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Àug» 6, 1946 ML cAwElN 2,405,290 TELEVISION SIGNAL - AMPLIFIER Filed March 29, 194s 3 Sheets-Sheet l INVENTOR Aug. 6, 1946. T M. cAwElN ' TELEVISION SIGNAL AMPLIFIER 1 Filed March 29, 1943 3 Sheets-Sheet 2 HG2A CUPRLAETN 30 _ '-I- , GRID VOLTAGE ÄVOLTAG-E IO+ SIG`NAL ^` 29 J PLATEVOLTAGE IO +. CATHODEVLTAGE I O + A2,405,290 ' TELEVISION SIGNAL AMPLIFIER ‘ ' Filed Marsh 29, 1943 s sheets-Sheet s ~ INVENToR MADISON ¿cAwErN Patented Aug.~6, 1946 ` .¿ ._„ ._ ` UNÃTED STATS l i 2,405,290 Í or rie, 2,405,290> ` ' , Y 5 TELEVISION SIGNALAMPLIHER \ Madison Cawein, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation, a corporation of Delaware Application March 29, 1943, serial No. 480,978 1 ~ 10 Claims. (Cl. 178-7.5) 2 This invention relates to amplifiers and par ' grid adjusts itself to a direct current signal level suchpthat the maximum positive peaks of one` oi ticularly to amplitude discriminating amplifiers for expanding non-linearly one of the compo nents of a composite television signal. the signal components produce a zero bias of the ‘ grid.- According' to conventional television practice, a carrier wave is modulated by both video and necessary to amplify non-linearly one of the com which is retained, ponents and sometimes to simultaneously sup press the other component. When the video component is so amplified, itis possible to eilect disadvantage of this latter type of limiter is that plitudein order that the 'signal component which is toibe suppressed have a voltage which will elîect the cut-off of plate current in the tube at the desired point of the signalfwave. For ex diñers from the relative contrasts between other parts of the picture. For example, it may be clesirable to deemphasize the contrast between the darker parts of the picture while at the same time to emphasize or accentuate the contrast between the lighter parts of the picture. The 'gamma control may be accompanied by a separa tion of the video signal component from the syn ample, _where it ‘is desired to recover the 'syn chronizing Vcomponent to the exclusion of video componentv fromva" composite 'television signal 2 constituted in accordance with the present stand ards adopted by RadioV lManufacturers Associa tion in the United States,`the~ grid .voltage to eiïect plate currentcut-oif should correspond to chronizing component of the composite television signal. For the synchronous operation of the receiving the television signals representing the black por f tions of the picture. Wherever possible it is' de- ' 439 one or moreampliñers for the demodulated sig sired to recover. the video component to the ex «- clusion of the synchronizing component >andto amplify non-linearly` the video component for the purpose of gamma control, it is desirable to accomplish this result before the> video com "le this typeof limiter is that it can be used only in cases where the direct current signal level of the picture to be reproduced has been established. 45 input circuit of the tube in such a manner that one of the signal components drives the potential '- of Athe grid of the tube beyond lthe point of plate current Acut-off and the other signal component is` amplified in the output lcircuit o_f thetube. This type of limiter can be arranged so that the sirable to _separate the video signal component from the synchronizing signal component in the signal Wave derived from the detector Vofithe're ceiver. Hence, for the operation of a limiter of the grid cut-off xtype,'it is necessary to provide nals in order to obtain the required amplitude in the case of weaksignals. Also, where it is de , Another type of limiter employs a tube which ’ it requires television signals of considerable am between some parts of the reproduced picture is biased to effect plate current cut-ofi.r The 'com posite television signals are impressed upon the ‘ ' As .is well understood in the art, however, a a gamma control such that the relative contrast Numerous circuits have been devised for oper ating limiter tubes. One expedient is to bias the tube for saturation at a predetermined signal level. The composite television signals are ap plied to the input circuit of the tube so that one of the signal components drives the tube to satu ration while the other signal component is am plified in the output circuit. A disadvantage of ' tion and reproduce in ampliñed form, withoutk substantial distortion, the signal component television signal.'v At the receiver, it frequently is press any desired one of the components- ' ployed heretofore effect the desired signal separa synchronizing components to form a composite apparatus, it also is necessary to separate the synchronizing and video components in such a manner that the syncëhronizing component is re tained and the video component is suppressed. The separation of the two components of the television signal may be accomplished by means of limiter tubes. Such tubes may beV utilized to separate the two signal components and to sup ' Limiters of these two types and others em ponent is amplified.- . _ Aníobject of the present invention, therefore, is to provide,A in a'television system, an improved signal separator, the operation of which is inde pendent of the signal strength or the directA current signal level, _ ` ’ Another object of the invention is to provide an ampliiier for obtaining non-linear- distortion of a portion of a composite television signal,- whereby the distorted portion may beV separated from other portions of the _television signal. _ _: Stilljanother object, ofthe invention is to pro vide, in'a‘television receiver, an improved signal limiter lwherein there is employed a yregenerative voltage feed-back_'irom the output ¿circuit "cia triade to _theginput çircuitiliereef.. whereby te 4 of a, cathode ray tube I3 for controlling, in a conventional manner, the shading of the picture to be reproduced. The demodulated video and synchronizing separate the synchronizing and video compo nents of weak as well as strong composite te e vision signals. A further object of the invention is to provide an improved non-linear feed-back amplifier which is discriminatory of signal amplitude, signals are coupled by means of a condenser I4 to the input circuit of a space discharge device, such as a triode vacuum tube I5.» The input cir whereby only the synchronizing component of cuit includes the cathodefand the control grid of the tube which are interconnected by resist a composite television signal is ampliñed. A still further object of the invention is to provide a non-linearly distorting amplifier, whereby the video component of a composite television signal may be recovered‘in' 'such> form" ance means. This means comprises a series ar rangement of a grid leak resistor I6 and a cathode follower resistor I1, and the junc tion4 point of these resistors is grounded. The condenseris connected to the grid leak re “sist'or portion-of the input circuit resistance ele to eiîect a gamma control of a reproducedïpicture. In accordance with the-~ present `Vinvention, - there is provided a space discharge device suclrv ment. There also is provided a variable resistor I8 vconnected between the anode of the tube and the ’cathode follower resistor Il. A battery I9 is connected through a resistor 2B to the anode as a three element vacuum tube which employs a self-biasing grid circuit. Th’ecathode'circ'uit of the tube is made degenerative and‘thereîis“ coupled to the cathode a'direct current regenera tube. ' tive feed-back?circuit.- The arrangement is such 20 of Itthe Will appear from subsequent -portions of the that the regenerative component is large relative description that there is developed at the cathode tothe degenerative component so as to eiîect a of the tube I5 a, square-topped impulsive voltage discrimination in signal amplitude, Wherebyall at the synchronizing signal frequency. This signals of less than a predetermined positive voltage is applied to conventional synchronizing amplitude are suppressed inthe 'outputcircuih 25 and scanning control apparatus'ï‘l, the'purpose Also‘the signals' which are reproduced in the of which is to separate the horizontal and verti output circuit are ampliñed in a non-linear man cal synchronizing signals so as- to control the ner with respect t0 the corresponding signals im scanning generators. ’The horizontal` and verti pressed upon the input circuit.' The magnitude of the distortion varies directly> as a Lfunction 4of the impressed signal7 amplitudeï n so Whether the reproduced signals correspond to the video or synchronizing components de cal generators are connected‘to*4 the Yrespective sets of deflector elements 22 and 23;»'whereby'the usual defiections of the electronßbeamrmay be eifected.l ‘ Y The signal which appears in the output 'circuit vision signal as it isimpressed upon the' input -cir 35 of detector vI I has afor'm‘ such as that illustrated by the curve 2E of Fig. ‘3; This signalv consists cuit of the‘ampli'i'ler; Y of video and synchronizing`> components vand va For a betterI understanding of theV invention, ries in polarity so that the negative portions together withl other’and. further objects-thereof, thereof> constitute some‘ of the video signals'and reference" 'is ‘ had tov the" following»` description the positive'portions thereof constitute the-re-taken in connection with vthe- accompanying mainder of the video signals and the synchroniz drawings, and its -scope will be pointedout inthe ing impulses. If such a*l signal were applied to appended ‘ claims. ` ' a prior art limiter, having a' tube'biased toy ei'r‘ect 1n the ^ accompanying >`drawingsâ Fig; 1 is a> circuit diagram of» a'television re- _. plate current cut-on, the signal would have the relation~ of the curve 25'iof Fig. 2, with respect ceiver, partially schematic, embodying .the- pres to the-plate current versus gridV volt'age'charac ent‘linv‘ention, as-ernployedto recover the syn teristic of the tube I5. chronizing component from- a composite tele This would be the case in the present ampli-A vision signal; fier if it were not for the'regenerativefeed-back Fig. 2 is a graphic representation of typical provided. It is seenthat the tube has a television signalsin relation to the plate~ cur 50 voltage zero grid-to-cathode' bias' corresponding' to 'the rent versus grid voltage characteristic of a'vacu maximumY positive peaks vof the signal; If’the um tube; ~ signal applied to the input circuit ofthe'tube Fig. 3 illustrates typicalV television signal volt' is relatively weak,Y the‘iarnplitude is-insuii‘icient ages appearing in the circuits of Fig. 1 which are to carryv the grid sufficiently' negativeV to' effect 55 of assistance in explaining the operationl'of-l the cut-01T of the' plate >current below the VbasesV ci illustrative embodiment of the invention; and the pedestals of the synchroniaingimpulses. The Fig. 4 is a circuit diagram of a television re result produced in such a >case vis >illustrated in ceiver, partially schematic, embodying the pres Fig. 2 in which the curve 26 represents the plate pends upon the polarity of the composite tele ent invention as employed tof recover the video component from a composite television-*signal in a form suitable for Veffecting a gamma-«control of the reproduced picture. Y _ N , current derived from a weak »signal‘applied‘ to a‘signal limiter of the grid cut-off type. Thus, separation of the video and synchronizing sig nals by such a device is impossible, unless the sig Referring now more particularly to> Fig; 1 of the drawings, there is shown> schematically-_a nal strength is increased by amplification before superheterodyne receiver embodying-the present the signal is applied to the limiter. invention in a preferred form. Thetelevision Y y _ i Á ' , The apparatus in accordance ¿with the present signals which are interceptedV by an antenna I0 are ,converted into an intermediate,` frequency invention combines the amplifying ¿and limiting operations in the circuits associatedv with'av single video signals andthe synchronizing impulses are tube then would' nuctuate'asiiiustrated‘pyftrie three-element tube. Assume for the~ moment and demodulated by conventional radiantvenergy when a compositeftelevision signal having receiving apparatus I I , labeled inthe drawings ¿as 70 that, theY form of theï'curve 24'of 3Tis1appl'ied'to “Frequency converter, L'. F. amplifier, detector.” the input circuity of`the'ftube I 5*,"l the'` feed-back The rectified television signals consisting Aof the circuit is inoperative.V The ' anodefvol'tage‘ ->ofthe ampliiiediby a video amplifìerfIZ.` . Theampli fled yideo signals are impressed upon the'grid _ curve" 21 and the‘cathode' voitagè‘as‘ shown by ...2,405,290 6. Vthe curve 28. _The _ranges ofvariation of the nal strength. @anode and cathode voltages are substantially dif .maximum amplitude variation under these, con ferent by reason of ' the constants selected for -the resistors Il and .29. Satisfactory operation V.using the tube of the 6J 5 type has been secured ï with the resistor I1 having a value of a few thou sand ohms and the resistor 25 `having a value - ofapproximately twenty-five times the value cf ditions. . cathode increases as the amplitude of the signal increases in a positive direction. 'I'he amplitude of the positive anode potential decreases as the amplitude of the signal increases in a positive direction. >As would Ibe expected, the voltage _ developed in the cathode follower resistor Il has a degenerative eiiect upon the input circuit of . « ’ . , , ,_ signal component will be positive and the syn chronizing signal lcomponent will be negative. By the action of the amplifierthe positive Video signals will be amplifiedA in the output circuit of the tube distorted in amplitude and the negative synchronizing signals will be suppressed. Also, since ,the magnitude of the distortion of the amplified Video signals depends upon the ampli tude of these signals, the signals representing . `Actually, however, a portion of the anode volt age is applied to the cathodeiof the tube by means of a direct current coupling comprising ‘the vari-. able resistor I8. This feed-back voltage has a regenerative effect upon the input circuitof the - ` i5 in reverse polarity to that described, the video The amplitude of the positive potential of the the-tube I5. , It is evident that,> if the composite television signal is applied'to the input circuit of the tube the resistor Ii. When using circuitelements having these constants, the .battery I9 has a po tential of 300 volts. The 4anode voltage has a greater « zo the darker .portions of the picture are distorted less than the signals representing the lighter por tions of the picture. Consequently, it is seen that the ampliiier embodying the present inven tion is also susceptible of operation, whereby to eñîect a gamma control of the reproduced picture. Such a use for the ampliiier embodying the instant invention is illustrated in Fig. 4. Circuit elements corresponding to similar `elements of Fig. 1 are given identical reference characters in i tube and, by means of a suitable adjustment of 25 the two iigures.` The demodulated composite tele the resistor> I 8, the regenerativecornponent is vision signals are impressed upon the input cir made large relative to the degenerative compo cuit of the tube I5 in such a mannerthat the nent. . video signals are of positive polarity and the .The result produced by the regenerative feed synchronizing signals are of negative polarity. .back voltage is an amplitude distortion or expan '30 The output circuit including the resistor I1 is . sion of the signal applied to the input circuit of connected to the input circuit of the video am the tube I5. Also, by reason of the described com , pliñer I2... Also theldemodulated >signals, are im-bination of regenerative and degenerative com ponents which have materially diiierent ranges 35 pressed upon a synchronizing and video signal separator 33 which may be of the type illustrated of variation, there is produced a net feed-back in Fig. 1, if desired. v voltage to the input circuit which is non-linear . with respect to the signal voltage. The eifect of The operation of the distortion amplifier as a .device for effecting a gamma control of the re the feed-.back voltage is much greater in the region of the positive signals than it is in the» 40 produced picture is believed to be obvious in view of the detailed description of its operation as a region of the negative signals. Consequently, the limiter. By reason of the described polarity of ~positive portions of the signals are expanded the composite television signal as applied to the ' more than the negative portions. input circuit of the amplifier tube I5 of Fig. 3, Also, by reason of the direct current coupling ' the video component appears in the output cir between the output and input circuits of the tube, 45 cuit-in a non-linearly distorted form. Such a a portion of. the positive potential _of the battery signal, when applied to the grid of the cathode I9 appears as a steady direct current voltage at ray tube I3, controls the reproduction of the pic the cathode. The effect of this arrangement ture in a manner to vary the contrast in the light is to shift the average bias ofthe tube further and dark portions of the picture. toward the point of plate current cut-oli. 50 While there has been described what at present f Thus, when a signal such as that represented is considered the preferred vembodiment of the by the curve 25 of Fig. 2 is impressed upon the invention,k it will be obvious to those skilled in resistor IS of Fig. l, the resultant expanded sig the art that various changes and modifications nal applied to the input circuit of the tube by the may be made therein without departing from the ' apparatus in accordance with this invention is invention and, therefore, it is aimed in the ap represented :by the curve 29 of Fig. 2. It is seen pended claims to cover all such changes and that the synchronizing signal portions have been modifications as fall Within the true spirit and amplified considerably more than the video sig scope of the invention. nal portions. Also, it is seen that the bases of ~ What is claimed is: the pedestals of the synchronizing impulses coin-` 60 cide approximately with the point of plate cur rent cut-oli. Such signals produce plate current in the tube I5, having the form illustrated by the curve 35. Thus, the video signals are sup pressed completely in the output circuit. The Voltage ñuctuations actually occurring at the anode of the tube I5 are represented by the curves 3I of Fig. 3. Similarly, the actual voltage 1. Apparatus for effecting a non-linear ampli- Y fìcation of a composite televisionsignal including video-'and synchronizing components comprising - a single vacuum tube having input and output circuits, means for impressing said composite television signal upon said input circuit, a re generative feed-back connection between said output and input circuits to eifect a non-linear amplitude distortion of said amplified signal, and means including said feed-back connection to are illustrated by the curves 32. Either or both 70 reproduce in said output circuit onlyone of said of these twoV voltages may be utilized for synchro composite television signal components. nizing purposes. However, it is preferable to 2. In a television receiver, means for receiving use the voltage derived from the cathode for the a composite television signal including video and reason that it does not vary in its maximum fluctuations occurring at the cathode of the tubel amplitude appreciably with variations of the sig synchronizing components, an ampliñer tube hav 75 ing input and output circuits, means for im l2,405,290 '7 pressing said received television signal .upon the input circuit of said ampli'iier tube in such ‘a manner that said video component is positive and said synchronizing component is negative, means for regeneratively coupling said input >and out putcircuits to produce in the‘output circuit of said amplifier tube non-linearly distorted video signals, and means including said coupling to effect a gamma control of a picture reproduced 8 including saidcoupling to `reproduce said syn chronizing lcomponent in said output circuit to thev exclusion of said video component. 7.' An amplifier for alternating current signals comprising a vacuum tube having an anode, a cathode and a control grid„a resistance element connected between said grid and said cathode, means for impressing said signals upon a portion of said resistance element, and means including 10 a resistor for impressing upon another portion from said video signals. ' of said resistance element a potential derived 3. -Apparatus for separating lthe video and syn- , from said anode, whereby to increase the `poten chronizing components of a composite television tial of said grid with respect to said cathode suf signal comprising a vacuum` tube having an ficiently to suppress in said output circuit signals anode, a cathode and a control grid, an input of less than a predetermined positive amplitude. circuit including said cathode and said grid, an 8. An amplifier for alternating currentsignals output circuit including said Ycathode and said anode, means comprising a direct current cou pling including a resistor between ,said anode and said cathode to effect a non-linear regenerative feed-back from said output circuit to said input circuit, and means including said coupling to re produce said synchronizing component in said output circuit to the exclusion of said video component. 4. Apparatus for separating the video and syn chronizing components of a composite television signal comprising, a vacuum tube having -an anode, a cathode and a controlV grid, an input Y circuit including said cathode and said grid, an comprising a vacuum tube having an anode, a cathode and a control grid, a series connection of a grid leak resistor and a cathode follower re sistor, means for impressing said signals upon said grid leak resistor, and means rincluding a variable resistor for impressing upon said cath , ode follower resistor a potential derived from said anode to increase the potential of said grid with respect to said cathode, said anode-derived po tential having a> magnitude determined by said variable resistor whereby to suppress in said out put circuit signals of` less than a predetermined positive amplitude. .9.. A combined signal limiter and ampliñer for output circuit including said cathode andsaid 30 separating the synchronizing component from a anode, means comprising a direct current cou composite television signal including video and pling between said anode and "said cathode to synchronizing components comprising a single effect a regenerative Yfeed-back-from said output space discharge device having input and output circuit toY said input circuit, and means includ means responsive to signals having ing said coupling to reproduce said synchroniz 35 circuits, greater than predetermined amplitudes for auto ing component in said output circuit to the ex matically biasing said input circuit to effect out clusion of said video component. put current cut-off of negative portions of said 5. Apparatus for separating the videoV and signals, means coupling said output and input synchronizing components of a composite tele vision signal comprising a vacuum tube having 40 circuits responsive to signals having lesser than said predetermined amplitudes for regenerating an anode, a cathode and a control grid, an input circuit including said cathode and said grid, an output circuit including said cathode and said anode, a resistor connected to said cathode in se ries with said input and output circuits, a direct current coupling including a resistor between said anode and said cathode, and means includ ingV said coupling to reproduce said synchroniz ing component in said output circuit to the exclu sion of said video component. 6. Apparatus for separating the video arid synchronizing components of a Composite tele vision signal comprising a vacuum tube having an anode, a cathode and a control grid, an in put circuit including said cathode and said grid, an output circuit including said cathode and said Lanode, self-biasing means in said input circuit including a condenser in series'with said input circuit and a leak resistor in shunt with said in put circuit, a resistor connected Vto said cathode in series with said input and output circuits, a direct current coupling including a resistor be tween said anode and said cathode, and means said last named signals, and means including said coupling means, to eiiect said output current cut-oit of the video component of said last ., named signals. Y l0. A combined signal limiter and amplifier for separating the synchronizing component from a composite television signal including video and synchronizing components comprising a vacuum tube having input and output circuits, means in cluding a condenser in series with said input cir cuit and a resistor in shunt with said input cir cuit and responsive to large amplitude signals for automatically biasing said input circuit to effect output current cut-off of negative portions of said signals, means including a resistor cou pling said output and said input circuits for re generating small amplitude signals, and means including said coupling to effect said output cur rent cut-off of the video component of said small amplitude signals. MADISON CAWEIN.