US2240533A - Automatic control of television receiving systems - Google Patents

Description

J. C. WILSON May 6, 1941.

Filed March 15, 1938 4 u m+ m+ n u Ill lwl I-l b fi 0E H h A 3 Maw v an A W on 3 8 55.26 2 553%. v was: m z an or %z m uz m d .o 2.. g 3:... mwmwag rm. z wumfiw m 5. .6 .3523 a m @8580: z 3 G 3053 mew uww amamwi 5.3468 655.. 2 nk .1 mi Nu INVENTOR JOHN '0. WILSON W 4 ATTORNEY y 1941- J. c. WILSON 2,240,533

AUTOMATIC CONTROL OF TELEVISION RECEIVING SYSTEMS Filed March 15, 1938 2 Sheets-Sheet 2 i A Mm; UE m2. 5; W=Ql;fl=JL-- FIG.3.

Dnffmm Vohogo IN'VENTOR J 0. WILSON ATTORNEY Patented May 6, 1941 UNITED," STATES... PATENT OFFICE AUTOMATIC CONTROL OF TELEVISION 1 RECEIVmG SYSTEMS John O. Wilson, Bayslde, N.

Hazeltlne Corporatio ware Y., asslgnor to n, a. corporation of Dela- Applicatlon March 15, 1938, Serial No. 195,955

10 Claims. (Cl. 178-43) This invention relates to television system and, more particularly, to the receiving automatic control of one or more operating characteristics respond to initiations of successive lines and fields in the scanning of the image.

At the receiver, a beam is so deflected as to scan and illuminate a target in a series of fields of parallel lines. The synchronizing components of the received signal are utilized to-control the scanning apparatus of the receiver so as to synchronize its operation with that of similar apparatus utilized at the transmitter in developing the signal. trolled by the light-modulation components, thereby to reconstrrct the image.

In certain proposed television systems, the video-frequency modulation developed from the image is so impressed on the carrier that increases in the carrier amplitude correspond to increases in brightness of the image, this being termed positive modulation; the background illumination is transmitted directly, that is, by variation of the average carrier intensity during the The intensity of the beam is conamplification control. More particularly, since the average intensity of the. carrier is varied in accordance with background light-modulation components, neither the average nor the peak carrier intensity can be utilized to eflect satisfactory automatic amplification control in the conventional manner, which requires some controlling effect dependent solely upon the average trace intervals directly in accordance with the low-frequency or steady background light variations. Betweentrace periods,,a blanking signal is transmitted which corresponds in amplitude to black or blacker than black and establishes a predetermined blanking level. The synchronizing pulses are impressed on the carrier duringthe blanking intervals in the opposite sense to the light-modulation signals, ordinarily periodically reducing the carrier amplitude to zero.

While systems utilizing the type of signal just mentioned have, in general, given satisfactory results, they have been objectionable in that it has been difficult or impossible heretofore to provide satisfactory automatic control of the system characteristics in accordance with the average received carrier amplitude and independent of light modulation, as, for example, automatic relatively fixed amplitude,

carrier intensity as determined by the power of the transmitting station, its distance, fading, etc., and independent of the degree of modulation. Nor can the amplitude of the'synchronizing pulses .of such a signal be made readily available for this purpose, as it can in the case of negative modulation where these pulses are transmitted on outward peaks of modulation at 9.

since, in positivemodulation systems, the synchronizing pulses are ordinarily transmitted on inward peaks of modulation corresponding substantially to .zero amplitude of the carrier wave. .Various arrangements have, however, been devised heretofore for providing-an automatic control of an operating characteristic of a positive-modulation television system in accordance with the average amplitude of received carrier wave and independent of light however, these systems in that they were detheir operation highly critical.

It is an object of the present invention, therefore, to provide a television receiver including improved means for automatically controlling one or more operating characteristics of the receiver in accordance with the average intensity of the received modulated-carrier signals and independently of-the light-modulation components. It is a further object of the invention to provide an improved automatic amplification control system for television receivers adapted for the reception of positively-modulated television signals including background and high-frequency light-modulation components.

It is a further object of the invention to provide a. system of the character described which operates independently of any timing impulses.

In accordance with the present invention, a television receiver adapted for the reception of positively-modulated television signals, including background components, synchro components, and video-frequency light-modulation components, comprises asynchronous control means responsive to a predetermined intermediate amplitude level of the modulation envelope of the signals, and including a time-constantcirare further provided for which means may i her. A stabilizer is preferably coupled to the output circuit of the 1 frequency amplifier .l

components on the output acontrolling effect. Means said synchronizing thereof for developing eifect for adjusting an of the receiver.

In a preferred embodiment of the invention, a television receiver "comprises means for detectingthe signal to derivethe modulation envelope,

be a conventional diode rectidetecting means for establishing a-fixed level for the peaks of the synchronizing pulses and developinga signal component varying in accordance with the blanking level of the f modulation envelope. Suitable rectifying means, for example, a diode rectifier, is provided fordeveloping a unidirectional control-bias volt-- age fromthe peak value .of the voltage representing the difierence between the stabilized de- The unidirecbe utilized to tected signal and a fixed voltage. tional voltage thus developed may system, thereby to ma tain the signal-output intensity within a relatively'narrow range for a wide range of signal-inputintensities.

For a better understanding of the present invention, togethenwith other and further objects cuit for substantially eliminating the effect of control elements The intensity of of the generators l8 and. I. the electron beam of the tube I5 is thus modulated or controlled in accordance utilizing this controllin operating characteristic 'quency generators l8 with the video-frequency voltages impressed upon the control electrode of the tube in the usual manner. Saw-tooth current or voltage-scanning waves are generated in the lineand field-freand I9, which are controlled by synchronizing-voltage impulses supplied from control the amplification in various stages of the thereof, reference is had to the following description taken in connection w'th the accompanying drawings, and its scope will be p0 ted out in the appended claims.

In the accompanying drawings, Fig. l is a circuit diagram, partially schematic, of a cathoderay tube television receiving system including circuits embodying the present invention; Fig. 2 is a diagrammatic illustration of a positively-modulated television carrier wave, to aid in the understanding of the invention, 6 are groups of wave forms representing the derived signal developed at different points in the receiver of Fig. 1.

Referring now more particularly to the drawings, the system there illustrated comprises a receiver of the superheterodyne type including an antenna and ground. system Ill-II connected to a radio-frequency amplifier l2 to which is connected in cascade,-in the order named, an oscillator-modulator l3, s an intermediate-frequency amplifier l4, a'detector l5, a video-frequency amplifier 16,. and a cathode-ray signalreproducing-tube ll. A line-frequency generator l8 and a field-frequency generator l9 are also coupled to the video' f requency amplifier and connected to the scanning elements of the cathoderay tube in the-conventional manner. The stages or units Ill-l9, inclusive, may all be of conventional'well-known'construction so that detailed illustrations and descriptions thereof are deemed unnecessary herein; 1 1

Referring briefly, however, to the operation of the system described above, television signals intercepted by the antenna circuit Ill-ll are selected and amplifiedin the radio-frequency amplifier l2 and supplied to the oscillator-modulator while Figs. 3', 4, 5, and

signal are derived ,mined intermediate The tube 20 may be of any suitthe video-frequency amplifier l6, and applied to the scanning elements of the cathode-ray tube I! to produce electric scanning fields, thereby to deflect the ray vertically and horizontally so as to trace a rectilinear scanning pattern on the screen of the tube and-thereby reconstruct the transmitted picture.

Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention, for the purpose of developing a control-bias voltage there is provided an asynchronous control means' including a vacuum-tube repeater 20 connected fier 2|, and a direct 22, as shown, which current reversing amplifier is responsive to a predeteramplitude' level of the received signal. able type having an inputcircuit, comprising a coupling condenser 23 and nected to the output circuit of the detector l5. An output circuit is provided for the tube 20 comprising a resistor 25 and a parallel condenser 26 having a predetermined time constant which is long compared to the duration of the line-synchronizing pulses and blanking intervals, but short compared to the field blanking intervals. For conventional systems utilizing, for example, frames of 441 lines per second, the resistor 25 may be of the order of 100,000 ohms and the condenser 26 -of the order of 0.006 microfarad, providing a time constant of 0.0006 second. The diode rectifier 2| is coupled across the output circuit of the tube 20 and is provided with a load circuit comprising a parallel-connected resistor 21 and condenser 28 having a large time constant. This time constant should be considerably greater than the field-scanning period, which may, for example, be %0 second, in which event the time constant may be of the order of 6 second. The input electrodes of the reversing amplifier are connected across the resistor 21, while its anode circuit includes a load resistor 29. Operating potentials are applied to the anodes of the tubes 20 and 22 by way of their respective resistors 25 and 29 from suitable sources, indicated at +3. The less positive end of the resistor 29 is connected, by way of a suitable filter including series resistors30 and shunt condensers 3| and conductors 32, to the control electrodes of one or more vacuum tubes included in the amplifier l2, oscillator-modulator l3, and amplifier M, as shown. The lead 32 may include a negative-biasing battery 33 to compensate for the high potential of the anode of the tube 22 from which the volume control potential is derived. However, if the to act as a stabilizer, a recti-' leak resistor 24, con-- The lesser amplitude, as might be caused by occasional fading. In Figs. 3-6, inclusive, corresponding wave portions are shown, illustrating the formof the signal derived at different parts of the system.

The points of the wave of Fig. 2 at which the carrier amplitude is reduced to zero, certain of which are indicated at L, represent line retrace periods or synchronizing pulses. The portions between the pulses L correspond to the trace portions of successive lines and are modulated, as. indicated at M1 and M2, by the high-frequency and background light components; During the frame retrace periods, part of one of which is indicated at YY, the amplitude of the wave is independent of light modulation. The, frame retrace periods ordinarily include the line pulses, as well as additional broad pulses, not shown, which latter constitute collectively a frame-synchronizing pulse.

It is well known that, with the positive .type of modulation, such as is represented in Fig. 2, increases in light intensity of the image being scanned are represented by increases in the amplitude of the carrier amplitude and the average intensity of the signal is varied in accordance with the low-frequency or unidirectional background-illumination components of the image. Thus, in the particular portions of the wave shown, during the line-trace modulation periods represented at M1, the amplitude of the carrier is such that it reaches the white level, whereas this does not occur during the modulation periods represented at Me. Since the average intensity of the carrier is thus varied in accordance with background-illumination variations, these carrier variations cannot be used for the purpose of ampliflcation control. Nor, as stated above, are there in this type of wave any synchronizing pulses which could be utilized for the purpose in question, since the signal is reduced to zero during the duration of line-retrace or line-synchronizing pulses. However, the modulation envelope, inchides a certain representative level, for example, the blanking level, at which the signal may represent black or blacker than black and which .appears in the signal not only for black pictures, but also during the frame retrace periods. This level affords a measure of the intensity of the carrier wave which is independent of light modulation and of other signal characteristics, such, for example, as its periodicity or synchronization. This level may, therefore; be utilized automatically to control a characteristic of the receiver, such as its amplification.

In the operation of the present invention, therefore, the modulated-carrier wave is detected by the detector l5 to derive a voltage wave corresponding to the modulation envelope, which voltage is impressed upon the input electrodes of the stabilizing tube by way'of the non-conductive circuit including the grid condenser 23 and leak resistor 24. The grid condenser and leak serve to so vary the bias of the control grid, which, in the absence of a signal, is provided with zero bias, that the signal is stabilized; that is, to ensure that the peaks of the synchronizing pulses are at a substantially fixed level as impressed on the control grid. Where the connection between the control circuit and the detector issubstantially a direct one, so that the unidirectional component of the signal is not lost, the signal may be sulficiently stabilized without the use of the stabilizing tube 20. However, where a nonconductive coupling intervenes between the detector and the control circuit, as in Fig. 1, the unidirectional component is suppressed and the signal tends to center itself about the zero axis and to appear as illustrated by the curves of Fig. 3. In this case, therefore, it must be stabilized as explained.

The characteristics of the tube 20, also, are preferably such as to cut off or limit at least a part of the video-frequency portion of the wave, so that the wave form of the current in the anode circuit of the tube 20 is as illustrated by the wave forms of Fig. 4. Due to the time constant provided by the anode load circuit comprising resistor 25 and condenser 26, which is long compared with the duration of the line-synchronizing pulses and line-blanking intervals, but short compared with the field-blanking intervals, the voltage developed across the condenser 26 is not of the same form as the anode current, but is of the form shown in Fig. 5. In other words, the output circuit time constant is such as substantially to eliminate the effect of the line-synchronizing components and may be said to cause the system to ignore signal levels lower than that desired, for example, the line-synchronizing pulse level, because of their short duration, that is, the

, condenser 26 may be considered as by-passing the synchronizing pulses or components. The portions m, m of the wave forms of Fig. 5 are representative of, and vary with, the signal level during the field blanking intervals. However, these portions are minima in the waves of Fig. 5 so that they cannot be measured directly.

In order to measure these voltage minima. there is obtained the difference voltage between the voltage across condenser 26 shown in Fig. 5 and the operating voltage or +3 voltage, which difference voltage is developed across the resistor 25 with wave forms such as shown in Fig. 6. It is noted that this difference voltage has amplitude maxima p, p corresponding to the minima m, m of Fig. 5 and occurring during, and dependent solely upon, the blankinglevel during the field blanking intervals. This difference voltage, therefore, constitutes a control voltage which has a peak amplitude corresponding to variations in the average intensity ofthe received carrier and independent of light modulation, this being indicated at p and p in Fig. 6 for the signals of different intensities. Moreover, the control signal is derived in response to a predetermined level of the modulation envelope occurring for a predetermined duration greater than the duration of a line-synchronizing pulse, in the present case, the blanking level, and independently of other characteristics thereofparticularly, independently of pulses dependent upon the periodicity of synchronization of the signal. This control signal is thereupon rectified by the peak rectifier or diode 2|, thereby developing across its load resistor 21 va control-bias voltage dependent solely upon the portions p and p of the waves of Fig. 5 while the difference voltage is as shown in Fig. 6, since operating potential is applied to the anode of diode 2| from the source +B by way of resistor 21, this potential is also on the control grid of the tube 22. A biasing battery is, therefore, included as shown in the cathode circuit of the tube 22 to compensate for the +3 voltage. The difference or control-bias voltage is applied negatively to the grid of tube 22 wherein it is amplified with a reversal in polarity. This rectified and amplified control-bias voltage across the load resistor 29 of tube 22 is fairly steady and is proportional to the average intensity of the carrier and independent of light-modulation components. The filter comprising the resistors 30 and condensers 3l serves to remove residual fluctuations from the bias voltage. The resultant unidirectional-bias voltage is impressed negatively on the control electrodes of one or more of the tubes in the stages l2, l3, and H to control the amplification of these stages inversely in accordance with variations of the average carrier intensity and independently of light-modulation components. Thus, the output-signal intensity of the channel, including the stages l2, l3, and i4, is maintained within a relatively narrow range for a wide range of received signal intensities.

As explained above, the amplifier 22 simply serves to reverse the polarity of the unidirectional-bias voltage developed by the rectifier 2| to develop a voltage which increases negatively with increasing carrier amplitude and independent of light-modulation components. This is, of course, the required polarity for effecting automatic amplification control. Various other embodiments of the invention will be readily apparent to those skilled in the art, in some of which embodiments a reversing amplifier will be unnecessary. For example, the tubes and 2| may be so connected that the voltage developed in the output circuit of the tube 2! increases negatively with increasing carrier amplitude, as by including a resistor equivalent to resistor 25 in the cathode circuit instead of the anode circuit of tube 20.

It will be noted that where .the expression wide range of signal-input intensities isemployed herein and in the appended claims, it refers to such intensity variations of the received carrier as are due to fading, to the difierent field strengths of different signals, and the like, and are independent of light-modulation components, and not to the normally smaller average carrier intensity variations which are caused by variation of the averagebackground illumination.

It will be appreciated that the present invention may be utilized automatically to control various characteristics of a television receiver other than its amplification, for example, its selectivity or its tuning. 1

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A television receiver adapted for the reception of positively-modulated television signals including background components, synchronizing components, and video-frequency lightmodulation components, comprising asynchronous control means responsive to a predetermined intermediate amplitude level of the modulation envelope of said signals and including a time-constant circuit for substantially eliminating the-effect of said synchronizing components on the output thereof for developing a controlling effect, and means for utilizing said controlling effect for adjusting an operating characteristic of the receiver.

2. A television receiver adapted for the reception of positively-modulated television signals including background components, synchronizing pulses of a given type, and video-frequency light-modulation components, comprising asynchronous control means responsive to a prea predetermined determined intermediate amplitude level of the modulation envelope of said signals occurring for duration greater than the duration of one of said synchronizing pulses and during any portion thereof for developing a controlling effect, and means for utilizing said controlling effect for adjusting an operating characteristic of the receiver.

3. A television receiver adapted for the reception of positively-modulated television signals including background components, synchronizing components, and video-frequency light-modulation components and having a predetermined blanking level, comprising asynchronous control means responsive solely to the blanking level of the modulation envelope of said signals and including a time-constant circuit for substantially eliminating the effect of said synchronizing components on the output thereof for developing a controlling effect, and means for utilizing said controlling effect for adjusting an operating characteristic of the receiver.

4. A television receiver adapted for the reception of positively-modulated television signals including background components, synchronizing components, and video-frequency light-modulation components and having a wide range of input intensities, comprising asynchronous control means responsive to a predetermined intermediate amplitude level of the modulation envelope of said signal and including a time-constan't circuit for substantially eliminating the effect of said synchronizing components on the output thereof for developing a controlling effect, and means for utilizing said controlling effect to maintain the signal-output intensity of the receiver within a relatively narrow range.

5. A television receiver adapted for the reception of positively-modulated television signals including background components, synchronizing components, and video-frequency light-modulation components having a predetermined blanking level occurring during retrace periods and having a wide range of signal-input intensities, comprising a signal-amplifying channel, and asynchronous control means responsive to the blanking level of the modulation envelope of said signals and including a time-constant circuit for substantially eliminating the effect of said synchronizing components on the output thereof for developing a controlling effect, and means for utilizing said controlling effect to maintain the signal-output intensity of said channel within a relatively narrow range.

6. A television receiver adapted for the reception of a television signal carrier positively modulated during trace periods by background and video-frequency light components and umnodulat-ed by light components during retrace periods and modulated by synchronizing pulses occurring during said retrace periods, comprising means for deriving the modulation envelope of said signal with the peaks of said synchronizing pulses thereof at a fixed level, asynchronous control means for developing a controlling effect from said modulation envelope variable in accordance with variations in a predetermined intermediate amplitude level thereof and including a time-constant circuit for substantially eliminating the effect of said synchronizing pulses on the output thereof, and means for utilizing said controlling effect to adjust an operating characteristic of the receiver.

'7. A television receiver adapted for the reception of a television signal carrier positively modulated during trace periods by background and retrace periods, comprising means for detecting video-frequency light components and unmodusaid signal to derive the modulation envelope, a

lated bylight components during retrace periods vacuum tube for stabilizing said detected sigwith synchronizing pulses occurring during said nals having an input circuit. non-conductively retrace periods, comprising means for detecting 5 coupled to said detecting means and an output said signal to derive the modulation envelope, circuit having a predetermined time constant for means for stabilizing the detected signal to essubstantially eliminating the eifect of said syntablish a fixed level for the peaks ,of said synchronizing pulses to develop an asynchronous chronizing pulses, asynchronous control means control signal from said stabilized detected sigfor developing a controlling effect from said stal0 nal variable in accordance with variations in a .bilized detected signal variable in accordance predetermined intermediate amplitude level with variations in a predetermined intermediate thereof, a peak rectifier coupled to said output amplitude level thereof and including a timecircuit for developing a'unidirectional-bias voltconstant circuit for substantially eliminating the age variable in accordance with variations in the eflect of said synchronizing pulses on the output peak amplitude of said control signal, and means thereof, and means for utilizing said controlling for utilizing said bias voltage to adjust an opereffect to adjust an operating characteristic of ating characteristic of the receiver.

the receiver. 10. A television receiver adapted for the re- 8. A television receiver adapted for the recepception of a television signal carrier positively tion of a television signal carrier positively modmodulated during trace periods by background ulated during trace periods by background and and video-frequency light components and unvideorequency light components and unmodumodulated bylight components during line and lated by light components during retrace periods field retrace periods and having a predetermined and having a predetermined blanking level with blanking level with synchronizing pulses occursynchronizing pulses occurring during said rering during said retrace periods, comprising trace periods, comprising means for detecting means for detecting said signal to derive the said signal to derive the modulation envelope, modulation envelope, a vacuum tube for stabilizmeans for stabilizing the detected signal to esing said detected signals having an input circuit tablish a fixed level for the peaks of said synnon-conductively coupled to said detecting means chronizing pulses, asynchronous control means and an output circuit having a predetermined for developing a controlling eflect from said statime constant greater than the duration of said bilized detected signal variable in accordance synchronizing pulses but less than the normal with variations in the blanking level and induration of said field retrace periods for developcludlng a time-constant circuit for substantially ing an asynchronous control signal from said eliminating the effect of said synchronizing stabilized detected signal variable in accordance pulses on the output thereof, and means for utiwith variations in the blanking level thereof, a lizing said controlling efiect to adjust an operpeak rectifier coupled to said output circuit for ating characteristic of the receiver. developing a unidirectional-bias variable voltage 9. A television receiver adapted for the recepin accordance with variations in the peak amplition of a television signal carrier positively modutude of said control signal, and means for utilated during trace periods by background and lizing said bias voltage to adjust an operating vid -irequency light components and unmoducharacteristic of the receiver. lated by light components during retrace periods 1 with synchronizing pulses occurring during said 1 JOHN C. WILSON.

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