US2178234A

US2178234A - Television system

Info

Publication number: US2178234A
Application number: US713279A
Authority: US
Inventors: Ralph S Holmes
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1934-02-28
Filing date: 1934-02-28
Publication date: 1939-10-31
Anticipated expiration: 1956-10-31
Also published as: DE931053C

Description

Oct. 31, 1939. R. s. HOLMES 2,178,234

' TELEVISION SYSTEM Filed Feb. 28, 1954 Second Detector Impedanc (Tube 13) Input Signal an grid of /7'ube Sif not loaded.

Input Signal on 7 05052654 13 showing synchronizing impulse leveling.

INVENTOR:

Ralph/ 5'. Holmes BY R'T'T'ORN Y Patented Oct. 31, 1939 PATENT OFFICE ramvrsrou SYSTEM Ralph S. Holmes, l-laddonfield, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Application February 28, 1934, Serial No. 713,279

4 Claims.

, My invention relates to television systems and it has particular relation to methods and means for obtaining accurate synchronism between the scanning operation at a transmitting station and .3 the scanning operation at a receiver.

In substantially all television systems it is necessary to periodically modulate the carrier wave to peak values, recurring at the line frequency and at the picture-frame frequency, for synchroi nizing purposes. If a Nipkow disc is used at the transmitting station for scanning the transmitted view, these impulses may be introduced through the use of a series of auxiliary openings circumferentially disposed around the periphery of the disc through which light from a fixed source periodically passes during rotation of the disc to impinge upon a photoelectric cell. The output current from the cell, after suitable amplifica- 'tion,-is utilized to modulate the carrier to peak values, in the direction corresponding to dark portions of the view, between lines and frames of the said view, givingimpulses which, at the receiver, may be utilized for synchronizing. As a rule, at the receiver, the impulses are separated -35 from each other by means of frequency and waveshape discriminating circuits and, thereafter, are made use of to maintain a plurality of local oscillators, respectively, in step therewith. These oscillators, when the receiver includes a cathode ray tube, control the horizontal and vertical de fiection of the electron beam as it sweeps over the fluorescent screen to reproduce the received picture impulses as a visible image.

In order that the received picture, as it appears upon the screen, shall have smooth boundaries the synchronizing impulses impressed upon the deflection oscillators must not vary in amplitude. Such variations in amplitude may be caused by dirt in the synchronizing openings in the scanning disc, by non-uniformity in the size of the openings, by selective fading of the carrier wave or they may be occasioned by various conditionsin the receiver itself. Irrespective of the causative factors, however, if the impulses are not of substantially invariable amplitude the edges of the reproduced view are irregular and, under extreme conditions the edges may assume a saw-tooth contour that is very objectionable.

It is, accordingly, the principal object of my invention to provide a television receiving circuit that shall compensate irregularities in the amplitude of the synchronizing impulses, whether such irregularities are introduced at the transmitter or at the receiver, and one that 'shall deliver synchronizing impulses at constant amplitude to the local synchronizing oscillators or to other utilization devices.

The foregoing objects, and other objects ancillary thereto, 1 accomplish, in a preferred embodiment of my invention, by providing devices 5 for cl pping off the peaks, so to speak; of the synchronizing impulses before delivering them to the aforesaid utilization devices. Specifically, I make use of the fact that the grid-impedance of a grid-leak biased thermionic tube is extremely 10 low for positive impulses applied to the grid thereof and I connect such a tube, in parallel with a normally biased synchronizing frequency amplifying tube, across the output circuit of an amplifier tube that handles both picture and syn- 15 chronizing frequencies. In the event, therefore, that the synchronizing impulses drive the grid of the grid-leak biased tube positive, the said tube functions substantially as a short circuit across the output resistor of the preceding tube 20 and prevents the peaks of the impulses from materially altering the potential of the grid of the synchronizing frequency amplifying tube.

The novel features that I consider characteristic of my invention are set forth with particu- 25 larity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific 30 embodiment, when read in connection with the accompanying drawing, in which:

Figure 1 is a diagrammatic view of a portion of a television receiver including a preferred embodiment of my invention, and 5 Figure 2 is a graph to which reference will be made in explaining the operation of my improved circuit.

Referring now to Figure 1 of the drawing, a television receiver to which my invention may '40 advantageously be added is illustrated as being of the superheterodyne type, including a second detector I. Obviously, as will hereinafter appear, receivers of other types may be utilized. The stages preceding the second detector have not been illustrated since they are immaterial to the operation of my invention.

The output from the second detector is impressed upon the input terminals of a synchro- 5O nizing frequency amplification channel exemplified in the drawing by a plurality of resistancecondenser coupled thermionic devices 3, 5 and I which may be of the screen grid type, as shown. The synchronizing amplification channel may in- 55 clude means (notshown) for removing the picture signals from the synchronizing signals.

The output circuit of the synchronizing frequency channel includes a choke-coil 8, across 5 which the horizontal impulses appear and a series connected capacitor il across which the vertical impulses, at a comparatively low frequency as compared with the horizontal impulses, appear. Suitable circuit connections are included whereby 10 the impulses may be impressed upon the local oscillators, (not shown) to keep them in step.

Utilizing only the circuit thus far described, the synchronizing impulses appearing across the choke coil and the capacitor would vary in am- 15 plitude with variation in the output from the second detector. In accordance with my invention, therefore, I provide further means for compensating such variation which, for purposes of convenience will be described with respect to the go horizontal impulses.

The specific compensating means I prefer to utilize, as shown in the drawing, is an unbiased picture-frequency amplifying tube i3, the input circuit of which is connected in parallel with 25 the input circuit of the synchronizing frequency amplifying tube 5 across the output resistor i5 of the first thermionic tube 3 in the synchronizing frequency channel. The output circuit of the picture-frequency amplifying tube is resistor-ca- 3o pacitor coupled to the input circuit of the receiving cathode ray tube H which need not be illustrated in detail since its structure is now well known to those skilled in the art.

Referring now to Figure 2 of the drawing, the

5 picture-freqency amplifying tube i3 is not supplied with a fixed bias but operates at a bias determined by the amplitude of the synchronzing impulses in the positive direction with respect to their alternating current axis. Such operating bias is indicated in the drawing by the position 40 of the line B and is determined by the grid-current drawn by the tube when the synchronizing impulses are applied thereto. More specifically stated, the bias is a function of the capacity of the coupling capacitor l9 and the resistance of u the grid-leak 2! connected across the input terminals of the picture frequency amplifying tube. The capacitor should be sufficiently large to hold substantially all of its charge over the periods between successive horizontal synchronizing impulses, thus assuming a working potential which will bias the tube to a good operating point for the picture signal and perhaps 75% of the synchronizing impulses. The picture frequency amplifying tube l3, by reason of its low gridimpedance to positive impulses in excess of the potential coresponding to the lower bend in the characteristic grid-impedance curve, will then load the plate circuit of preceding amplifying tube 3 during the occurrence of the synchronizing im- 60 pulse peaks and will level the impulses off to perhaps 75% of the average amplitude of all of the synchronizing impulses.

In a satisfactorily operating system constructed according to my invention the coupling condenser 66 It has a value of .01 mid. and the grid-leak of the picture frequency amplifying tube a magnitude of 2 megohms.

In order that the vertical synchronizing impulses shall not disturb the operation of the sys- 70 tern, it is desirable, in general, that their average amplitude shall not exceed 75% of the average amplitude of the horizontal synchronizing impulses. This may be accomplished through proper adjustment of the synchronizing impulse 7i generators at the transmitting station.

It might also be noted at this point that I have not found it necessary to provide a separate tube to level of! the vertical synchronizing impulses. Such a tube, however, could be provided subsequent to the output circuit of the synchronizing 5 frequency amplification channel.

From a consideration of the foregoing it will be apparent that I have provided a simplified system for clipping the peaks of the synchronizing impulses in a television receiver. In addition, my 10 improved system has the further advantage that the picture-frequency amplifying tube is automatically operated at the point on its characteristic curve where it gives the best picture signal amplification.

Although I have chosen a specific embodiment of my invention for purposes of explanation other modifications within the scope thereof will be apparent to those skilled in the art. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a television receiving system, an amplifying device having an output circuit, an amplify- 5 channel adapted to handle synchronizing impulses, an input circuit for said channel, a picture-frequency amplifying tube having a control grid and a uni-potential cathode and having an input circuit constituted only by a grid-leak, said control grid being connected directly to said unipotential cathode through said grid leak and means for connecting both of said input circuits in parallel across the first mentioned output circuit, whereby the picture frequency amplifying tube functions as a load across the first mentioned output circuit and serves to maintain constant the amplitude of positive synchronizing impulses applied to the input circuit of the synchronizing impulse amplifying channel.

2. In a television receiving system, an electric discharge tube having an output circuit, an amplifier for amplifying synchronizing impulses, said amplifier having an input circuit, an electric discharge tube having an input circuit constituted only by a grid leak, and means for connecting both of said input circuits across said output circuit, the connection between said last input circuit and said output circuit including a condenser which has suflicient capacity to integrate successive synchronizing impulses.

3. In a television system of the type in which picture signals and synchronizing signals are transmitted as a composite signal and in which the synchronizing signals are of greater amplitude than the maximum peak amplitude of picture signals of like polarity, a receiver comprising a cathode-ray tube having a control electrode, a picture signal amplifier tube having a control grid and a cathode, an output circuit and an input circuit for said tube, said output circuit being coupled to said control electrode, said input circuit including a grid leak resistor connected between said control grid and said cathode, a grid condenser, and means for impressing said picture signals and synchronizing signals upon said input circuit through said grid condenser with the synchronizing signals in the positive direction whereby they periodically drive said control grid positive to charge said condenser and apply a negative biasing potential to said grid between synchronizing impulses, the time con stant of the circuit including said grid condenser and said grid leak resistor being such that the charge on said grid condenser holds over a substantial amount between positive synchronizing impulses.

4. In a television system of the type in which picture signals and synchronizing signals are transmitted as a composite signaland in which the synchronizing signals are of greater amplitude than the maximum peak amplitude of picture signals of like polarity, a receiver comprising a cathode-ray tube having a control electrode, a picture signal amplifier tube having a control grid and a cathode, an output circuit and an.

input circuit for said tube, said output circuit being coupled to said control electrode, said input circuit including a grid leak resistor connected between said control grid and said cathode,

a grid condenser, and means for impressing said picture signals and synchronizing signals upon said input circuit through said. grid condenser with the synchronizing signals in the positive direction whereby they periodically drive said control grid positive to apply a negative biasing potential thereto between synchronizing impulses, the time constant of the circuit including said grid condenser and grid leak resistor and RALPH S. HOLMES.