US2178736A - Television apparatus - Google Patents

Description

Nov. 7, 1 939. R. L. CAMPBELL TELEVISION APPARATUS Filed July 27, 1935 VEH'I'ICHL 1 3 72 HORIZONTAL g 70 snw- TOOTH H I E k.61

IN VENTOR Q Richard L; Campbell a'r onus? Patented Nov. 7, i939 Unitas sT Tas want i arrears TELEVISION arrana'rus l Richard L. Campbell, Philadelphia, lPa., assignor to Radio Corporation of America, a corporation of Delaware My invention relates to the transmission and reception of pictures and particularly to a method of and means for obtaining proper synchronization between scanning at the transmitter and scanning at the receiver.

Specifically, my invention relates to a television system in which, at the transmitter, a horizontal synchronizing impulse is transmitted at the end of each scanning line and a vertical synchronizing or framing impulse is transmitted at the end of each picture frame, each framing impulse be ing of longer duration than a horizontal synchronizing impulse. A system of this general type is disclosed in French Patent No. 742,671, published March 14, 1933.

An object of my invention is to provide an improved method of and means for synchronizing the scanning at the transmitter and receiver of a television system.

A further object of my invention is to provide an improved receiver for use in a system of the above-mentioned type.

A still further object of my invention is to provide means for isolating, at the receiver, the horizontal synchronizing impulses and the framing impulses.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawing, in which the single figure is a circuit diagram of a television receiver embodying my invention.

\ Referring to the drawing, the picture receiving apparatus comprises a radio receiver l which supplies picture and synchronizing signals through amplifiers 3 and 5 to a cathode ray receiver tube l. The cathode-ray tube l is of well known construction and includes an electron gun comprising a cathode t, a control grid H, and a first anode it. The tube is also provided with a second anode l5 and a fluorescent screen ll. Suitable deflecting devices, such as deflecting coils l9 and 2 I are provided for deflecting the electron beam both vertically and horizontally for scanning the fluorescent screen l'l.

It will be noted that both the picture signals and the synchronizing signals are impressed upon the control grid ll of the cathode-ray tube. The

presence of the synchronizing signals in the control grid circuit is not objectionable, and, in fact, is desirable since by giving them a negative polarity they are made to block the electron beam during the return line periods.

The signals appearing in the output circuit of the amplifier tube 3 have been generated at the transmitter in accordance with the teachings of the above-mentioned French patent. An important characteristic of these signals is that the framing impulse is of considerably greater dura tion than the horizontal synchronizing impulses.

Also, the framing impulses are of substantially the same amplitude as and occur at a much lower frequency than the horizontal synchronizing impulses. For example, in the system for which the receiver shown in the drawing is designed, the framing impulses occur at the rate of 24 per second, while the horizontal synchronizing impulses occur at the rate of 2880 per second.

The above-described signals appearing in the output circuit of the amplifier 3 are impressed upon the input circuit of an electric discharge tube 29 which is so adjusted that it separates most of the picture signals from the framing and horizontal synchronizing impulses. In the circuit illustrated, this separation is accomplished by biasing the tube close to the cut-oil. point. Since thesynchronizing impulses are impressed upon the input circuit of the separating tube 29 with a positive polarity, they appear in the tube output circuit while the more negative picture signals or not passed by the tube.

Referring now to the deflecting circuits which are to be controlled by the synchronizing im-- pulses, the vertical deflecting circuit includes an oscillator 3i and a saw-tooth current generator It will be understood that the oscillator it may be of any suitable type such as the dynatron illustrated. The oscillator 3! comprises an electric discharge tube having a cathode 371, a control grid as, a screen grid it and a plate 63. The plate at is supplied through a choke coil 65 with a suitable positive potential from a voltage divider ill. The screen grid d! is supplied with a higher positive potential from the voltage divider M. The control grid tit is connected through two resistors 58 and 55 connected in series to a point on the voltage divider 5?. which point is positive with respect to the cathode ti. It may be noted that so far as my invention is concerned. the control grid it of th dynatron may be at any bias potential which permits the dynatron to oscillate weakly enough to permit pulling it into synchronism. The theory of operation of a dynatraton oscillator is well known and need not be explained here. The oscillator is adjusted to oscillate at a frequency slightly lower than the frequency of the i'raming impulses whereby the impulses may pull the oscillator into synchronism with the vertical deflection at the transmitter.

The impulses appearing in the output circuit of the dynatron oscillator 3| are impressed upon circuit is a fllter or integrating circuit which consists of a series resistor 68 and a shunt condenser The output circuit of the separating tube 28 is coupled through a coupling condenser 5'! and the series resistor 53 to the junction point of the two resistors 49 and M. The shunt condenser 55 and the resistor 5!" are connected in parallel whereby the grid circuit of the dynatron oscillator has impressed thereon the voltage which appears across the condenser 55. In the grid circuit, the resistor 59 functions to prevent the control grid from becoming too positive, while the resistor 5| functions both as a coupling resistor and as a grid leak resistor.

The horizontal deflecting circuit is similar to the vertical deflecting circuit and comprises a dynatron oscillator 59 and a saw-tooth current generator ii. The oscillator 59 is adjusted to oscillate at a frequency slightly lower than the 20 frequency of the horizontal synchronizing im pulses, whereby it may be pulled into synchronism with the horizontal deflection at the transmitter.

The horizontal synchronizing impulses are impressed upon the grid circuit of the oscillator 59 substantially to the exclusion of the framing impulses. This is accomplished by providing a filter circuit consisting of a condenser 63-having small capacity which is connected in series with a high impedance resistor 65. The output circuit of the separating tube 29 is connected through the coupling condenser 51 to one terminal of the condenser-resistor combination 63-65, while the other end of the condenser-resistor combination 6$65 is connected to ground whereby the comparatively high frequency synchronizing impulses will pass through the condenser 63 and the comparatively low frequency framing impulses will be excluded. The grid circuit of the oscillator 59 is coupled across the resistor 65 by means of a coupling condenser ill to receive the horizontal synchronizing impulses. The condenser 10 and the dynatron grid resistor 12 form a second filter section which may be omitted, if desired.

The characteristic of the vertical separating .circuit is that it passes the comparatively low frequency components of the synchronizing impulses and excludes the higher frequency com.- ponents, while the characteristic of the horizontal separating circuit is that it passes these higher frequency components which are excluded by the vertical circuit and excludes the lower frequency components. these characteristics substantially isolate the framing impulses and the horizontal synchronizing impulses, since impulses having the general character shown in the above-mentioned French patent are characterized by the fact that most of the energy in the framing impulses appears ina frequency band below a certain frequency (in general, the horizontal synchronizing frequency), while most of the energy in the horizontal synchronizing impulses appears in a frequency band above this frequency.

It has been found that for the best operation of a television receiver the framing impulses and the horizontal-synchronizing impulses should be isolated. By following this procedure the fram-.

ing impulses cannot produce undesirable transient effects in the horizontal deflecting circuit, and likewise, the horizontal synchronizing impulses cannot produce similar efiects in the vertical deflecting circuit.

Specific values in ohms, megohms. and micro- Separating circuits which have.

farads have been indicated on the drawing, but it is to be understood that there may be a wide departure from these values in practicing my invention.

The saw-tooth current generators indicated in Figs. 1 and 2 may be of any suitable type, such as those disclosed in French Patent No. 738,475, published December 26, 1932. It will be understood, of course, that if the cathode-ray tube is provided with deflecting plates instead of deflecting coils, suitable saw-tooth voltage generators will be provided;

I claim as my invention:

1. A television receiver comprising means for intercepting picture signals accompanied by horizontal and vertical synchronizing impulses, the vertical synchronizing impulses being of longer duration than, and of substantially the same amplitude as, the first impulses and occurring at a substantially lower frequency, a cathode-ray tube, horizontal and vertical deflecting circuits therefor, means comprising an electric discharge tube for separating said impulses from said picture signals, said tube having an input circuit and an output circuit, means for impressing said impulses upon said input circuit, a resistor and a capacitor connected in series to form a horizontal separating circuit, a resistor and a capacitor connected in series to form a vertical separating circuit, said two separating circuits being connected in parallel across said output circuit, said horizontal deflecting circuit having its input circuit connected across the resistor of said horizontal separating circuit, and said vertical defleeting circuit having its input circuit connected across'the capacitor of said vertical separating circuit, the capacity of the capacitor in said horizontal separating circuit being so small that the horizontal synchronizing impulses are supplied to the horizontal deflecting circuit substantially to the exclusion of said vertical synchronizing impulses.

2. In a television receiver wherein a composite signal is received, said signal comprising picture signals and periodically recurring synchronizing signals to said synchronizing signals being of greater amplitude than picture signals of like polarity, a cathode ray tube having deflectin means associated therewith for deflecting the cathode ray, a picture signal channel through which said composite signal is supplied to said cathode ray tube, a synchronizing signal channel having its input circuit coupled to said picture channel and having its output circuit coupled to said deflecting means, an electric discharge tube having a cathode, a control grid and a plate and being located at a point in said synchronizing channel where said synchronizing signals appear on said control grid with a negative polarity, two resistor sections connected in series between said control grid and said cathode, the signals in said synchronizing channel being impressed

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