divider network to which the rectiiied sync vpulse output of said eiectronvdischargedevice is applied, the connection lto said intermediate-frequency amplifier stage being made intermediate the capacitors Vof said vpair.

8. In a television receiver'Which'includes aseoond detector preceded by an intermediate fre quency amplifier, the Ycombination'of a video 'amplier :having an 'output impedance lfanii -an vinput circuit direct-coupled to said second detector to provide across said impedancev a signal which contains sync pulses and retains the second-detector direct-current component. an electron discharge device having a cathode and a second electrode, means .conductively connecting said cathode #and second electrode in shunt with said impedance so that .said electron discharge device recties that portion .of said signal representing sync pulses, .means Vfor maintaining said second electrode at .a substantially constant potential for steady signal amplitude and .superimposinz on said potential the rectified output potential of said electron discharge device, and means ifor applying the `variations of said superimposed 'D0- tentials .to the intermediate frequency ampliiier to control the .gain thereof.

9. The combination defined by .claim 8 in which said last-mentioned means comprises `two capacitors connected in series between a 4iixed potential point and said second electrode, the common point rbetween said capacitors being connected to said :intermediate ,frequency amplifier.

10. The combination defined by claim 9 wherein .the ratio of ,capacity of said capacitors is approximately four to one, the smaller capacitor being .connected to said second electrode.

11. In a television receiver having a second detec'tor preceded by an intermediate frequency ampliier, the combination of an automatic gain control and sync separator comprising an 'impedance :across which a complete video signal is developed including sync pulses and directcur rent :information yfrom the second detector, a triode 'having a cathode and a control electrode connected :respectively to spaced points Vof said impedance through `decoupling resistance whereby .the Vpeak voltage of `the sync pulses is measured, ,means connected to said control electrode for .deriving a gain-control voltage from .said measured peak voltage, a capacitor shunted across said means for 'making the gain control fast actin-g; said triode also having an anode, and means for Vmaintaining a low direct-current potential difference rbetween said cathode and anode whereby .the -sync pulses are separated and amplied.

ALFRED C. -SCHROEDER .REFIERE-N CES .CITED UNITED .STATES PATENTS Number Name Date `1,890,454 Bruce Dec. 13, 1932 vv2,018,982 Travis Oct. 29, 1935 12,144,605 Beers Jan. 24, 1939 2,194,278 George Mar. 19, 1940 2,196,364 Schlesinger Apr. 9, 1940 2,227,001 Schlesinger Dec. 31, 1940 2,408,821 Stearns Oct. 8, 1946 FOREIGN PATENTS Number Country Date 521,465 Germany Apr. 13, 1933

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