US2326907A - Television system - Google Patents

Description

2 Sheets-Sheet l '4mmn attorney K. R. WENDT TELEVISION SYSTEM Filed NOV. 27, 1940 l D .mi Wm NG um SSL 555i Y quiv. kei UNHKNQ Nbitws Aug. 17, 1943.

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Aug. 17, 1943.

K. R. W'ENDT TELEVISION SYSTEM Filed Nov. 2v,v 1940 2 sheets-sheet? N; R955 mllb :19M/.70A .mam/1a Zaunz? klonu @HSR 3mm: or Karl E. Wendl;

'v Gttomeg Patented Aug. 17, 1943 TELEVISION SYSTEM Karl n. wendt, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Dela- Ware Application November 27, 11940, serial No. 367,324 7 Claims. (Cl. T28-7.1)

This invention relates to. .television systems and more particularly to picturetransmission systems.

In television systemswhere1n a cathode ray tube is used to transform an optical image into a, train of signal energy representative of the optical image, it is necessary to amplify the train of signal energy in order to increase its power to an amount necessary to modulate a radio transmitter. 'I'his s-usually accomplished by passing the sign-als through an alternating current amplifier. It is well known, however, that, when a Vtrain of signals, such as picture signals which vary in magnitude and not in polarity, are passed throughI an alternating current amplier with its associated capacity coupling, an alternating current axis is established,

and it depends for its position on a reference established by zero signal.

In the case of a train of signals representing an optical image, the established alternating current axis in the amplifier is therefore not controlled by the predetermined reference light unit necessary for the proper background or average overall illumination in the reproduced picture, but is caused to vary in accordance with the overall amplitude of the signal.

to produce a reference voltage which remains constant (that is, whichdoes not, for example, have noise superimposed thereon) f'or the period of time necessary to produce a uniform background. However, this has been attempted with some success. It has been proposed to establish a reference voltage which represents blacker than black and, in addition, provide a clipping circuit to clip the signal uniformly at a voltage representing a substantially "black picture. This method eliminates any noise that may be present in the blacker than blac region. One example of this method is described in Bedford f Patent No. 2,192,121. The said method as practiced in the system described in the present application and as applied to alow velocity beam pickup tube is described and claimed in application Serial No. 367,888, filed Nov. 30, 1940, inthe name of Alva V. Bedford, and entitled Television systems. Heretofore, the clipping circuit comprised a. mul-tielement tube which did not have a uniform response due'to the inherent curvature, particularly in the lower portion of its response characteristic curve. So, by using only the upper portion of the response curve, a more uniform amplitude response may be had. According to this invention, the direct current reference voltage representing a blacker than black picture is inserted previous to a clipping circuit which comprises thecombination of a clipper tube and a diode 'in series with the anode circuit of the clipper tube. The anode potential` and the bias across thegdiode is so adjusted that the diode cuts oi the current flow in the output circuit when the impressed signal is in the curved region of the response curve of the clipper tube. By such combination only the straight portion of the response curve is utilized whereby all signals within the desired range are passed uniformly and, furthermore, the uniformity may be controlled in order to produce what is known as gamma correction. I

In order to have satisfactory gamma co trol,

it is necessary to provide a system having maximum nonlinearity in the region of maximum gain. I-Ieretofore, gamma control devices have used a number of tubes in parallel, each of the tubes havingl a different cut-off voltage. By

so doing, it was possible to produce the desiredmaximum nonlinearity in rthe region of maximum gain.

According to-this invention, this is accomlplished in a single tube by using a tube connected in an inverted manner as an impedance device.

Accordingly,- the primary object of this inventin is to improve a'transmitted video signal.

Another object of this inventionfis to provide signal transmission system including a circuit i diagram of one form of this invention,

Figure 2 is an equivalent circuit diagram showing the fundamental operation of one form of this invention,

Figure 3 is a circuit diagram showing another Iorm of this invention,

Figure 4 is a circuit diagram showing still another form of this invention, and

Figure 5 is a graphical illustration showing the operation of this invention.

Referring in more detail to Figure 1, a pickup camera I, which may vbe of the type such as described by Iams and Rose in an article entitled, Television pickup tubes with cathode ray beam scanning," published in the Proceedings of the Institute of Radio Engineers, August, 1937, feeds a train of signals representative of the picture being televised to amplifier 3 which in turn impresses the amplified signal upon the control electrode 5 `of the discharge device 1 through the coupling condenser 8.

A bianking and deflection signal generator 9 supplies a camera bianking signal to the pickup I camera I which extinguishes the electron beam during that period of ltime which is occupied by thetprocess o f returning the electron beam to a .position where it isready to start its next scanning line or frame. This'signal is utilized to prevent any light interference on the image to the operation of` the camera bianking signal I on the pickup camera.

The blanking and deflection signal generator performs still another function in supplying a signal like that of the camera bianking signal to a D.C. inserter circuit II, such as shown and described in my copending application Serial No. 362,311, filed October 23, 1940, whose output is also applied to the control electrode 5 of the discharge device 1. The D.C. inserter supplies a reference voltage to the video-signal at a time during which the electron beam is extinguished bythe camera blanking signal. This reference' voltage is preferably chosen to be equivalent to a signal voltage that would producev a blacker than black picture at the receiver, that is, the signal voltage is greater in the "black direction than is necessary to produce a black image. As explained in my aboveidentified copending application, the potential on the control electrode 5 is the same at the start of each scanning line.

The discharge device 'I includes a cathode I3, an auxiliary electrode I5, and an anode I1. 'I'he anode 'I1 is supplied with a D.C. potential through lresistance I9 and variable resistance 2|. The output signalof the anode I1 of the discharge device 1 is impressed upon the cathode 23 of the diode discharge device 25, whose anode 21 is connected to the line amplifier 29 which is in turn connectedto the transmitter or trans-I mission system 3I.

A positive potential is also supplied to the auxiliary'electrode I5 through resistance 33 and to the anode 21 of the diode 25 through output resistance 35. The potential on the auxiliary. electrode I5 and the potential on the anode 21 is maintained at a constant potential by the circuit including voltage regulator tube 31, lter condenser 39, and resistor 4 I The operation of this form of the invention will now' be explained in more detail. When the pickup camera I is focused on an optical scene, the deflection signals from the blanking and deilection signal generator 9 causes the electron beam in the pickup camera I to scan the lightsensitive mosaic and provide a train of picture signals which are amplified in the amplifier 3 and impressed on the control electrode 5 of the discharge device 1. The picture tube bianking signal is inserted in the signal prior to the coupling condenser 8, and the D.C. inserter II supplies the train of picture signals with a predetermined reference voltage between the coupling condenser 8 and the control electrode 5.

As previously explained, the reference voltage supplied by the D.C. inserter II is such that. the optical image represented thereby would be a blacker than black image. The signal applied to the control electrode 5 is preferably such that black is in the negative direction.

It will thus be understood that, as the signal 'increases in the white direction, the current flowing through the discharge device I will increase by reason' of the fact that the potential on the control electrode 5 increases in a positive direction. It follows that the potential on the anode I'l will drop or go in a negative direction because of the increased voltage drop through the resistances I9 and 2I under a larger current flow.

The potential of the cathode 23 oi.' the diode 25 will follow the potential of the anode I1 and, as the potential drops or goes more negative, the diode 25 will become conducting when the potential of the cathode 23 goes more negativethan the potential of the anode 21.

The relative potentials on the cathode 23 and anode 2'I are so adjusted by variable resistance 2| that they will become equal when the signal voltage impressed upon the control electrode 5 is equivalent to that voltage which is necessary t? produce a black picture at the receiving stat on.

Accordingly, during the time that the reference voltage is being established at "blacker than blac or during that time in which the signal voltage applied to the control electrode 5 is held in the region of blacker than black in order to remove extraneous interference or noise, the potential on the control electrode 5 is sufficiently great in the negative direction to cause small current flow through the discharge device 1, with a corresponding large positive potential on anode I1 and cathode 23, so that no current flows between the cathode 23 and anode 21 by reason of the fact that the potential on the cathode 23 exceeds, in a positive direction, the potential on the anode 21. only signals representing a picture between the range of black and white are transmitted through the diode 25, and all those signals representing a picture in the region of `"blacker than black are eliminated.

It has been known prior to this invention that a diode can be utilized as a signal limiter. The response characteristic of thediode is, however,

curved in a manner somewhat similar to that of the multielexnent discharge device. According to this invention, however, a diode is used in combinationl with a multielectrode discharge de- Therefore, it may be seen that vice providing a. clipper circuit to givea uniform response over the entire range of amplitude of signals to be transmitted. This may best be explained by reference to the' equivalent circuit diagram shown in Figure 2. Let e, a source of potential, represent the discharge device 1 of Figure 1, and Zr the impedance of the discharge device 1. The voltage in the source e is supplied to a shunt circuit which comprises impedance ZB and representing the anode resistors I9 and 2| of Figure 1. ZS is connected in parallelwith a series circuit comprising an output impedance Zn, resistance 35, and the impedance representing the diode 25Zd. The current flowing through the circuit is divided between the legs comprising impedance ZS and the series circuit comprising impedances Zd and Z0. Impedances Z1 and ZS are greater than the sum of Zd and Zo, and

`consequently most of the current ows through the leg including the impedances Zd and Z0. It follows that any change in impedance Zd would therefore have little effect on the amount of current flowing through Zd and Z0. Therefore, even at low levels of input signals, the diode acts as substantially a linear clipper because the 'inherent change in impedance of a` diode operating in the range of low signal level has less tendency to cause amplitude distortion.

Turning now to Figure 3, there is shown another form f this invention, wherein like numeralslrepreseniasimilar parts. As the potential on the cathode 23 of the diode 25 exceeds in the positive direction the potential on the anode 21, there is no current ow through the diode 25. It is, therefore, possible for this potential on 'the cathode 23 to greatly exceed in a positivedirection the potential on the anode 21 during the time the blacker than black signal is being clipped oi'. Due to the interelectrode capacity of the diode, the -high frequency portion of this The operation of the gamma control circuit may be explained by anillustration of what occurs at various input signal amplitudes.

The circuit constants are so adjusted that when, for example, a signal representing a black picture is impressed upon the control electrode 5, there is no potential diierence between the cathode 23 and the anode 21 ofthe diode 25. It, therefore, follows that there is no-current flowing through the resistor 35 and the only current flowing through the tube 53 is that predetermined current which flows through the tube 53 and resistances 65 and 61. The amount of current owing through the tube 53 under conditions of a black picture may be adjusted'by variable resistance 61.

As the signal impressed upon the control grid changes to a signal representing a white picture, the diode 25 is made conducting by reason of the potential on the cathode 23 increasing in a negative direction with respect to the potential on the anode 21. This causes a drop in a negative direction ofthe potential on the anode 21. vPart of this potential drop occurs across the resistorA 35, and the remainder on the cathode 55 of tube 53, lwhich is therefore operating as a portion of the output impedance of tube 1. As is well known in the art, the impedance looking into the cathode of a discharge device is approximately equal to the reciprocal of the mutual conductance of the device in parallel with the actual resistor (65 and 61 i'n this case) occurring between the spurious signal is passed on to the line amplifier 29. In order to eliminate this interference, a diode 33 is connected in parallel with the input circuit to the dode 25 so that the anode 45 is connected to the diode 23, and the cathode 41 is connected through resistance t9 to the voltage supply source and' to the end of a resistor 35 opposite to that end of the resistor which is connected to anode 21. A bypass condenser 5l is inserted from the cathode 41 to ground.

As the potential on the cathode 23' exceeds in a positive direction the potential on the anode 21, so that the anode d5 of the tube 53 becomes substantially positive relative to its cathode lil, the diode d3 will become conducting Ito provide a low impedance path to ground, preventing the voltage on the cathode 23 from exceeding by a great amount in the positive direction the voltage on the anode 21.

Referring now to Figure 4, there is shown still another form of this inventionin which an auxiliary discharge device 53 is included in the circuit to provide gamma control. Tube 53 contains a cathode 55, an anode 51, a control electrode 59 and an auxiliary electrode 6l. 'Ihe cathode is connected to the output resistor 35 at the end opposite to that end which is concathode and ground, Also, as is well known, the mutual conductance varies in a nonlinear manner with the cathode current of the device. In the present case, the current through tube 53 is the controlling parameter, and the impedance of tube 53 is therefore controlled by the current through it. The greatest change or maximum nonlinearity occurs near cut-off of tube 53, which occurs at black level, which is a desired condition for proper gamma correction. Tube 53 therefore functions as a nonlinear output impedance for tube 1 with the maximum nonlinearity occurring in the black region. Therefore, a small increment of signal is amplified much more in the black region than in the white region. The nonlinearity is controlled and ground. Resistance and variable resist- 'ance 61 is connected between the cathode 55 andv ground. A source of potential is supplied to the anode 51 and the auxiliary electrode El.

by the value of resistor 61. If desired, the value of resistor 35 may be reduced to zero, such that the entire output impedance `occurs in tube 53. This will decrease the gain in the white region, but affect the gain in the blacks only slightly, thus increasing the nonlinearity of thedevice.

The point at which the'diode 53 starts to function by reason of its anode d5 going positive with respect to its cathode 41 may be adjusted by varyinglthe potentiometer 63 In a preferred form of the system, the cathode 41 may be set approximately 2 volts positive relative to the potential of cathodeV 55 under no-current conditions through diode 25. This voltage, comprised of initial velocity and contact potentials in diodes 25 and 43, allows current to start flowing through diode 43A at approximately the same voltage as current ceases in diode 25. Y

Referring now to Figure 5, "the nonlinearity of the response characteristic of a multiple element discharge device is clearly indicated and the position at which the diode cut-oifvoltage intersects the`response characteristic curve of the discharge device is shown to form a relatively sharp corner at the point of intersection so that 'it may be seen that the portion of the response shown by the line a, it may be adjusted to produce a straight response characteristic, thus producing a uniform amplification over the range .of picture signals representing a picture from black to white.

While several systems for carrying this invention into eilect have been indicated and dereference level, a peak voltage limiting circuit v comprising the combination of a discharge device having'i'nput and output circuits, means for im.

' pressing said signals on said input circuit wherescribed, it will be apparent to one skilled in the art that this invention is by no means limited to the particular organizations shown and described, but that many modiiications may be made without departing from the scope of this .invention as set forth in the appended claims.

I claim as my invention:

1. .In a picture transmission system including means for transforming an optical image into a train of picture signals having -a predetermined -reference level, a peak yvoltage limiting circuit comprising the combination of a diode having a cathode and a diode anode, means for .impress- `ing said train of picture signals between said cathode and a point of iixed potential, means for maintaining a predetermined bias voltage on said diode anode whereby only those of said signals which are in one. direction of polarity from voltage limitingcircuit comprising the combination of a rectiner having `input and output circuits, means for impressing said picture signals by said signals appear in said output circuit, a rectifier having an input circuit connected to said output circuit, said rectifier having an output circuit and adapted to pass current to its output circuit only when the voltage across its associated output circuit is greater than the voltage across its associated input circuit, means for maintaining a predetermined bias voltage across said rectier output circuit whereby only those of said signals which are in one direction of polarity lfrom said predetermined reference level appear in said rectier output circuit, an auxiliary discharge device having a cathode, an anode, and a control electrode, the cathode-anode impedance of lsaid auxiliary discharge device being connected in series with said rectiiier and said rectier output circuit with the cathode of said auxiliary discharge device connected to said rectier output circuit, means for maintaining the anode of said auxiliary discharge device at a constant potential, and means for maintaining the control grid of said auxiliary discharge device at a constant potential which is negative with respect to said constant anode potential.

5. In a picture transmission system including means for transformin ian optical image 'into a train of picture signals having a prede- "termined reference level, la peak voltage limiting circuit comprising the combination of a discharge device having input and output circuits, means for impressing said signals onsaid input circuit whereby said signals appear in said output circuit, a rectifier having an input circuit connected to said output circuit, said recand synchronizing pulses on said input circuit,

said rectiiier having an output circuit and adapted to pass current to its output circuit only when the voltage across its associated output. circuit is greater than the-voltage across its associated input circuit, means in shunt with said rectifier input circuit adapted to pass current only when the voltage across said rectiiier input circuit is greater than the voltage across said rectiiier output circuit, and means :for maintaining a predetermined bias voltage across said rectier output circuitwhereby only those ofsaid signals which are in one direction of polarity from saidv predetermined reference level appear in said rectifier output circuit.

tier having an output circuit and adapted to pass current to its output circuit only when ythe voltage across its associated output circuit is .greater than the voltage across its associated input circuit, means inshunt with said rectiiier input circuit adapted to pass current only when the voltage across said rectier input circuit is greater than the voltage across said rectiiier output circuit, means for maintaining Va predetermined bias voltage across said rectifier output circuit whereby only those of said signals which are in one direction lof polarity from said predetermined reference level appear in said rectifier output circuit, and an auxiliary electron discharge device connected in shunt with. said rectiiier output circuit and adapted to change its internal impedance in accordance with the signals appearingin said rectifier output circuit.-

.3. In a picture -transmission system including.

means for transforming an optical image into a train of -picture signals, a peak voltage limiting circuit comprising thecombination vor a rectifier and output circuits, means for imhaving input pressing said train of nieturesignals on said input circuit, said rectie'r having an output circuit f and adapted to pass current to its output circuit I said .rectiner input circuit is greater than the voltage across said rectifier output circuit.

6. In a picture transmission 'system including means for transforming an optical image into a train of picture signals having a predetermined reference level, a peak voltage limiting circuit comprising the combination of 'a discharge device having input and output circuits, means for impressing said signals`on said input circuit whereby said signals appear in said voutput circuit, a rectifier having an input cirfcuit connected tosaid output circuit, said rectiiier having an output circuit and adapted to Cmpicture-un system including means for .transforming an opticalimage into a trainofpicturesignalshavingapredtermirted'w pass current lto its output circuit only when the voltage across its associated output circuit is greater than the voltage across .its' associated inputv circuit, means in shunt with said recti iier input'circuit adapted to ypass current only when the voltage across said rectifier input circuit is-greater than the voltage across said rectier output circuit, means for maintaining a aaaaeov predetermined bias voltage across said rectiiier output circuit whereby only those of said signals which are in one direction of polarity from said predetermined reference level appear in said rectiiler output circuit, and an'auxiliary electron discharge device having a cathode, a control electrode and an anode means for maintaining constant the potential of the said control electrode and anode of said auxiliary device and a connection between the said cathode of said auxiliary device to said rectifier output circuit.

'7. A hunting circuit comprising in combination, a discharge device having a cathode, an anode and a control electrode, means for applying a signal between saidcathode and said control electrode, a load impedance for said discharge device, means for applying an operating pass current when said signal exceeds a predetermined volt'age level, and said load impedance having connected in parallel therewith a surge limiting circuit including an impedance serially connected with a Asecond rectifying device biased and polarized to pass current through said second rectifying device when said rst mentioned rectifying device ceases to pass current.

KARL R. WENDT.

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