US2208374A - Television receiving system - Google Patents

Description

July 16, 1940. H. M. LEWIS TELEVISION RECEIVING `SYSTEM Filed Aug. 28, 1937- llll S la M. LE ff ATTORNEY Patented July 16, 1940 PATENT OFFICE TELEVISION RECEIVING SYSTEM Haroldl M.- Lewis, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Dela- Application August 2s. 1937. serial No. 161,424

4 Claims.

'Ihis invention relates to television receiving systems and, more particularly, to such systems including provisions for controlling the `background illumination of the reproduced scene.

In accordance with present television practice, a transmitted signal comprises pictureor video-frequency components representative of varying light values in successive areas of an image being transmitted and synchronizing-frequency components which correspond to initiations of successive lines and frames in the scanning of the image. The latter components ordinarily have amplitude values outside of the amplitude range of the video-frequency components.

Il There is usually developed at the transmitter a unidirectional voltage which corresponds to the average background illumination of the scene, but this voltage is lost when the signal is passed through the alternating current amplifiers of the transmitter. However, the signal as received includes a component from which the unidirectional voltage representative of background illumination may be reproduced.

A signal-reproducing device, such as a cathode-ray tube, is employed at the receiver and the cathode ray is deected therein to scan, in series of parallel lines, a iluorescent screen upon which the image of the transmitted scene is reproduced. The synchronizing-frequency components of the received signal are utilized to control the scanning apparatus so as to synchronize its operation with that of similar apparatus utilized at the transmitter in developing the signal. The intensity of the cathode ray is controlled by the pictureor video-frequency components of the signal, as Well as by the unidirectional background illumination voltage developed in the receiver, thereby to reproduce the scene.

Essential functions which must be performed in a television receiving system of the type described, therefore, include deriving from the signal separate line-frequency and frame-frequency synchronizing impulses and a unidirectional bias voltage representative of the average background o illumination. Various methods have heretofore been devised for performing these functions, but, in general, they have involved relatively complicated and/or unstable apparatus, or have been 50 otherwise unsatisfactory.

It is an object of the present invention to provide, in a cathode-ray tube television signal-reproducing system Wherein the received signal includes combined pictureor video-frequency components, background illumination components,

and synchronizing components having amplitude values outside the amplitude range of the picture components, improved means for deriving from the received signal the synchronizing impulses and a unidirectional control-bias voltage repre- 5 sentative of the background illumination.

In accordance with the invention, a television signal-receiving system for reproducing a signal including combined picture components and synchronizing components having amplitude values 10 outside the amplitude range of the picture components, the background illumination being represented by the peak value of the signal on one side of its zero axis, comprises a source of bias voltage and a signal-translating stage having an 15 input circuit to which the signal is applied. A rectifier, preferably a diode, is coupledldrectly to the input circuit for deriving from the signal a bias voltage proportional to the peak value of the signal representing the background illu- 20 mination. There is inclu-ded in the circuit of the rectifier impedance means for deriving from the signal impulses corresponding to the synchronizing components. The system also includes means for utilizing the derived impulses for syn- 25 chronizing the scanning operation of the system and means respective jointly to the two bias voltages for controlling the background illumination of the signal translated by the stage.

For a better understanding of the invention,' 30 together with other and further objects thereof, reference is had to the following description taken in Aconnection with the accompanying drawing, and its scope will be pointed out in the appended claims. 35

In the accompanying drawing, Fig. 1 is a circuit diagram, partially schematic, of a cathode-ray tube television reproducing system including circuits embodying the present invention and Fig.

2 is a group of curves representing certain op- 40 erating characteristics of the system of Fig. 1 to aid in the understanding of the invention.

Referring now more particularly to the drawing, the system there illustrated comprises a receiver of the superheterodyne type including an antenna and ground system Ill--II connected to a radio-frequency amplier I2 to which is connected in cascade, in the order named, an oscillator-modulator I3, an intermediate-frequency namplifier I4, a detector and A. V. C. supply I5, a. video-frequency amplifier I6, and a cathode-ray signal-reproducing tube I1, constituting the main signal channel of the receiver. The elements or components I 0-I6 may all be of conventional wellknown construction so that detailedillustrations and descriptions thereof are deemed unnecessary herein.

The cathode-ray tube I1 comprises `the usual envelope lI8 containing, in the order named, a cathode 20, a main or video control grid 2l, a screen 22, a rst anode 23, a second anode 24', andthe usual fluorescent screen 25 at the end of the tube. Line-frequency and picture-frequency electromagnetic deecting or scanning coils 26 and 2`|are disposed about the tube for deflecting the beam in two directions normal to each other. The output of the video-frequency amplier is connected to the control grid 2l by Way of a coupling condenser 28 and leak resistor 29 for controlling the intensity'oi the cathoderay in accordance with the video-signal components, as in usual practice. Operating potentials are applied to the electrodes of the tube I'| from a suitable source, such as thebattery 30. The

unidirectional bias voltage developed by ther A. V. C. supply is applied by suitable leads to one or more of the control grids of the tubes in the preceding stages of the system in the conventional manner.

For developing saw-tooth current waves for the coils 26 and 21 to eiect scanning of the target 25 by the cathode ray,` there are provided a linefrequency generatorl and a frame-frequency generator 3 I The output of the video-frequency amplifier I 6 is coupled to these generators, for maintaining them in proper synchronisrn with the scanning at the transmitter, bypmeans of rectifying apparatus indicated generally at 32, a synchronizing impulse amplifier 33, and a synchronizing impulse separating means indicate-d generally at 3d. The apparatus indicated at 32, 33, and 33 embodies, or is directly associated with, the present invention and will be hereinafter described in detail. The generators 3| and 3|' may be of any well-known or suitable design, such, for instance, as that described in United States LettersPatent No. 2,052,184, granted August 25. 1936. y

Since the system thus far described is, in general, conventional and well understood in the art, a detailed explanation of its general operation is deemed unnecessary. Briefly, however, signals intercepted by the antenna circuit |0|I are selected and amplified in the radio-frequency amplier I2 and supplied to the oscillator-modulator I3, wherein they are converted to intermediate-frequency signals which. in turn. rare selectively amplified in the intermediate-frequency amplifier I8 and delivered to the detector I5. The modulation components of the signal are derived by the detector I5 and are supplied to the video-frequency amplifier. i6, wherein they are amplied and from which they are supplied to the control grid 2| of the cathode-ray tube. The A. V. C. bias voltage developed by the detectox and A. V. C supply I5 andappled to the tubes in the preceding stages of the system serves to maintain the signal amplitude at the output of the amplier I4 within a yrelatively narrowr range, or substantially constant for a wide ranger of received signal intensities.

With proper. operating potentials supplied to the electrodes of the tube Il, an electron beam is emitted from the cathode 20 and its intensity is modulated or controlled by the grid 2| in accordance with the video-frequency voltages irnpressed thereon. The screen 22 serves to control the average intensity of the beam, While the first and second anodes 24 and 25 co-operate to `the well-known rectilinear scanning pattern on the fluorescent screen 25 to reproduce the transmitted scene.rr

' The generators 3i and 3|" are synchronized with the corresponding scanning apparatus at the transmitter by the synchronizingimpulses appearing in the output circuit of the ampliijer I6, theseimpulses being derived by theA apparatus 32, 33, and 3d embodying the present invention. A unidirectional bias voltage representative of the background illumination is also developed by this apparatus and applied to the control grid 2| to control the average illumination of the reproduced scene.

mferring now more particularly to the part of the system embodying the present invention,the rectifying means 32 serves to develop from the signal both the background illumination control unidirectional bias voltage and the synchronizing impulses. The rectifying means 32 comprises a diode 36 connected across the output circuit of the video-frequency amplified I6 through an impedance, which may comprise resistance means only, but preferably comprises a resistor 3T and inductance 38 in series, as shown, and a source of biasing potential, indicated by the battery 39. Theelectrical values of the condenser 28 and resistor 29 are very large and provide a large time constant for the rectifier circuit. The battery 39 provides an initial negative bias for the control grid of the cathode-ray tube to ensure proper operation thereof in the absence of a signal, while the impedance elements 31, 38 provide a load or outputy circuit `across which are developed synchronizing impulse voltages free from the picture-frequency voltages, as will bepresently further explained.

The amplifier 33` comprises a 'conventional pentode tube 5U having its input circuit connected across the impedance elements 31 and 38 'by Way of coupling condenser Il and leak resistor d2, as shown. The output circuit of this tube includes a blocking resistor 83, a coupling sistor 4T and condenser 48. .The inputcircuits of the frameand line-frequency generators 3|' and 3| are connected across the condenser 48 and resistor 46, respectively, to receiver the synchronizing impulses. Operating potentials are applied to the electrodes of the tube 40 from suitable sources, such as batteries 40a. Preferably the scanninggenerators are designed to operate without synchronization at frequencies slightly lower `than the desired operating frequencies, so o that they may be easily synchronized and will be unaffected by impulses which occur at irnproper times. f

The operation of the system in accordance with the present invention may best be explained with reference to the curves of Fig. 2. In these curves. which represent the .wave forms of particular signals developed at different points in the system, only a few line-frequency cycles are indicated, the broken lines indicating omissions. Curves W, G, and B represent the signal voltage which, in the absence of the background control embodying the invention, would appear between the grid 2l and cathode 20 of the cathoderay tube l'l4 for: a white background having a vertical black bar; a grey background having two vertical bars, one white and one black; and a i black background having a vertical white bar, respectively, each line including a sudden change from positive to negative to represent a sudden change from white to black, or vice versa. At thel end of each line period there is a negative `impulse or pedestal, as indicated at N, occurring during the line retrace or so-called block-out vscanning period of the cathode ray. A synchronizing impulse I occurs at the initiation of each retrace period. Thus, the picture components are combined with the synchronizing components and the latter have amplitude values which are outside of the amplitude range of the picture components. The signal represented by the curves W, G, B, does not include a unidirectional voltage component representing changes in background illumination, since the alternating current wave`centers itself about its axis and the unidirectional voltage is lost when the signal is 'passed through alternating current amplifier stages of the system. However, this signal does include a component representative of the background illumination, this component being the peak value of the signal on the side of its axis representing black.

Curve A of Fig. 2 illustrates the grid voltage (eg)-vs.cathoderay current (im) characteristic of the reproducing tube I1, it being apparent that there is a substantially linear relation between the cathode-ray current and its controlgrid voltage over the range of operation.'` -It is assumed that the luminosity of the fluorescent screen is proportional to the current in the ray and that point b, near cutoi, represents the bias voltage (eg) required for black, the point g, that required for gray, and the point w, near zero bias, that required for white. For maximum sensitivity, the video modulation voltage is applied to the grid with suiiicient amplitude to control the cathode-ray current between the limits b, w, thereby to produce the variation from black to white. It `will be obvious that a unidirectional control voltage is required for establishing the bias at the points w, g, and b for white, grey, and black subjects, respectively, in order that proper background illumination of the reproduced image may be obtained. That is, the signal when applied to the control grid 2l should be of the form indicated at W1, G1, B1. It will be seen that, with such a background illumination cornponent superimposed on the video signal, operation of the signal-translating stage comprising tube il is, at all times, automatically maintained over substantially the entire useful portion of the tube characteristic.

For this purpose, the control grid 21 is ini-l nal.

Curve B of Fig. 2 illustrates the characteristios of the rectifier 36, the abscissae representing the applied alternative voltage (ed) and the ordinates representing the load current (is). It will be seen that the diode passes current only during the occurrence of the synchronizing impulses, this being determined by the large time constant circuit comprising condenser 28 and resistor 29. Hence, during each of the synchronizing impulses, current flows through the impedance elements 31, 38 in series with the diode 36 and there is developed across these elements synchronizing impulses or voltages free from the picture components of the signal.

Assuming the Well-known type oi' synchronizing signal to be employed, commonly known as the serrated signal, the line impulses and frame impulses occur continuously and independently, the frame impulses being interrupted or serrated to provide intervals for the line impulses. The impedance network 34 included in the output circuit of the tube 40 is designed to effect the separation of these line and frame impulses. In accordance with the Well-known principles of differentiation, double impulses at the line, irequency and free from the frame impulses are developed across the resistor 46. These impulses tageous for the reception of signals in which this component istransmitted and retained throughout the receiving system including rthe videofrequency amplifiers. In this case utilization oi the invention permits the operation of the cathode-ray tube l1 with its cathode grounded, under which condition the direct current component is ordinarily lost due to the coupling condenser which must be employed between the last videofrequency amplier stage and the grid of the cathode-ray tube. In such an arrangement the present invention is utilized to restore the direct current component by means of the background control including the recter 3S, as described above. It is emphasized that the curves of Fig.

I 2 illustrate the necessity of having the direct current component present prior to both the application of the signal to the grid of the cathoderay tube and priorto the separation of the synchronizing signals from the video-frequency signals. The diode circuit arrangement of this invention is effective to insert the direct current component for both of these purposes and simultaneously accomplishes the separation of the amplitudes, they may beseparated by ampli-l tude responsive means, as, for example, a properly biased vacuum tube arrangement. f It will be clear to those skilled in the art that the arrangement may be simplified to omit amplier 33 and incorporate the circuits for separating the frame impulses from the line impulses directly in the circuit of the diode 35, that is, replacing the elements 3l, 38.

It will be seen from the above description that the present invention provides an emcient and relatively simple system whereby a single means serves simultaneously to develop from the detected signal a background illumination control-bias voltage as Well as synchronizing impulses.

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

What is claimed is:

1. A television signal-receiving system for reproducing a signal including combined picture components and synchronizing components having amplitude values outside the amplitude range of the picture components, the background illumination being represented by the peak value of the signal on one side of its yzero axis, comprising a source of bias voltage, a signal-translating stage having an inputcircuit to Which said signal is applied, a diode rectier coupled directly to said input circuit for deriving from said signal a bias voltage proportional to said peak value of said signal, impedance means in circuit with said rectiiier for deriving from said signal-impulses corresponding to said synchronizing components, means for utilizing said synchronizing impulses for synchronizing the scanning operation of the system, and means responsive jointly to saidbias voltages for controlling the background illumination of the'signal translated by said stage. e

2. A television signal-receiving system for reproducing a signal including combined picture components and synchronizing components having amplitude values outside the amplitude range of the picture components, the background illumination being represented by the peak value of the signal on one side of its zero axis, comprising a, source of bias voltage, a signal-translating` stage having an input circuit to which said signal is applied, a. rectifying means 'for deriving from said signal a bias voltage proportional to saidpeak value of said signal comprising a diode rectifier coupled directly to said input circuit, an input circuit including a large capacitance element and an output circuit including a high resistance element, impedance means in series with said rectifier for deriving from said signal impulses corresponding to said synchronizing components, means for utilizing said synchronizing impulses for synchronizing the scanning operation of the system, and means responsive to the resultant of said two bias voltages for controlling the background illumination of signal translated by said stage.

3. A television signal-receiving system for reproducing a signal including combined picture components and Asynchronizing components having amplitude values outside the amplitude range of the picture components, the background illumination being represented by the peak value of the signal on one side of its zero axis, comprising a source of bias voltage, a signal-translating stage having an input circuit to which said signal is applied, ra diode rectifier coupled directly to said input circuit for deriving from said signal a bias voltage proportional to said peak value of said signal, impedance means comprising a resistance and an inductance in series with said rectifier for deriving from said signal impulses corresponding to said synchronizing components, means for utilizing said synchronizing impulses for synchronizing the scanning operation of the system, and means responsive to the resultant of said tvvo bias voltages'for controlling the background illumination of the signal translated by said stage.

4. A television signal-receiving system for reproducing a signal including combined picture components, and synchronizing components having amplitude values outside the amplitude range of the picture components, the background illu- `Inination being represented by the peak value of the signal on the side of its axis corresponding to black, comprising a time constant circuit for translating said signal including a series connected condenser and resistor across Which-the composite signal is impressed, a signal-reproducing device having input electrodes coupled directly to said resistor, a circuit connected in parallel with said resistor including adiode rectifier for developing a, bias voltage across said resistor proportional to said peak value of said signal and an impedance element for deriving from said signal impulses corresponding to said synchronizing components separate from said picture components, means for utilizing the picture components impressed across said resistor for reproduc` ing 'a scene and for utilizing said bias voltage for controlling the background illumination thereof, and means for utilizing said separate synchronizing pulses for synchronizing the scanning operation of the syste HAROLD M. LEWIS.

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