US2485310A - Video black clipper - Google Patents

Description

Oct. 18, 1949. s. R. PATREMIO VIDEO BLACK CLIPPER Filed Dec. 12, 1946 Patented Oct. 18, 1949 VIDEO BLACK CLIPPER Salvatore Robert Patremio, Rochelle Park, N. J., assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. J a corporation of Delaware Application December 12, 1946, Serial No. 715,727

This invention relates to a device for keeping the black portions of the video information and spurious signals in a television system from interfering with the synchronizing signals for the system. The device can be located in a video Camera chain and thus improve contrast and synchronization.

In carrying out the invention the composite blanking ignal is reversed in phase and applied through a delay network across a potentiometer where a portion of it is utilized to apply a positive bias to a cathode follower stage during the active time of the video signal. In this Way a bufier zone is provided into which the black video information cannot penetrate and interfere with the synchronizing signals.

The invention may be understood from the description in connection with the accompanying drawing, in which:

Fig. 1 is a schematic diagram of an illustrative embodiment of the invention.

Fig. 2 is a, time diagram of a composite video signal without this invention, and

Fig. 3 is a time diagram of the corresponding composite video signal with this invention.

In the drawing reference character I indicates a vacuum tube to the control grid 2 of which blanking signal A is coupled through a capacitor 3. Its grid leak resistor is shown at 4. The cathode 5 of this tube l is connected to ground through a resistor 6 with a capacitor 1 connected across it. Plate 8 is connected to a source of positive potential B through resistors 9 and connected in series, with a point between them connected to ground through a capacitor I I. The plate 8 is also coupled by capacitor l3 to the control grid I 2 of the tube l5. This rid I2 is also connected to ground through grid leak resistor I4. The cathode 16 of this tube I is connected to ground through a resistor l1. This cathode is also connected to a delay network 51.

The plate l8 of tube i5 is connected to a source of positive potential through resistors I9 and 20 connected in series with their common connection coupled to ground through a capacitor 2|. The plate I8 is coupled by condenser 22 to the control grid 23 of a pentode tube 24 having a grid leak resistor 25 that is connected to ground. The cathode 26 of tube 24 is connected to its suppressor grid. It is also connected to ground through resistor 21 which has a capacitor 28 connected across it. The screen grid 30 of tube 24 is connected to a source of positive potential B through the resistor 3| and to ground through the capacitor 32. Its plate 29 is connected to a 2 Claims. (Cl. 1787.1)

source of positive potential through load resistor 33 and is coupled into the chain of video amplifiers 31, 38, 39, 40 and 4|.

Th output of this chain of video amplifiers is applied to the control grid 42 of the triode 43. The plate 44 of this tube 43 is connected to a source of positive potential. The cathode 45 of this tube is connected to ground through resistor 46, potentiometer contact 41 and a portion of resistor 48. This cathode 45 is also connected to the input of additional stages of amplification (not shown) through resistor 49. It is also coupled to the grid 5| of tube 50. The plate 52 of this tube is connected to a source of positive potential. The cathode 53 is connected to ground through a resistor 54 and to the input of additional stages of amplification, not shown, through resistor 55.

The operation is as follows:

The blanking signal A is inserted at the grid 2 of tube l and is taken mi in amplified form at the plate 8 and applied in reversed phase to the grid l2 of the tube 15. It appears in the same reversed phase across the cathode load resistor l1 and in its original phase across the plate load resistor l9. It is coupled by condenser 22 from plate l8 to the grid 23 of the tube 24 where it is taken off in amplified form and reversed phase at the plate 29 and inserted in the video signal amplifier chain at some point such as the grid of stage 31. The video signal comprises the picture signal and the synchronizing signals, as usual. After amplification in a number of stages the composite signal appears in the form shown in Fig. 2 in the output of stage 4! and at the grid 42 of the tube 43.

At the same time the blanking signal appears across cathode bias resistor I! in reverse phase, that is, with negative pulses during the active time as shown at 56. This signal is delayed by the time delay network 51 and delayed by an amount equal to the delay introduced by the video amplifier chains 31, 38, 39, 40 and 4| so that it will appear at exactly the same phase relationship as the blanking signal component of the output of this amplifier chain. The amplitude of the signal from the cathode 45 of tube 43 is adjusted by the potentiometer 41, 48 which applies the desired positive bias to the cathode 45 of the tube 43. This positive bias prevents the blackest portions of the video signal and spurious signals in the black and blacker than black region from appearing in the synchronizing signal.

The buffer area is shown in Fig. 3 as the area between the blanking level and the dotted line 3 d. The black signal a of Fig. 2 is cut ofi at the line 11 in Fig. 3. This prevents interference by the blacker than black portions of the video Signal with the synchronizing signals.

Tube 50 and its connections are merely another output for the video signal appearing across the load 46 of the cathode 45 of tube 43.

What is claimed is:

1. In a television transmission system, a series of vacuum tubes adapted to amplify the video signal of said system, a vacuum tube amplifier adapted to amplify a blanking signal, means'to apply said blanking signal to said vacuum tube amplifier, means comprising another vacuum tube having a resistance in its anode circuit and a resistance between its cathode and ground to obtain outputs of opposite polariti'eszfromtthe anode and cathode respectively of said second named vacuum tube, the control grid of said second vacuum tube being coupled to the plate of the first named vacuum tube, means comprising a third Vacuum tube coupled to the plate of said second vacuum tube to amplify and reverse the phase of said blanking signal and apply it to the gridof one-0f said series of Vacuum tubes adapted to amplify the video signal of said system whereby said blanking signal is mixed with said video signal, means to-delay said output from the cathode of said second named vacuum tube, a fourth vacuum tube coupled to the output of said series of vacuum tubes and adapted to be used as a cathode follower stage, means to cause said delayed output to be applied to the cathode of said cathode follower stage thereby limiting its output voltage swing in the negative direction.

2. In a television transmission system, a series of amplifiers adapted 'to amplify the video signal of said system, a vacuum tube amplifier adapted to amplify and reverse the phase of the blanking signal of said system, a second vacuum tube having its control grid coupled to the plate of said amplifier and having plate and cathode resistors, a third vacuum tube having its control grid coupled to the plate of said second vacuum tube and its plate coupled to the grid of one of the amplifiers of said series, a lead from the cathode of said second vacuum tube, a delay network between the cathode of said second vacuum tube and one end of awesistorwhich has its other end grounded and a slideable tap on said resistor con- .nected through another resistor to the cathode "of a fourth vacuum tube to the grid of which is cnnnectetl the output of said series of amplifiers, the .plate of said fourth vacuum tube being con- :nected .to asource of positive potential, the output being takeni'irom the cathode of said fourth vacuum tube.

"SAINACRQRE ROBERT PATREMIO.

REFERENCES CITED The following references are of record inthe file of this vpatent:

TILNZIZ'IE'D STATES PATENTS Number Name Date 2,171,536 Bingley Sept. 5, 1939 2,207,048 Campbell July 9, 1940 2,212,199 Browne Aug. v20, 1940 2,244,239 Blumlein June 3, .1941 12,295,330 Blumlein Sept..8,.1942 2,305,902 "Schroeder Dec. 22, 1942 2,321,343 Went June 8, 1943 2,366,358 Schlesinger J an. .2, 1945 2,445,040 Schade July 13, .1948

FOREIGN PATENTS Number Country Date 472,401 Great Britain Sept. 20, .1937

OTHER REFERENCES Proc. =IRE,'-May 1941, pp. 261-262.

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