US2291277A - Television signal-translating channel - Google Patents

Description

July 28, 1942.

IMAGE REPRODUCING DEVICE VIDEO- DETECTOR N. P. CASE TELEVISION SIGNAL TRANSLATING CHANNEL Filed Nov. 18, 1939 INTERMEDlgAJE' INTERMEDIATE WERE? iii Frequency "i '9 LL.

# Frequency INVENTOR NELSON P. CASE A'ITORNEY Patented July 28, 1942 TELEVISION SIGNAL-TBANSLATING CHANNEL Nelson P. Case, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation t Dela- Application November 18,1939, s ns] No. 305,049

4 Claims.

This invention relates to television signaltranslating channels adapted to translate a composite television signal including a video-modulated carrier wave and an adjacent audio-modulated carrier wave.

In accordance with present television practice, a transmitted composite television signal comprises a carrier wave modulated during successive intervals or trace periods by high-frequency and unidirectional components representative of light variations in an image being transmitted and of its background illumination, respectivelv, and modulated between the trace periods, that is, during retrace intervals, by synchronizing components which correspond to initiations of successive lines and fields in the scanning of the image. The transmitted composite signal of a conventional television transmitter comprises also an audiomodulated carrier wave adjacent the above-mentioned video-modulated carrier wave.

The above-mentioned composite television Signal is usually received by means of a receiver the first amplifying stages of which have a pass band sufficiently broad to transmit at least one modulation sideband of each of the above-mentioned carrier-frequency signals of the composi e signal. In this way only one tuning operation need be effected at the receiver in order to condition the receiver to receive both modulated signals. If the receiver is correctly tuned with reference to one of the carrier-frequency signals of the composite signal, tuning is also correct for the other of the carrier-frequency signals. Such receivers thus generally comprise selective circuits which have a response characteristic which is relatively I flat over a wide frequency range including both the video-modulated carrier Wave and the audiomodulated carrier wave to be received.

For television reproduction with high definition, it is necessary to transmit a relatively Wide range of video-frequency signals, for instance, signals of a range of the order of three and onehalf megacycles. However, the smaller cathoderay reproducing tubes used in less expensive television receivers are incapable of utilizing all of the informationwhich is conventionally transmitted by a modern television transmitting station; for instance, such tubes may have a resolving power corresponding to a band width of the order of only one megacycle. Obviously, in receivers utilizing such cathode-ray reproducing tubes, it is unnecessary to translate the entire modulation sideband of the received video-modulated signal. However, it is particularly desirable of distortion. Furthermore, it is desirable to efiect this high degree of amplification in as few am-,

I plifying stages as possible, in order to reduce the cost of the receiver to a minimum. Such results are typical of high-definition television receivers but have not been ordinarily'realizable in lowcost, low-definition receivers.

It is an object of the present invention, therefore, to provide an improved low-definition television receiver adapted to reproduce a compositetelevision signal including a video-modulated carrier wave and an adjacent audio-modulated carrier wave.

It is a further object of the invention to provide an improved low-definition television receiver of the type under discussion which embodies one or'more of the above-mentioned advantages of high-definition television receivers.

In accordance with the present invention, a television signal-translating channel adapted to translate a composite television signal including a video-modulated carrier wave and an adjacent audio-modulated carrier Wave comprises, a bandpass selector in the channel having a continuous pass band including the video-modulated carrier wave and the audio-modulated carrier wave. This band-pass selector includes tuned circuits considerably over-coupled to provide a response at the secondary tuned circuit which varies with frequency and is double-peaked With a substantial valley between the peaks. The reactive constants of the selector are so proportioned that one of the peaks coincides substantially with the carrier frequency of the video-modulated carrier wave to be translated and the other of the peaks coincides substantially with the carrier frequency of the audio-modulated carrier wave to be translated.

For a better understanding'of the invention, together with other and further objects thereof, reference is had 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, partly schematic, of a cathode-ray tube television receiving system including a signaltranslating channel embodying the present invention; while Figs. 2 and 3 are graphs illustrat ing certain of the operating characteristics of the that there should be a high degree of amplifica- 5 receiver of Fig. 1.

antenna system I0, connected ing, the system there illustrated comprises a receiver of the superheterodyne type including an aradio-frequency amplifier 12 to which are connected in cascade, in the order named, 'an oscillator-modulator l3, an intermediate frequency amplifier I4, and a television signal-translating channel I! embodying the present invention. The signaltranslating channel It is adapted to translate a composite intermediate-frequency television signal including a video-modulated carrier wave and an adjacent audi-modulated carrier wave in a manner more fullydescrlbed hereinafter. Connected in cascade to an output circuit of signal-translating channel It, in the order named, are-a video intermediate-frequency amplifier 18, a video-modulation detector 11, a video-frequency amplifier l8, and an image-reproducing device l3. Connected toanother output circuit of signal-translating channel II, in the order named, are a sound intermediate-frequency amplifier 20, an audio detector 2|, an audlo-frequencg amplifier 22, and an audio-reproducing device 23. d, v

The stages or units just described, with the exception of unit 15, may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are deemed to be unnecessary herein. Referring briefly, however, to the operation of the system described above, composite television signals including a videomodulated carrier wave and an adjacent audimodulated carrier wave intercepted by antenna circuit 10, are selected and amplified in radiofrequency amplifier l2 and supplied to the oscillator-modulator l3, wherein they are converted to a composite intermediate-frequency termediate-frequency amplifier ll havin an 'output circuit in which there is coupled a bandpass selector 3| having a continuous pass band including the intermediate-frequency video-modulated carrier wave and the audio-modulated carrier wave and which includes inductivelycoupled tuned circuits 33 and 34, which are considerably over-coupled to provide a response, at the secondary tuned circuit which varies with frequency and is double-peaked with a substantial valley between the peaks. A second. vacuumtube amplifier 32 is provided in signal-translating channel I 5 having input electrodes coupled to the signal which, in turn, is selectively amplified in the intermediate-frequency amplifier I4 and delivered to signal-translating channel l5, wherein the'signal is further amplified and the signals of the two types are separated in a manner which will be described more fully hereinafter. The video intermediate-frequency signals are further amplified in unit 16 and the video-modulation V components of the received television signal are derived by detector l1 and supplied to the videofrequency amplifier l3, wherein they are amplified and from which they are supplied to the control element of the image-reproducing device I 9.

It will be understood that suitable synchronizing circuits are necessary for image-reproduc ing device IB-and that such synchronizing circuits, which may be entirely conventional, have been omitted from the circuit of Fig. 1 in the interest of simplification. Detected synchronizing-signal components derived at detector l1 may be utilized to synchronize the operation oi imagereproducing device I! in a manner well understoodin the art.

The sound intermediate-frequency signals delivered by signal-translating channel l5 to sound output terminals of band-pass selector 31. 'Amplifier 32 includes an output electrode in circuit with which there isincluded a relatively broadly tuned selector circuit 35,31 having a pass band including the carrier frequency of the videomodulated carrier wave of the composite received signal. The tube 32 also includes a screen electrode in circuit with which there is included a relatively sharply-tuned selector 33,33 resonant at the carrier frequency of the audio-modulated carrier wave of the composite received signal. The selector circuit 35,31 is coupled to the input circuit of intermediate-frequency amplifier I6, while selector circuit 33,33 is coupled, to the input circuit of intermediate-frequency amplifier 20.

It will be understood that radio-frequency amplifier l2 and oscillator-modulator 13 may be tuned to any one of a plurality of composite television signals by means of any suitable tuning control such as that schematically shown by unicontrolled condensers 40,". v

Coming now to the operation of the circuit of Fig. 1, it will be assumed that band-pass selector 3| has a coupling substantially greater than the critical value so that it has fa marked doublepeaked frequency-response characteristic with a substantial valley between the peaks, such as that illustrated by curve A of Fig. 2 of the drawing. The reactive constants of selector 3| are so proportioned that one of the peaks of curve A coincides substantially with the frequency f: of

the intermediate-frequency video-modulated carrier wave while the other of the peaks coincides substantially with the frequency h of the intermediate-frequency audio-modulated carrier wave. Tuned selector 36, 38 is designed to have a sharply-peaked frequency-response characteristic, such as that illustrated by curve B of Fig. 3, while tuned selector 35, 31 is designed to have a frequency response characteristic sufiiciently broad to pass the rang of video-frequency signals to be utilized as represented by curve C of now to the portion of the system in- Fig. 3. Under the conditions assumed, it is seen that a substantial gain at each of the intermediate carrier frequencies f1 and fa is provided by the stage of amplification including tube 30 and selector circuit 3t. The video-modulation signal output or band-pass selector 3| is amplified in a conventional manner by vacuum tube 32 and selector 35, 31 to provide the intermediate-frequency input for amplifier l6. Amplification of the audio-modulation output of selector 3| is also provided due to the action of the screen.

circuit of tube 32, in which is included selector 38, 38. The tuned circuit 36, which is resonant at the intermediate audio-modulated carrier frequency, is also effective to act as a trap for the video-frequency channel of the receiver. In this manner, the two intermediate-frequency carrier waves are effectively separated for. separate utilization.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modiflcations 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 signal-translating channel adapted to translate a composite television signal including a video-modulated can'ier wave and an adjacent audio-modulated carrier wave comprising, a band-pass selector in said channel having a continuous passband including said video-modulated carrier wave and. said audiomodulated carrier wave, said band-pass selector including tuned circuits considerably over-coupled to provide a response at the secondary tuned circuit which varies with frequency and is double-peaked with a substantial valley between the peaks, the reactive constants of said selector being so proportioned that one of said peaks coincides substantially with the carrier'frequency of said video-modulated carrier wave and the other of said peaks coincides substantially with the carrier frequency of said audio-modulated carrier wave.

2. A television signal-translating channel adapted to translate a composite television signal including a video-modulated carrier wave and an pass selector including timed circuits considerably over-coupled to provide a response at the secondary tuned circuit which varies with frequency and is double-peaked with a substantial valley between the peaks, the reactive constants of said selector being so proportionedthat one of said peaks coincides substantially with the carrier frequency of said video-modulated carrier wave and the other of said peaks coincides substantially with the carrier frequency of said -audie-modulated carrier wave.

3. In a television receiver of the super-heterodyne type tunable to a plurality of frequency bands, an intermediate-frequency signal-translating channel adapted to translate a composite television signal including a video-modulated carrier wave and an adjacent audio-modulated carrier wave comprising, a band-pass selector in said hannel having a continuous pass band including said video-modulated carrier waveand said audio-modulated carrier wave, said bandpass selector including tuned circuits considerably over-coupled to provide a response at the secondary tuned circuit which varies with frequency and is double-peaked with a substantial valley between the peaks, the reactive constants of said selector being so proportioned that one of said peaks coincides substantially with the carrier frequency of said video-modulateda continuous pass band including said videomodulated carrier wave and said audio-modu-- lated carrier wave, said band-pass selector including tuned circuits considerably over-coupled to provide a response at the secondarytuned circuit which varies with frequency and is doublepeaked with a substantial" valley between the.

peaks, the reactive constants of said selector being so proportionedthat one of said peaks coincides substantiallywith th carrier frequency of said video-modulated carrier wave and the other of said peaks coincidesv substantially with the carrier frequency of said audio-modulated carrier wave, a broadly-tuned selector circuit coupled to said selector and having a pass band including the carrier frequency of said videomodulated carrier wave for selecting the videomodulatedcomponent of said translated composite signal, and a sharply-tuned selector cir-- cuit coupled to said selector and resonant at the carrier frequency of said audio-modulated car- 'rier wave for selecting the audio-modulated component of said translated composite si nal.

unison P. CASE.

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