US2136810A - Television system - Google Patents

Description

NOV. 15, 1938. W VVANCE 2,136,810

TELEVISIN SYSTEM Filed June 28, 1954 5 sheetssheet 1- ivy/9.1.. l

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I/vcoM/NG SIGNAL REPHEEA/T/NG STE/wy STATE- VERY `JEH-'nenn' v/Ew IN VEN 'ro i? Hrthu W Vance TIME Patented Nov. 15, 1938 UNITED STATES TELEVISION SYSTEM vArthur w. vence, Philadelphia, Pa., assigner to Radio Corporation oi' America, a corporation of Delaware Application June 28,

z claims.

My invention relates to television systems and more particularly to lmethods of Aand means for maintaining synchronism between transmitting and receiving devices and .for obtaining automaticcontrol of the average brilliance of a received view.

At a television transmitting station, it is expedient to use alternating current amplifiers for the purpose of amplifying the varying currents, representative of the view, before utilizing them to modulate a carrier wave for radio or line transmission. Also, in a receiver, it is convenient to utilize such amplifiers to amplify the demodulated incoming television signals before applying them to devices which translate the signals into visible images of the view. In the case of a transmitter of the type referred tp above, therefore, the direct current compo/nent of the picture-currents, representing the average illumination of a view over a relatively long space of time, does not appear. in the carrier wave and, even were it present in thev incoming signal at a receiver, it would be lost if the signais pass through but a single alternating current amplifying stage after demodulation.

In view of the foregoing, if the television receiver includes `one or more alternating current amplifying stages wherein the picture-signals are amplified before they are applied to the translating device which gives them visibility, changes in the average illumination of the view at the transmitter, if they occur at a relatively slow rate, are not reproduced at the receiver. For this reason, television receivers have been provided withmanually adjustable' devices, by which the brilliance of the received picture may be altered from time to time, or with automatically functioning background control devices such, for example, as those disclosed in the copending application of Ralph S. Holmes, Serial No. 658,894, filed February 28, 1933, and assigned to Radio Corporation of America.

'I'he Holmes system, while giving entire satisfaction from the operating standpoint, necessitates the use of a rather complicated circuit and it is, accordingly, an objectA of my invention to provide a simpler system that shall, at the same time, function equally as well.

Although I shall, hereinafter, explain my improved system in connection with a television receiver of the type including an alternating cur-A rent amplifying stage, sight should not be lost of the fact that atleast one phase of it is equally as `well applicable tov a television transmitter. That is to say. it may also be expedient to rein- 1934, serial No. 732,747

(ci. ris- 1.5)

troduce the direct current component, representing the average illumination of a view, into the carrier wave radiated from a transmitter ofthe type wherein the synchronizing and pictureimpulses are amplified in alternating current amplifiers before the modulation stage. This can be accomplished by automatically varying the average amplitude of the radiated wave proper-'- tionally to the average illumination of the view being transmitted. Such automatic variation may be had through automatic gain-control of the radio-frequency amplifying` stages, the oscillator or the modulator, by applying grid-biasing potentials thereto which vary in accordance with v the average illumination of the view. It is, therefore, a further object of my invention to provide means for automatically deriving unidirectional vpotentials proportional to the average illumination of an object or view being "televised which potential, at the transmitting station, may be utilized to cause the radiated power to vary in accordance with the said average illumination.

It has also been pointed out by others, at various times, that when picture and synchronizing signals are simultaneously .transmitted through any system with a `definite low-frequency vcutoil, such as an alternating current amplifier or a modulator, the peak potential of the synchronizing impulses, with respect to ground, is a function of the average intensity of illumination of the view being transmitted. 'I'his phenomenon, hereinafter, will be designated as modulation" of the synchronizing impulses.

As a result of the modulation of the synchronizing impulses, at the receiver, the impulses accompanying a bright picture are stronger than those accompanying a dark picture. Unless steps are taken to level-off the synchronizing impulses, proper synchronization of the scanning in the receiver with scanning at the trans- 40 mitter cannot be maintained. r

It is, accordingly, a further object of my in'- vention to provide a system whereby synchronizing impulses may be so leveled off before utilization to control the ray deflection oscillators associated with a cathode-ray receiving tube that exact synchronization may be maintained.

A still further object off my invention is to provide unitary means for automatic background illumination control in a cathode-ray receiver 50 tube and for leveling off the synchronizing impulses.

In a preferred embodiment of my invention, the foregoing objects and other objects ancillary thereto I attain as follows: 'I'he carrier wave,

modulated both by picture-signals and by synchronizing impulses is ampliiled, demodulated, further amplified in an alternating current ampliiler and then applied to the input terminals of an electron tube having a self-bias resistor many times greater in magnitude than usual. A relatively large capacitor isconnected in shunt tothe resistor. To colmteract the biasing effect of such a large resistor the grid return of the tube is brought to a highly positive point on the potential source that supplies the tube. The tube is provided with an output resistor across which the leveled-of! synchronizing impulses appear, as will be explained in"detail later.

By reason' of the unusual magnitude of the self-bias resistor a high potential doats across it but the actual grid-to-cathode bias is maintained at substantially the cut-oi! point. The number of amplifying stages preceding the ,tube is such that the synchronizing impulses tend to drive its grid positive and the tube draws just enough current on each peak to have nearly the same average plate current as when no-signals are being received. When the amplitude of the synchronizing impulses increases, however, with respect to ground, the direct current potential drop across the self-bias resistor increases and when the amplitude decreases the potential drop also decreases. Buch potential, therefore, represents the average illumination of the view being transmitted and I utilise it for supplying grid-bias to the cathode-ray receiving tube, the connections being such that the bias becomes less negative when a bright picture is being received, and vice versa. V

The novel features that I consider characteristic of my invention are set forth with particularity 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 embodiment, when read in connection with the accompanying drawings. in

which:

Figures 1, 2 and 3 are greatly simpliiled graphs representing television carrier waves under differing conditions of illumination of the transmitted view, and

Figure 4 is a diagrammatic view of a portion of `a television receiver in which a preferred embodiment of my invention appears:

Figures 5 and 6 are graphs to which reference will be made in explaining the operation of a television receiver in which my invention is included.

Before explaining the method I have provided for obtaining automatic synchronizing-impulse leveling, automatic background control and the apparatus I prefer to utilize, it is probably best to recapitulatesomewhat the manner in which a television transmitter may operate. To this end, attention might well be directed yto the Bates Patent No. 1,940,838 as exemplifying a television transmitter of the type wherein a. scanning disc is utilized for the purpose of deriving picture-representative impulses and for providing synchronizing impulses at the vend of each scanned line, which impulses, at the receiver, may be utilized for controlling the scanning operation.

'Ihe Bates patent discloses a television transmitting system wherein, during each interval between scanning lines, when the photo-cell is not receiving light from the view, a synchronizing impulse is introduced by exposing the same photo-cell momentarily to light from a xed source. Bygeason of the use of a common photo-cell and commonV amplifier for both picture-signais and synchronizing impulses, obviously the radiated" carrier wave is modulated in the same sense vbythe synchronizing impulses and by bright views or by bright portions of a yview being transmitted. Inv other well known television transmitting systems, the synchronizing impulses cause the radiated carrier wave to be modulated in the same sense that it is modulated by dark portions of the transmitted view.

My invention is applicable to the reception of signals from television transmitters of both types, but for purposes of convenience, I will hereinafter explain it in connection with the reception of signals of the type transmitted by the last referred to systems.

To simplifythe explanation of my invention, it is assumed that the dark direction of modulation of the carrier for dark views and for the synchronizing impulses is "up" and that the light direction is down". That is to say, it is assumed that the instantaneous carrier amplitude increases in response to synchronizing impulses and to signals representative of dark portions of a view and decreases in response to signals representing light portions of the view.

It should be understood, however, that the relative sense of modulation is immaterial since, in my improved receiver, the proper utilization of picture and synchronizing impulses depends A merely upon the number of stages of amplification and connections therebetween, it being well known to those skilled in the art that a single resistor-condenser coupled stage provides output potentials that are 180 degrees out of phase with the input potentials supplied thereto.

The manner in which the synchronizing irnpulses may be introduced and, in fact, the entire transmitting equipment forms no part of my present invention. It might be stated, however, that such modulation may be occasioned by connecting a synchronizing impulse modulator in parallel with the picture frequency modulator, thus adding their effects upon the carrier wave in proper phase.

If the transmitter is so adjusted that the synchronizing impulses cause the carrier amplitude to periodically rise to ypeaks much in excess of the maximum modulation thereof caused by black portions of a view being transmitted, the instantaneous carrier wave, for a few lines of a very black view, if plotted with respect to the zero axis, would have an envelope somewhat like that shown in Figure 1 of the drawings. Similarly, the graph of an instantaneous carrier wave for a few lines of an all-white picture might be exemplified by Figure 2, and that for an allgray picture, or a picture having medium illumination, might be illustrated as shown in Figure 3.

It will be noted, from an inspection of the drawings, that out of each scanning line period, nine-tenths of the time is devoted to scanning the view and that one-tenth of the time is allocated to the horizontal synchronizing impulse. The vertical synchronizing impulses have not been illustrated but it is to be understood that whenever the phrase synchronizing impulses is used hereinafter both horizontal and vertical impulses are included.

In the receiver, after demodulation and amplification in an alternating current amplifier, the amplified alternating signal currents, or signal zero axis as indicated by the axes Oi iniflgures l, 2 and 3./ In.` each case, the peak ampll ude of fthe synchronizing impulses, indicated at drawings, with -respect to the new reference axis 1 'ff 9,186,810 I A -potentials,,instantaneously available, m I be 4 rel)- resentedbyone-,halffot the envelope of carrier wave redrawnmith reference to a new A. C.

in the is diiferent. the amplitude after the "whi lines in the picture being greatest, the amplitu e after the "blacki lines in the picture being` le t, and that corresponding tofthe graypicture having an intermediate value. Aocordingly, even vthough thedirect current component of the signal has V been lost after denipdulation, the proportionality of the peak amplitude ofthe synchronizing impulse to the 4illumination of the transmitted view remalns, and, according to one phase of my invention, I utilize such roportionality to automatically correlate the b ghtness of the received picture to that of the picture being transmitted.

, At this point, itis not amiss to reiterate that Figures 1,2 and 3 are highly simplified graphs of the transmittedA signal. In actual transmission,

' the synchronizing impulses might not appear as 'indicated for the reason that in transmitters of certain .types an additional impulse in-the dark" direction' is introduced at the end of each scanning line for the purpose of suppressing parasitic frequencies which tend to appear in the intervals between\lines. Such being' the case, the composite impulse introduced between scanning lines may have a rather complicated wave form. No necessity is, seen, however, for 'illustrating the 'actualwwave shape since my invention is more concerned with the amplitude of the impulse than with its outline. l At the receiver, referring now to Figure 4 pf/the drawings, the incoming carrier wave, 'Idulated by picture-impulses and by regularly 'recurring synchronizing impulses, may be amplified, hetero- 'dyned, amplified at the intermediate frequency. and then demodulated to give a series of impulses corresponding tothe envelope of the wave, which 'impulses are amplified in a pictureffrequency amplifier of the alternating current type and applied to subsequent receiver elements.4 Since the preamplifiers, detectors, etc., of the receiver form no part of my invention, I have merely indicated them in the drawings by a rectangle having a plura'lity of output terminals I and 3.'

It lis important to note at this time thatthe number of 'stages of picture-frequency amplification preceding the output terminals is so chosen that the synchronizing impulses tend to drive the terminal l periodically positive with respect to the terminal 3. Thereason for this polarity will be clearly apparent hereinafter.

The amplified combined signal appearing across the terminals I and 3 .is impressed in parallel across the input terminals of a furtherpicturesignal amplifying tube, preferably of the type known as Radiotron,2-A3, and the input terminals of .a tube which functions simultaneously to level off the synchronizing impulses and to provide a unidirectional potential for automatic the fluorescent screen of the cathode-ray tube., For convenience,the second mentioned tube will be calledthe control tube, hereinafter. Little need be said about the picture-signal amplifying tube except that it amplies and inverts the negative picture-signals applied to its input terminals and impresses them upon the grid of a cathode-ray receiving tube. Also, the picture-signal amplifying tube inverts the positive synchronizing impulses and` applies them as nega;

tive impulses to the grid of the cathode-ray tube, Sinne the picture-signals andrsynchronizing are sequentiallyl received. and not simultaneously,

there is no interference with they reproduction f the picture.' Y i, l

The controljtube, preferably, is ofthe s'creengrld typecomm'ercially known asRadlotron R57,

anode. The `amide of the tube and the screen grid thereof are connected to appropriate positiv@ pointseon apotential source. exemplied inthe drawings by'a bleeder resistor. an Vimpedance device such as a cho -e or an output resistor being included in the an de connection. The' cathode of the tube is connected to the negative vterminal of the bleeder resistor through an impedance device which maybe a` choke coil or a self-bias 4resistor having many times the. resistance of resistors usually utilized for self-biasing purposes.

In a preferred embodiment of my inventlonthe self-bias resistor has a resistance of 500,000 ohms,

Vhaving a cathode, a control grid, a screen grid and providing decided degeneration", although fairly the bleeder resistor. This point is so chosen that thertube, in the absencq of incoming signals. draws just suflicient current to maintain the grid at a potential of -6 volts, which is slightly above the cut-off potential. This lconnection and the unusually high resistance of the self-bias resistor, are important features of my invention.;

To provide automatic background control, in the manner later to be explained more in detail. a conductive connection including .a resistor extends from the cathode end of the self-bias resistor associated with the control tube tothe grid of the cathode-ray tube. An adjustable' connection extends from the cathode of the cathode-ray tube to a positive point on the bleeder, the negative bias on the grid of the tube being-the algebraic sum of the potential developed across the self-bias resistor and the,l potential between the negative end of the bleeder resistor and the point thereon to which the cathode is connected'. 4

As wilt also vbe explainedl in more detail. the leveled oi synchronizing impulses appear as potentials across the output resistor of the control tube. Such potentials may be impressed on subsequent separating networks or`v devices (not 55 shown) in any suitable manner, exemplified in thel drawings by the conductors extendingI from.

the anode of the control tube and the cathode thereof. l

In explaining the operation of. my invention to provide automatic background control and leveling off of the synchronizing'impulses attention may new advantageously be directed to Figure of the drawings. VIn the figure, the horizontal full line represents grid volts and time, and vthe intersecting fullvertical line represents `output current from the control tube as well as,V time. The first dotted line, 'Bm appearing to the left of the vertical linevrepresents the algebraic sum of the drop in potential across the self-bias'res'istor,

Vin the rio-signal condition, and the potential between th negative end of the bleeder resistor and the point thereon to which the grid is Vconfnected. In theexarnple chosen, the net bias on the grid of thetube is -6 volts, this being the 4 difference between the self-bias potential, -200 volts, normally appearing across the self-bias resister, and the positive potential from the bleeder resistor, +194 volts. -Under these conditions, with 450 volts anode potential and 300 volts screen grid potential, substantially .0004 ampere flows in the self-bias resistor.

In the example under discussion, the cathode of the cathode-ray tube is shown as being connected to a point on the bleeder resistor at 220 volts positive which provides bias potential of volts on the grid of the tube in the no-signal condition. v

Assume, now, that steady state signals representative of a very dark or black view are being impressed across the input terminals of the control tube, the polarity of the impulses being positive with respect to the normal bias potential and that portion of the signal representing the picture being negative with respect to the said bias potential.h This condition is indicated by the irregular vertical line appearing to the leftof the v full vertical line in Figure 0 oi th'e drawings.

. sistor to cause a unidirectional potential to appear thereacross in such direction as to tend to render the grid of the control tube more negative. Such being the case, the synchronizing impulse -pote`ntials appearing across the output resistor assume a definite amplitude correspondencey to the dark picture, as is also indicated in Figure 6.

Assume next that the picture changesl to one` with high illumination and that, as hereinbefore explained, the synchronizing impulses are so modulated thereby that their peaks are much more positive with respect to ground than in the case of the black picture.` Under these conditions, the peaks of the synchronizing impulses would tend to draw much more current than in the'dark view case were it not for the fact thatthe bias on the grid of the controltubeis driven more and more negative byreason of the action of the self-bias resistor and the shunting condenser. As a matter of fact, the bias across the resistor does change greatly, but, since the grid-potentialplate-current characteristic of the tube approaches the cut-on' point asymptotically, the tube can never entirely cease to draw current. As a result of thedegenerative action described, however, the amplitude of the synchronizing impulse potentials appearing across the output resistor becomes substantially no greater than when a dark view is being received and the effect of the picture signal sistor and the grid 0f the cathode-ray tube, to.

control the average, or backgroundbrllliance of the reproduced view.

In operation,the grid-bias on the cathode-ray tube is the algebraic sum of three potentials, a

positive potential derived from the self-bias resistor of the control tube determined by its normal bias, a varying positive potential occasioned by the rectification 'of the changing synchronizing impulses, and a negative bias derived from the bleeder resistor. When receiving a view, therefore, the connection from the cathode of the cathode-ray tube to the bleeder resistor may be so adjusted that black portions of the view are truly black on the fluorescent screen. After this adjustment, the automatic control comes into play. 'Ihat is to say, if the average brilliance of the view increases, the grid of the cathode-ray tube receives from the self-bias resistor a potential which renders it less negative and, if the average illumination of the view decreases the potential supplied by the self-bias resistor is such that the grid of the cathode-ray tube becomes more negative than when first adjusted, the changes in grid potential being relatively large by reason of the extremely highresistance of the said self-bias resistor.

It will thus be apparent, from a consideration of the foregoing, that I have provided unitary means, the control tube having an unusually large self-bias resistor, for accomplishing two very important results in a television receiver of the cathode-ray tube type. namely automatic background control and leveling off of the synchronizing impulses that have been modulated by the picture signals.

Inasmuch as certain modifications of my invention will at once be apparent to those skilled in the art, the specific circuit chosen for purposes of illustration is not to be construed as restrictive and my invention is not to belimited except insofar as is necessitated by prior art and by the spirit of the appended claims:

I claim as my invention:

l. In a television system of the type wherein an area is scanned to develop picture signals and wherein synchronizing impulses are developed periodically, said impulses having a predetermined ilxed amplitude with respect to a picture signal representing black, a receiver comprising an electric discharge tube having a cathode, a control element, and an anode, a potential source, a connection including a self-bias resistor between said cathode and said source, said resistor having a magnitude at least of the order of 50,000 ohms, a connection between saidcontrol element and a point on said source which is positive with respect to the point to which said cathode is connected, a condenser connected in shunt to said self-bias resistor, a connection between said anode and a point on said source also positive with respect to the point thereon to which said cathode is connected, means'for impressing said picture signals and said synchronizing impulses upon said control element with said impulses oi positive polarity whereby there appears across said resistor a voltage having a magnitude which varies in accordance with the amplitude of said synchronizing impulses, said self-bias resistor and shunting condenser having a time constant such that the condenser loses only a small percentage of its charge between synchronizing impulses, av cathode ray tube having a cathode and a control element, and a direct-current connection from said cathode ray tube control element to said cathode ray tube cathode, said directcurrent connection including at least a portion of said self-bias resistor whereby the. bias on the control element of said cathode ray tube varies in accordance with variations in the amplitude of said synchronizing impulses.

2. In a television system of the type wherein anode and a point on said source also positive with respect to the point thereon to which said cathode is connected, said last connection including an impedance element, means for impressing said picture signals and said synchronizing impulses upon said control element with said impulses of positive polarity whereby there appears across `said impedance element synchronizing impulses having a substantially constant amplitude, said self-bias resistor and shunting condenser having a time constant such that the condenser loses only a small percentage of its charge between synchronizing impulses, a cathode ray tube, and circuit connections fortaking oi! .said

synchronizing impulses from said impedance element and supplying them to detlecting means for said cathode ray tube.

y ARTHUR W.-VANCE.

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