US2265090A - Television receiver - Google Patents

Description

Dec. 2, 1941. J. VAN DER MARK 2,265,090

TELEVIS ION RECEIVER Filed March 2, 1958 lNVENTOR JAN (/4 A505 MAR/ ATTORNEY Patented Dec. 2, 1941 UNITED STATES PATENT OFFICE TELEVISION RECEIVER poration of Delaware Application March 2, 1938, Serial No. 193,403 In Germany March 8, 1937 (C1. PIS-7.7)

1 Claim.

This invention relates to a television receiving circuit and particularly to the part thereof which serves for synchronizing the scanning movement in the receiver with the transmitter to be received.

For the purpose of synchronizing the transmitter and the receiver it has previously been proposed to transmit jointly with the picture currents line and frame synchronizing impulses, the first-mentioned impulses being produced in the transmitter at the end of the scanning of a picture line, the latter at the end of the scanning of an entire picture and usually the duration of the picture impulses including the scanning time of a few picture lines.

In a television receiver arranged for the reception of the synchronizing impulses transmitted in this manner jointly with the picture currents, the line and picture impulses respectively are supplied to a device which controls the scanning movement in a direction parallel and perpendicular respectively to the picture lines.

If a cathode ray tube is used as the reproduction device each of the scanning control devices will generally comprise a generator, referred to hereinafter as a sawtooth generator, for the production of a sawtooth shaped current or voltage and, in addition, one or more deflecting means interconnected via a coupling element that may contain an amplifier.

In order that the curved shape of the current or the voltage produced by the sawtooth generator may be retained, the coupling element which connects the corresponding sawtooth generator to the deflecting means is proportioned in practice in each of the scanning devices in such manner that the basic frequency and all the higher frequencies are amplified without distortion. The basic frequency is to be understood to mean here the frequency of the current or the voltage produced by the sawtooth generator. This means that the lowest frequency that is let through by the coupling element that forms part of the device referred to hereinafter as the horizontal scanning device that controls the scanning movement in a'direction parallel to the picture lines is the line frequency i. e. the number of lines scanned per second. The horizontal scanning device is controlled by the received line impulses, a few line impulses being, however, suppressed during the occurrence of a picture impulse at the end of the scanning of each entire picture.

The lowest frequency that is let through by the coupling element that forms part of the device referred to hereinafter as the vertical scanning device that controls the scanning movement in a direction perpendicular to the picture lines, is the picture frequency i. e. the number of pictures scanned per second. The vertical scanning device is controlled by the received picture impulses.

It has been found that in the use of a television receiving circuit of the kind above indicated the first picture lines of the reproduced picture are often laterally displaced.

According to the invention, this disadvantage is obviated by the coupling element that forms part of the horizontal scanning device being so dimensioned that the time constant of this element is of the order of magnitude of the periodic time of the sawtooth shaped current or voltage produced in the vertical scanning device.

In order that the invention may be clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing, in which Fig. 1 shows one form of construction of a television receiver according to the invention,

Fig. 2 is one form of construction of a coupling element that may be used in the circuit according to the invention,

The operation of the circuit according to the invention will be described in greater detail by reference to the curves of Figs. 3a, 3b, 3c and 311.

In the television receiver according to Fig. 1, the modulated high frequency oscillations received in an antenna I are supplied to a mixing tube 2 which is also devised for the production of local oscillations or is coupled to a local oscillator 3. The oscillations occurring in the output circuit of the mixng tube 2 are supplied via an intermediate frequency amplifier 4 to a detector 5 and the picture currents and synchronizing impulses obtained after detection are fed, in some cases via a low frequency amplifier 6, to the control electrode of a cathode ray tube 1, the received picture currents controlling the intensity of the cathode ray beam generated in the tube '1 and the cathode ray beam being suppressed by the received synchronizing impulses, or as is commonly termed, the return line is eliminated.

In order that the movement of the cathode ray beam in the tube 1 may be performed in synchronism with the scanning movement in the transmitter to be received, the synchronizing impulses and picture currents obtained after detection are fed, via a device 8 for separating picture currents and synchronizing impulses, to the scanning devices. If desired, the conductor which connects the device 8 to the vertical scanning device may include a filter 9 which lets through the vertical synchronizing impulses only.

The horizontal scanning device comprises a generator ID for the production of a sawtooth shaped voltage the frequency of which is equal to the line frequency. The vertical scanning device comprises a sawtooth generator H which produces a voltage the frequency of which equals the picture frequency. We will not enter into details of the arrangement of such a sawtooth generator since the generator per se does not fall within the scope of the invention. The scanning devices comprise in addition a pair of deflecting plates M and I respectively which are connected via a coupling element l2 and 3 respectively to the associated sawtooth generator Ill and II respectively. evident that magnetic deflection may be used, if desired.

The coupling element shown in Fig. 2, which is frequently used in the television receiver described, comprises a resistance 2|! which has supplied to it the sawtooth shaped voltage produced by the generator It or H and has connected in parallel with it the series combination of a condenser 2| and a resistance 22. The sawtooth shaped alternating voltage which is set up across the resistance 22 is supplied, in some cases via an amplifier (not shown), to the deflecting plates M or IE. It is also possible for the voltage produced in the generator IE or H to be supplied to the resistance 20 via one or more amplifier valves.

In the well known television receivers whose construction corresponds essentially to the diagrammatic circuit shown in Fig. 1 it is common practice to dimension th time constant of the coupling element l2 in such manner that the line frequency produced by the generator IO and all the higher frequencies are let through, whereas, the time constant of the coupling element I3 is such that this coupling element lets through the picture frequency and all the higher frequencies.

As already observed, such an adjustment presents a disadvantage in that the first scanning lines of the reproduced picture are laterally displaced. The invention is based on the recognition of the fact that this displacement is due to the suppression of the line impulses during the reception of a picture impulse, since when the line impulses are suppressed, the sawtooth generator It! will produce a sawtooth shaped voltage whose frequency is governed by the natural frequency of the generator and differs slightly from the frequency that is set up during the reception of line impulses. The adjustment of the sawtooth generator is such that in the absence of line impulses the frequency of the oscillations to be generated and also the amplitude of these oscillations increases, which may be readily seen when it is realized that the operation of a sawtooth generator is frequently based on a sudden discharge or charge of a condenser charged or discharged via a resistance and this discharge or charge ensues under the influence of a received line impulse. In the absence of the line impulses It is, however, clearly the condenser charge or discharge does not ensue until at a later time and in this case the condenser is charged to a higher voltage or discharged to a lower voltage. Thus, at the end of the scanning of each entire picture the sawtooth generator ID of the horizontal scanning device produces a voltage which as a function of time has a variation of the kind shown in Fig. 3a of the drawing, in which the curve shown represents the voltage set up by the generator |0 during the last two scanning lines, during the reception of a frame synchronizing impulse and during the first eight lines of the following picture.

When this voltage is supplied to the resistance 20 of a coupling element of the kind shown in Fig. 2 which lets through the line frequency and all the higher frequencies, it is found that across the resistance 22 of the coupling element is set up a voltage whose variation as a function of time is shown in Fig. 3b and results in the above described lateral displacement of the picture lines. Across the condenser 2| is set up a voltage whose variation is shown in Fig. 30. This phenomenon is due to the fact that the voltage supplied to the resistance 20 contains a low frequency component whose variation is shown in dotted lines in Fig. 3d and whose basic frequency corresponds to the picture frequency.

According to the invention, the time constant of the coupling element is dimensioned in such manner that even the picture frequency is let through without distortion. In this case the frequency referred to occurs across the resistance 22 so that the voltage across the condenser 2| is constant and the lateral displacement is avoided. The voltage occuring at the output terminals of the coupling element is shown in full lines in Fig. 3d.

Referring to Fig. 3b and 30, it has been assumed for simplicity that the charge and discharge of the condenser 2| has a linear course. In fact, however, the course of the Voltage across the condenser 2| differs slightly from the linear shape.

What I claim is:

In a television receiver wherein video, line synchronizing, and frame synchronizing signals are received, means for separating said signals, a cathode ray reproducing tube for reproducing the televised image, said tube including means for generating and accelerating an electron beam, means for deflecting the electron stream within said tube, an oscillator for providing a sawtooth wave substantially at the line frequency, means for impressing said line and frame synchronizing signals onto said line sawtooth wave generator the amplitude of said wave being greater during the receipt of frame synchronizing signals, and a circuit comprising a pair of resistive members joined through a condenser coupling said sawtooth wave generator to the line deflecting means within said cathode ray tube, said coupling circuit having a time constant substantially equal to the period of the frame synchronizing signals.

JAN VAN DER MARK.

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