US2259538A - Television receiver with automatic shade-level control - Google Patents

Description

Oct. 21,1941. H. A. WHEELER 2,259,538

TELEVISION RECEIVER WITH AUTOMATIC SHADELEVEL CONTROL Filed 96. s, 1938 INVENTOR AROLD AJNHEELER ATTORNEY Patented Oct. 21,1941

TELEVISION RECEIVER WITH AUTOMATIC SHADE-LEVEL CONTROL Harold A. Wheeler, Great Neck, N. 1., a-ignor to Haz'eltlne Corporation, a corporation of Dela- Application December 8, 1938, Serial No. 244,190

Claims.

This invention relates to television receiving apparatus and more particularly to automatic control systems for such apparatus. The invention is especially directed to the provision of an improved signal shade-level control system for television receivers, which is useful as a blacklevel adjusting device.

In accordance with present television practice, a transmitted signal comprises a carrier, modulated during successive intervals or trace periods by video frequency, and steady components representative of light variations in an image being transmitted and of its average background illumination, respectively. Between the trace periods, that is, during retrace intervals, the carrier has a predetermined amplitude level, referred to as a blanking level, corresponding to a given shade, usually black, and is modulated during a part of this retrace period by synchronizing components which correspond 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 or screen in a series of fields of parallel lines. The synchronizing components of the received signal are separated from the other modulation-signal components and 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. The intensity of the beam is controlled by the lightmodulation components, thereby to reconstruct the image.

An important requirement in a properly operating television receiver is the adjustment of the system so that the modulation signal is stabilized withrespect to a particular shade value or level, such as the blanking level. More particularly, the system must be so controlled-that a given signal level or amplitude corresponding to black or any predetermined shade value is effectively held fixed with respect to the signal-input brightness characteristic of the reproducing device of the system at the input circuit of this device, in order that signal components representing any given shade will at all times appear as the same shade in the reproduced image. In general, the system should be controlled so that the light gradations or shades from black to white represented by the various light-modulation components are all faithfully represented in the reconstructed image.

fore, to provide an improved signal shade-level adjusting system for a television receiver.

In accordance with the present invention, a

television receiver adapted to receive a signal containing synchronizing components and video-frequency components comprises a picture-reproducing device, means for deriving from the received signal and applying to the device a signal having a periodically recurring level corresponding to a predetermined shade value, and means connected in circuit with the device for developing a voltage varying with the current therein. The receiver also includes an auxiliary circuit, means for applying the above-mentioned developed voltage to the auxiliary circuit, means responsive to the synchronizing components for disabling the voltage-developing means whenever the current in the picture-reproducing device exceeds a predetermined value corresponding to the above-mentioned predetermined shade value, and means including the auxiliary circuit for superimposing on the signal applied to said picture-reproducing device a bias for maintaining the current in the device at a predetermined value upon the occurrence of said recurring level in said signal.

For a better understanding of the present invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing,.and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a circuit diagram, partially in block form, of a cathode-ray tube television receiving system including circuits embodying the present invention; Fig. 2 is a graph of the .envelope of a negatively modulated television carrier wave to aid in the understanding of the invention; w hile Figs. 3-5, inclusive, are groups of wave forms representing the derived signal developed at difierent points in the receiver of Fig. 1.

Referring now more particularly to Fig. 1 of the drawing, the system there illustrated comprises a receiver of the superheterodyne type including an antenna system lllll connected to a radio-frequency amplifier II, to which there are connected in cascade. in the order named, an oscillatormodulator IS, an intermediate-frequency amplifier I, a detector IS, a video-frequency amplifler l8, and a cathode-ray signal-reproducing tube ll.

The cathode-ray tube ll comprises the usual envelope l8 containing, in the order named, a

It is an object of the present invention, therecathode l8 having a heater 20, a signal-input or control 2|, a screen 22, a first anode 23, a second anode 24, and the usual fluorescent screen 25 at the end of the tube- Line-frequency and field-frequency scanning coils 28 and 21 are disposed about the tube for defiectingthe beam in The out two directions normal to each other. put circuit of the video-frequency amplifier is connected to the control grid 2| by way 01' a coupling condenser 23 and resistor 29. Operating potentials for the several electrodes of the device included in the cathode circuit of the tube a l I are supplied from a suitable source, such as the battery 30; v

A line-frequency generator 3| and a field-frequency generator 32 are also coupled to the output circuit of the detector l5 for synchronization and are connected to the scanning coils 26 and 21, respectively, of the cathode-ray tube in the conventional manner. Suitable means are included in the generators 3| and 32 for selecting the synchronizing pulses from the other modulation components and from each other. The stages or units just described may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly to the general operation of the system described above as a whole, television signals intercepted by the antenna circuit ||i-|| are selected and amplified in the radio-frequency amplifier i2 and supplied to the oscillator-modulator 3, wherein they are converted to intermediate-frequency signals which, in turn, are selectively amplified in the intermediate-frequency amplifier l4 and delivered to the detector ii. The modulation componentsof the signal are derived by the detector .l5 and are supplied to the video-frequency amplifier l6 wherein they are amplified and from which they are supplied to the control grid. 2| of the cathode-ray tube ll in a manner hereinafter explained in detail, which is in accordance with the present invention.

The detected signal components are also supplied to the generators 3| and 32 to synchronize the operation thereof. With properoperating potentials supplied from the battery 30 to the electrodes of the tube ii, an electron beam is emitted from the cathode. l9 and its intensity is controlled by the grid 2| in accordance'with the video-frequency voltages and control voltages impressed thereon. The screen 22 serves to controlthe average beam intensity while the first and second anodes 23 and 24 cooperate to accelerate and focus the beam. It will be appreciated that the tube should have a substantially linear response or signal-input brightness characteristic, as distinguished from the curved characteristics of conventional amplifier tubes, so that variations in its grid-bias voltage serve simplyto adjust the point on this characteristic at which the tube operates, substantially without affecting its gain, so as to determine the average brightness of the reproduced image for a given input-signal level.

The intensity of the electron beam of the tube i1 is modulated or controlled in accordance with the video-frequency voltages impressed upon the control grid of the tube in the usual manner. Saw-tooth current waves are generated in the line-frequency and field-frequency generators 3| and 32, which are controlled by the detected synchronizing pulses, and these waves are applied to the scanning elements of the cathode-ray tube I! to produce electric scanning fields, thereby to defiect the ray in two directions normal to each other, so as to trace a rectilinear scannin natvacuum-tube conductance 33, for example, a triode having its control grid connected to a suitable point in the generator 3|, whereby negative pulses are applied to this grid during'the lineretrace scanningintervals. Such waves are developed and conveniently available in all conven-' tional scanninggenerators. This grid connection may, if desired, be to the field-frequency generator 32, instead of the generator 3|, for applying negative pulses to the tube 33 during the fieldretrace periods, or connections may be made to both generators for applying both line-retrace and field-retrace pulses. In any of these cases, the principle of operation of the system is exactly the same as will be readily appreciated from the following description. The tube 33 is normally conductive and is rendered non-conductive by the applied negative pulses during the entire retrace period including intervals during which the signal amplitude corresponds to the blanking level. A rectifier circuit, including a diode 34 in series with a parallel-connected resistor 35 and condenser 36 having a predetermined time constant, which is much greater than the period of the linesynchronizing intervals, is connected across the tube 33 for developing a unidirectional-bias voltage varying in accordance with the current passed by the cathode circuit of the cathode ray tube I1 during the line-retrace periods when the tube 33 is non-conductive; that is, varying in accordance with the blanking level.

For applying the unidirectional voltage developed by the diode 34 and its load circuit to the control grid of the cathode-ray tube with greater amplitude and reversed polarity, there is provided an auxiliary or bridge circuit including a resistance network, one arm of which comprises a source of unidirectional voltage, for example, a battery 31, grounded at an intermediate terminal, and the other arm of which comprises a resistor 38 connected across battery 31. A repeater or a unilaterally-conductive device comprising, for example, a vacuum tube 39 is connected between points of the network normally effective to produce a current therethrough; specifically, it is connected between an intermediate point on the resistor 38 and an intermediate point of positive potential on the battery 31. A second intermediate point on the resistor 38 is connected by way of the resistor 29 to the control grid 2| 0: the

cathode-ray tube. The control grid of the tube 33 is connected to the positive end of the diode load resistor 35, thereby to apply the voltage developed across resistor 35 to the auxiliary circuit through the unilaterally-conductive device 39 to effect, in at least a portion of the above-mentioned resistance network, a variation in the unidirectional current therein to produce, in at least a part of the network, a current in opposition to the first current and varying in accordance with variations in the level of the received signal from a predetermined level, specifically, the blanking level.

The fixed bias provided for the tube 33 by virtue of its connection in the bridge circuit is such that it always passes current and the amount of this.

current is dependent upon the positive potential applied to its grid by the diode circuit 34, 3|, .0. The potentials at the intermediate points on the resistor 38, where the anode of the tube II is connected and where the control grid of the cathode-ray tube I1 is connected, thus vary in blanking level, as will be clear from the descrip tion of the operation 01' the system in connection with Figs. 2-5, inclusive, which iollows.

For the purpose of effectively stabilizing the modulation signal with respect to the peaks ,of the synchronizing pulses, there is provided a diode 40 connected in parallel with the resistor 29, as

shown, which operates to rectiiy the peaks of the signal voltage output oi! the video-frequency amplifier ll, thereby to develop a positive unidirectional .voltage equal to the peak value of the unstabilized signal in the black direction. This voltage appears across the resistor 28 and is applied positively to the cathode-ray tube control grid 2| in opposition to the other voltage applied thereto by way of this resistor and thus, in effect, so adjusts the axis or the signal wave that the peaks of the synchronizing pulses are substantially aligned.

The operation and results obtained by the system of Fig. 1 may best be understood by reference to Figs. 2-5, inclusive. In Fig. 2 there are illustrated the wave forms of portions of the complete carrier envelopes of television modulated-carrier waves of the negatively modulated type. The portion shown at A represents a wave of relatively large amplitude while that at B represents a similar wave difiering only in that it is of lesser amplitude, as might be caused by occasional i'ading. In Figs. 3-5 corresponding wave portions are shown, illustrating the form of the modulation signal derived at various points oi the system for the assumed signal inputs of different amplitudes A and B.

The portions oi the wave of Fig. 2 indicated at L indicate the line-retrace periods and the level.

indicated at Bl represents the periodically recurring blanking level corresponding to black or some other predetermined shade value. The portion of the wave at which the carrier amplitude is greatest during the retrace periods, certain of which are indicated at 8, represents line-synchronizing pulses. The portions between the retrace periods represent trace portions of successive lines, certain of these being modulated, as indicated at M1 and m, by the high-frequency and background-light components. During the line-retrace and field-retrace periods, part of one of the latter being indicated at Y-Y, the amplitude of the wave is independent oi light-modulation components. The field-retrace periods ordinarily include the line-synchronizing pulses, as well as additional broad pulses, not shown, which latter constitute collectively a field-synchronizing in accordance with the low-frequency or steady background or average illumination components oi the image. Thus, in the particular portions of the wave shown, during the line-trace modulation periods represented at M1, the amplitude Y of the carrier is such that it reaches the white level, whereas this does not occur during the modulation periods represented at Ma. Since the average intensity of the carrier varies in accordance with background-illumination variations, as well as iading, different field strengths oi the signal, and the like, there may be no fixed field level throughout the signal which is representative of a given shade value. However, 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 field-retrace periods. as indicated at Bl in Fig. 2, has different values for the different signal intensities in the signal portions A and B and is independent of light-modulation components. This blanking level, therefore, aiiords a measure .01 the intensity of the carrier wave which may be utilized automatically to control certain characteristics oi the receiver for the purpose of setting, or stabilizing with respect to, the black level in the signal to be reproduced.

In the operation of the present invention, therefore, the modulated-carrier wave is detected by the detector IE to derive a voltage wave corresponding to one-half the modulation envelope of Fig. 2, this voltage being impressed on the input circuit 01, and amplified in, the amplifier l 8. The

steady component of the modulation signal is lost when the signal passes through the condenser 28 so that it centers itself about its zero axis and tends to have a wave. form such as is illustrated by the curves of Fig. 3. The diode 40 derives from the signal output of the amplifier It a unidirectional voltage equal to the peak voltage equal to the peak value of the signal on the black side 01' the zero axis, which voltage appears across the resistor 29 and is applied, as previously explained, positively to the control grid of the oathode-ray tube in opposition to the bias voltage applied thereto from the bridge circuit 31, 38 by way of the resistor 29. This positive voltage serves automatically to vary the bias of the constantially a direct one, sothat the unidirectional component of the signal is not lost, the signal may be sufficiently stabilized with respect to the synchronizing pulses without the use of the stabilizing tube 40. However, where a non-conductive coupling intervenes between the detector l5 and the control grid circuit, such as the condenser 28, the steady component is suppressed and the' modulation signal tends to center itself about the zero axis and, as stated above, to appear as il lustrated by the curvesof Fig. 3. In this case, therefore, it should be stabilized as explained.

It will be seen from the curves of Fig. 4 that, even with the signal stabilized with respect to the peaks of its synchronizing pulses, if, by reason of fading or the like, the average signal intensity independent of light modulation varies the level or amplitude in the signal which represents black or any given shade value correspondingly varies, so that signal components representing the same shade value have different amplitudes depending on the signal intensity variation.

The control circuit of the present invention operates to correct this condition. More particularly, the cathode of the cathode-ray tube I8 is normally, during trace periods, connected to ground by way of the tube 33 and the grid bias of the cathode-ray tube is originally adjusted, by means of the location of the grid lead on the resistor 38, so as to correspond to dull gray; that is, to provide a beam current suflicient to cause this shade to be reproduced by the tube. negative pulses applied to the control grid of the tube 33 from the line-frequency generator 3| render the tube 33 nonconductive only for the durations of the line-retrace periods, and during the remaining intervals, that is, while the current in picture-reproducing tube l'I exceeds the blanking value, it effectively by-passes or disables therectifier circuit 34, 35, 36. The circuit of the diode 34 is energized, therefore, only when these retrace pulses are applied to the tube 33, and during these periods the rectifier circuit 34, 35, 36 conducts the small cathode current, corresponding to gray. The value of the cathode current of the tube I1 during the retrace periods is dependent upon the blanking level of the signal which, as may be seen from Fig. 4, is higher for signals of greater intensity. In other words, the

diode 34 and its load circuit are connected to the picture-reproducing device I! to develop a voltage varying with the current therein and serve to develop a bias voltage varying with the blanking level during the retrace periods and apply it positively to the control grid of the tube 39 which, with its associated auxiliary bridge circuit, operates as a direct current reversing amplifier. The space current of the tube 39 flows through the resistor 38 so that the potentials of the intermediate points thereof vary oppositely with variations of the biasvoltage output of rectifier 34, 35, 36. Therefore, the unidirectional voltages at the point on the resistor 38 where the anode of the tube 39 is connected and, hence, at the point where the lead to the control grid of the cathode-ray tube I1 is connected, shift negatively in accordance with any increase in the amplitude of the blanking or black level of the signal. This bias voltage is or proper polarity to provide a variable bias on the control grid 2i which serves to adjust the point of operation of the signal input on the response characteristic of the tube l1 and thereby maintain the signal level corresponding to black substantially at the cutoff point 01' the tube characteristic at all times, that is, to maintain the current in tube II at the predetermined value corresponding to the black shade level upon the occurrence of the recurring blanking level in the signal.

Thus, the signal as effectively applied to the control grid 2| has a wave form such as illustrated in Fig. 5, where, it will be seen, the blanking level BI is the same regardless of variations in the signal intensity. Preferably this level is, as stated above, at the cutoff point on the inputsignal'brightness characteristic of the reproducing tube. In other words, the net result of the control is to reduce the cathode current of the cathode-ray tube nearly to zero for a signal level corresponding to black. While it cannot be reduced entirely to zero since the operation of the The.

control requires some cathode current, itdoes, however, closely approach the ideal of having the signal level corresponding ,to black substantially at the cutoff point. Thus the image is recon-' ance with the present invention.

It is to be noted that, while the invention has been described above in connection with a receiver adapted for a negatively modulated signal, the invention is equally applicable in connection with a receiver adapted for a positively modulated signal, as will be readily appreciated by those skilled in the art.

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 to receive a signal containing synchronizing components and video-frequency components comprising, a picture-reproducing device, means for deriving from the received signal and applying to said device a signal having a periodically recurring level corresponding to a predetermined shade value, means connected in the circuit with said device for developing a voltage varying with the current therein, an auxiliary circuit, means for applying said developed voltage to said auxiliary circuit, means responsive to said synchronizing components for disabling said voltage-developing means whenever the current in said device exceeds a predetermined value corresponding to said predetermined shade value, and means including said auxiliary circuit for superimposing on said signal applied to said device a bias for maintaining the current-in said device at said predetermined value upon the occurrence of said recurring level in said signal.

2. A television receiver adapted to receive a sig- 'nal containing synchronizing components and video-frequency components comprising, a picture-reproducing device, means for deriving from the received signal and applying to said device a signal having a periodically recurring level corresponding to the value of black, means'connected in the circuit with said device for developing a voltage varying with the current therein, an auxiliary circuit, means for applying said developed voltage to said auxiliary circuit, means responsive to said synchronizing components for disabling said voltage-developing means whenever the current in said device exceeds a predetermined value corresponding to said black value, and means including said auxiliary circuit for superimposing on said signal applied to said device a bias for maintainng the current in said device at said predetermined value upon the occurrence of said recurring level in said signal.

3. A television receiver adapted to receive a modulation signal containing synchronizing components and video -frequency components comprising, a picture-reproducing device, means for deriving from the received signal and applying to said device a signal having a periodically recurring level corresponding to a predetermined shade value, means connected in circuit with said device for developing a voltage varying with the current therein, an auxiliary circuit including a resistance network, means for producing a unidirectional current in saidnetwork and a unilaterally-conductive device coupled between points of said network normally effective to produce a current therethrough, means for applying said developed voltage to said auxiliary circuit through said unilaterallyconductive device toeflfect in at least a portion 1 of said network a variation in said unidirectional current to produce in at least a part oi said network a current in opposition to said first current and varying in accordance with variations in said level of said signal from a predetermined level, means responsive to said synchronizing components or rendering said unilaterally-conductive device nonconductive whenever the current in said device should exceed a predetermined value co responding to said shade value, and voltage-utilization means for controlling'said device to stabilize said modulation signal with respect to said level and connected to a point on said network the potential of which is varied by said opposing current in accordance with the amplitude thereoi'.

4. A television receiver adapted to receive a signal containing synchronizing components and video-frequency components comprising, a picture-reproducing device, means for deriving from the received signal and for applying to said device a signal having a periodically recurring level corresponding to a predetermined shade value, means connected in circuit with said device i'or developing a voltage varying with the current therein, an auxiliary circuit including a bridge circuit the arms of which comprise a source or unidirectional voltage and a resistor rent therethrough in accordance with variations through which current normally flows, a vacuum- 0 in said level of said signal to produce in at least a part of the resistance arm oi said bridge a current in opposition to said first current and proportioned to variations of said level, means responsive to said synchronizing components for disabling said last-mentioned means whenever the current in said device exceeds a predetermined value corresponding to said shade value, and means including said auxiliary circuit for controlling said device to stabilize said modulation signal with respect to said level and con-' nected to a point on said resistor the potential of which is varied by said opposing current in accordance with the amplitude thereof.

5. A television receiver adapted to receive a signal containing synchronizing components and video-frequency components comprising a picture-reproducing device having a predetermined cutoff characteristic, means for deriving from the received signal and applying to said device a signal having a periodically recurring level corresponding to a predetermined shade value, means connected in the circuit with said device for developing a voltage varying with the current therein, an auxiliary circuit, means for applying said developed voltage to said auxiliary circuit,

'means responsive to said synchronizing compo- HAROLD A. WEEEIER.

Images