US2352488A - Video signal control system - Google Patents

Description

@H6275 1944- L. F. MAYLE VIDEO SIGNAL CONTROL SYSTEM Filed Sept. 19, 1942 3 Sheets-Sheet lI2 R ma Nl.- EVI VA NM F.

June 27, 1944;` L. F. MAYL I yVIDEO SIGNAL CONTROL SYSTEM Filed sept. 19, 1942 3 sheets-sheet 3 Y E N R O W A INVENTOR MAYLE K Patented June 27, 1944 UNITED STATES 1 12,352,488 VIDE() SIGNAL CONfrrtoL sYsfrEM Louis Fl Mayle, Fort Wayne, Ind., assigner to tf t Farnsworth Television and'RadionCorporation, j

a 4corporationoftl )e laware ,Y

^ Application september 19, 194g', lSarita 1\Io. "4 5s, 92 V' This invention pertains generally `to television systems and more Vparticularly to amethod and apparatus for controlling and adjusting the l gamma. of television signals.

It is known in the art that the quality of'a reproduced television image'may be impaired as a result of the factthatdetail effects such as are given toapicture by its natural color, are lost. Alsol television transmitting apparatus may be of Asuch a nature that video signals aredistorted and decreased in level to such an extent that the reproduced signal does not create a faithfulv image of the initial picture subject matter. v'Furthermorepv each element of a reproduced television image' may be exactly proportional'in brightness to the corresponding element of the original image and, therefore' the reproduced frequency s i'gnalwit'hout' affectingthne' overall magnitude ofthat signal.

A stm further, object ofthisfinvetiorifi to provide, in a television system; a signal trans'- lating channeljfor controlling they-anima "fof, a

video frequency signal andirnpartingoyerall v'gain to that signal'. Y `There-is provided, in accordancewith this 'in'- vention, a signal translating'channel in atelcvision system whichconsists'ofja multi-electrodev electronic device for controlling the'garnma'fof a video frequency signal. 4The'electronic'device image 'may appear' fiat and lack efficient contrast to be interestingjto an observer. This effect may be the result of4 the .fact that the contrast ratio andl average brightness of the reproduced -image is generally lower in comparison to the corresponding characteristics of the picture being transmitted Therefore, it is frequently desirable in television transmission that videol frequency signals be distorted in certain portions of their amp1itude range for providing better fidelity of reproduction.'V 'Such distortion is known as gamma control, more particularly delned 4as the slope of the 'input-output or stimulus response curve of the system plotted'to a logarithmic scale. "Thereare known in the prior art, various devices for controlling thegamm-a of video frequency signals to provide faithful reproduction cfjtele'vision images whereby the contrast between the light `and dark portions of a television image 'is increased. These devices are subject to 7 Another object of this invention is to provide',`

in a television system, apparatus for imparting a gamma to a video frequency signal which-may be either greater or less than unity.

It is a further object of this invention to provide, in a television system, a video signal translating'device for varying the ygamma of a video should preferably comprise at least ftwoctlfol Y electrodes and an anodafone' control electrode having an input voltage-anode current character'- istic which tends to provide a gamma greater unity, the othercbntrol "electrode havingfan in'- put voltage-anode'Y currentcharacteristic which tends 'to provide a gamma lessthan unity.' The gamma controlchan'ne'l is connected inthe video f frequency amplifier circuit' Aby means of a voltagedividing device Isuch as' a pair of oppositely connected potentiometers, oneipotentiometer' being connected'thrllfh its adjustable tanto one'control electrode of the electronic device and` f ,th,e other potentiometer being connectedthroughtits adjustable tap' ,to the other 'control electrodefof the electronic jdevice ther ebyto provide afmeanfs for dividing a video signaland lapplying'fa.por-

tion thereof to each controlelectrcde jof`theele`cftronic device. .The `adjustable taps 'on the oppositely connected potentiometers are `preferably 1 control which may be adjusted tofin lout in theappended yclaims mechanically ,connected tepnrorid" cr'easethe voltage'drop across onepotentioxneter'and simultaneou'sly decrease the potential acrossjthe other potentiometer. Thus, 'thelportion of video frequency signal impressed on each control 'electrode may be varied 'to exactly .apportio'n the video frequency'signalbetweenfthe two control electrodes, Since the` characteristic 'of Ione,` control electrode has a gamma or exponent. greaterj tha n unity and the characteristic of the'other. electrode has a gamma or exponent less than unity, each portion of the videofeguencylsignal may have im-` partedv thereto al gamma either greatcrfofr less than unity; l

For a better 1'"lnderstandingr of" the 'invention together withl further .objects and radvantages thereof,' reference is Imade yto the'following' description taken in connection with the accompanying drawings, and 'its'sc'ope will be 'pointed In'the'drawings: rf

Fig. `1 ,isa' circuit diagram of a complete tele.

l Common' vision transmitting system illustrating conventional portionsof the system in schematic form andincluding avideo frequency vsignaltranslating channel' embodying the present invention. `Fig. 2 is al graph; illustrating the grid voltage plate current characteristics of the control electrodes of the signal translating channel'.

Fig. 3 is a graph, illustrating the operation,

under certair'i-v conditions', of thesign'al translat-` ing channel shown inlFig 1I,

Fig- 4 S a gaph, illustratingthe operation,

under certain conditions, ofthe signal translating channel, shown in Fig; l.

1 The .sliding contacts'31y and 381, of potentiometers Referring to Fig. 1 of` the drawings, the'tele- .vision transmitting system includes arconventional picture pickup tube I; anf objecta'nd` a.`

lens system 3 for focussing an.;image on the" screen 5 o f tube I'. The tubev I is provided with electron gun 1 and a power supply i2 to provide an electron beam for scanningscreen-fS.' This f tube is illustratedby way of example, as conventionsiim' the television art, andy it is no tfintended that this invention shall bejl'imitedltouse with any particular type of pickup device; l v

`AThe' cathode" ray'b'eamipf tube tisdeflected to a source of` line scanning waves and'eld scanning waves I3-and Il respectively. `The sources t3 and'; tifare. controlled" by. the synchronizing pulselgnerator 'I6 in 'fa mannerwell known in the' prior art 'Ihe output Q`f screen 5in tube I `is irn'pr'es'sed across the output resistor Vi8 connected to"V the in-put circuitof. the picture signal y. .'I l"li ere is provided necte'dfto the Outputcircuit- 2210i picture'signal ampliiler' 2l and the output circuit 26 of channel is connected to `al conventional radiant'energy transmitter 3`0`wh`eein the video frequency signal v is impressed 'on a carrier and radiated vby antenriajli; Q

Signal translating channel 2 5 consists of `the multielectrode electronic device 32. and a voltage dividing device consisting of potentiome ers 3l and 35 mechanically connected in tandem. Such aconnectionfis, wellknow'n tb those skilled in the artand is, therefore ,not shown in detail. Electronic device 32 is chosen to have thegrid voltageanode Acurrentv characteristics shown in Fig. '2, Curve' A, 'the 'grid voltage-anodecurrent `characteristic of grid lfhasan exponent o r gamma less 'than"unity Curve B, the" grid voltage-anode currentcharacteristic of grid 4I, has an exponent or. gamrnaggreater thanunity. For ideal gamma controlin accordance with this invention, curves and`fBfof Eig. 2 'should be complementary, but satisfactory gamma control is obtainable eventhou'gh these curves are not exactly complementary," Curves A and .B are substantiallyl parabolic in forrfn and, therefore,

follow the'equation X41/ where exponent a 'is the gamma. i l

The `grid voltage-anode current vcharacteristics fof Agrids 40 and Il may be illustrated also-on a log-log scale by converting theequation yf-#Xa to its logarithmic formwhich is .log y=a log :c and plottingalogarithmic graph. Curves Aand B would then be vst'raigh 'tl lines the slopes of which would be determined by the gamrnaor. term a in thc logarithmic equationi, If gamma is chosen plliler'ZD, there Vbeing a connection 2ly pi'oatnland pictuefsignal amplifier for" impressing ablanking pulse onainplier 20 as is 'fwellknownintli prior art. 'n accordance'with this in "ventiqn, a' signal tranSIatingchannel 25 con 4 cated at 39, they may be adjusted simultaneously 34 and 35 resp ectively; are connected to the control electrodes 40 and`v H respectively, of tube v 32. Sliding contacts 3T andii are mechanically connected", as indicated at 39, to provide a coml'x'non control for pctentiometers 34 andl 35, suchy vthat each sliding .contact may be simultaneously moved in the same direction. Control electrodes and 4lv are biased through grid resistors 44 and respectively in `accordance with the'potential drops as determined by the adjustments of the taps oflbias resistor 46' which isfconnected toa f source of biasing potential, C-

The anode 42 ofy tube 32 is connected to transmitter 3U and to a source of anode potential 'B+ togetherwith a peaking circuit consisting of resistor` 4 6 and inductance 41 which is conventional in the prior art. lThe remainder of the signal translating channel consists of well known circuit elements and connections and. therefore, it is not necessary that they be explained in detail. It should 'be noted that signal translating'channel 25 may by a cathode ray beam tc impress a video ire-y quency vsignal consisting of a.chain of electrical ir'npulsesfacross the resistor I8 representative of the successively scanned elemental areas 'of -thc object 2. The video frequency signal appearng across vresis tor. -l"!l is amplified in a. conventional manner by picture signal amplier 2D and impressed on the output circuit 22.

Signal translating channel 25 imparts a gamma to the picture signaleither greater or less than unity dependingfupon the adjustment of the potentiometers 34 and 35, and upon` the gamma of the video signal delivered by the picture pickup device. Sincethe adjustable sliding contacts 31 and 38 areimechanically connected as indito `move upwardly or downwardly, increasing,rr the output oi one from zero 4to maximuinand de-.

creasing the output of theotlier from maximum to zero. Thus. for purposes of illustration, it may be assumed that the mechanical connection simultaneously moves sliding contacts 3 1 and 38 upwardly o'r downwardly. so that the signal appeering in the outputcircuit 22 maybe divided in any desired proportions between the Acontrol electrodes 4'0 and 4I.

Fig. 3 illustrates. by means of agrapli, the resultswhen the voltage across the output circuit 2 2 is impressed on the grid 4 0or grid 4I `of -tube 32. It also illustrates the 'results when one-.half ofthe voltage across output circuit 22 is impressed on each of. the grids 4U and 4|; Curve c in Fig. 3 represents the fu-ll video voltageY across output circuit22. For purposes ofzexample, curve.

:on `grid 54,0.

'.e represents a Avideofvoltage `wavefmoving eight equal: increments ofvbrillianceV from black vto gridA M, the .voltage resulting l in `the output 'of tube2,3 isrepresented by curveC.` Curve .B is assumed to have. an exponent` or gamma greater than'unity, jFor'purpose's ofzzexample, curve B- has anzexponent of 2. Thus, vthe Avideoou'tput at` 2.2' has had lits, gamma inci'easedby a factor-of 2.

Since 'curve ehadl'a gamma. of"1, curvellC hasfa gamma of ,2. -From'inspection of curve Citjis ap,-

par'entfthatthe lblaclfrfportion of the signal is `expanded .while thewhite portion is contracted.

, Thus :black'portions Yofth'e transmitted' picture areiemphasized with respect to its white portions.'

" 'It the potentiometer settingis `changed ysotliat both sliders 3l andf38 are moved upwardly to the fuppe-r limit of 'their movementpfulllvideo voltage e-,is impressed on theigridll' and thereiszero videoyoltage-ongrid M.l Again referring' to Fig.

3., there `is shown a projectionof curve e] to curve A," which is Athe characteristic of grid 40, and voltkageHlappears Vin the output of tube 32,?tbi`svoltage having-a gammacorrespondin'g to `Vthatof curve VA which'is Afory this-example', equalto 1/2. Inspectionof curve AH 'reveals that the white por-V tion ofthe signalisgexpanded and the black -por tion1 is contracted thereby 'to' emphasize v,the white 'portions of the'transmitted picturewith'respect i to lits blackportions. ".'Inthis case the -gaminaiof curve'ie Yhas,beenrchanged fromi to 1/2 as `repre- By adjusting V the Sliders ai-andxss ef the petentiometer to the half-way position, one-'halfbf 'the `voltage e-is impressed ion eachof thegi'ids-U `vanddl. Thereisillustratedin Fig.3 by the curve 1/'e.. `Projecting this ,curve to ytliefcurves A `and Bthereresults the` curves 1D andD respectively.

curve' D having' a gamma ,of k2". By` adding these two curves, there is obtained the output voltage which appears in the lanode circuit ,oftube`32. This." s=illust-ratedY by thecurve D-i-.D `whi'ch -has a gamma of approximately one since it lies in substantially a straight line. In this case there is substantially no distortion of signal.' Thus, the transmitted picture is not distorted as regards contrast between black and white.

Fig. 4 of the drawings illustrates the results when the potentiometer is adjusted to impress 3A of the output voltage in circuit 22 on one grid and 1A of the output voltage on the other grid. In

.the first instance, let it be assumed that the po-l tentiometer is adjusted downwardly to the point --where 135, of the output voltage in circuit22 is -a curve F-l-F' having a gamma greater thanunity and expanding in amplitude the black portion of the signal, and contracting the white portion, but

not as much as represented by curve C of Fig. 3.

Reversing the above procedure, the potentiom- Y eter is adjusted upwardly until the sliders'31'and 38 impress 1A; e. e being the output voltage in circuit `22, o n grid 4| and %Je ongrid 40. By

quencylsisnal.

I acter-istics'such asfthose 'illustrated l these illustrated iuris; may utilized' fi .s

. illustrated herein.

7 noty haveja gammagreater jecting the voltage l/l;l ecn vrnirye -Bthereis tained? a voltage componentG. By'a'dding'cu es- The lcurvSAfald B of 'Figs i, to have'an'rbitrary exponent or gflma'forpurposes' of fillu,stration,f.;but it willbey obvious" tOi-'those ,.suchf'rves isy .dependent MP6- he ,grid ,Chre ,eristics of thefpar'ticular 'tube edjin thejsignal 'translating lhannel;z It ha` ...blee fun'd that tube conventionally desifg'natedfas 'a' 'SLThas c,

One ohara'cteristichaving a gamrnai'gr ter. than unityand the other characterist j h'avin`g` a ygi-,im' ma lessthan unity.v Any multieelectrode v zzu tube whose, grid characteristics are similar-tol `signal translatingl channel4 for' the purposes complishin'gf the .objects of" this invention therefore', it ie 'not intended 'that shall be'lifmited tothe particu It should ybeunderstood;thatdifferent of' gamma control may'ibe 4intredueedingef television system tocompensatedistortion ofa video 4 frequencyl signal which' may result.; for example, from the 'particular type 'of' pictur'elpiclgup 'tube which` isused,y orffrorn the particular-transmit,- ting or receiving"channel.l Therciirves'A'a'n such Aas i1111stratedf '.Fig'. 2. 'o`f` thedrawings, ned ty'v less than' un'ity by `'reason' ofthe factjthat' distoif tions of "theisgnalA may require that one curve. have algam'maof. unitvfand the othe'r'have gamma" g'reater thany unity.' AIt lmay be 'fdesirable that both curves have agamma greater; than' unityforless than unity or it'may bdcslrable that a portion of each curve have la gamma of unity and another portion have a gam-ma greater or less than unity. From this it -will be obvious to lthose skilled in the art that it is within th'e scope of thisinvention to provide'agamma ad.-

justing channel for distorting a video frequency signal to any degree which 'may benecessary to bv one or moreeleent considered the'preferred embodiment of the invention, it will'l be obvious to those skilled in the art,.th'at various .changes and modifications 'l may befmade .therein without departing from the' invention," and 'it' is, therefore, aimed in the lap.-

pended claims tocover all such changes and modi.-

skilled `infthe art-that `the'shapeorexponent".of

a .circuit oi those 1.*,Atelevision signal translating systemfcomn prisinga signal channel, a Viirstimeansl for con- -vtrollinga ow of electrons, a second means `for controlling a owwofelectronathe input'voltageanode current `characteristic of said first means providing a gamma greater than unitythe input voltage anode current characteristic of said sec-4 ondmeansproviding a gamma less than unity, 'Y fand ervoltage ,dividing means coupled-between said channelz-and said rstand secondineansfor dividing the television signal andimpressing it on `each offsaid first .and 'second means-,to-'impart a Igamma to theresulting` signalwhich Vmay be either greater or less than unity.

j 2'.` Av television .Y signaltranslating usystem compriising a y signal channel, `a `inulti-(electrodei electronic-deinem the input yvoltage anode current `charanteristic of one electrodeprovidingfagamma greater than` unity, the input, voltage-anode. current characteristic of another electrode providing fa gamma lesszth'an unity, and a- .voltage dividing fmeans, coupled betweenk said channel and said electrodes for simultaneously dividing the teleyisiorr signal and impressingv iton each of said electrodes toinparta gamma tothe resulting signal which mayeither greater or less than 3. television signal transiatingsystemgcom- "lprising a signal channel, a linulti-,electrode ,electronic device, the input voltage-,anodecurrent anode current characteristic of another electrode providinga gamma lessvthan unityy a'lfst outn y 12,352,488 4 ilcations asl fall within thetrue spiritandvscope tronic n device,y 4the input `voltage-anode current ,ofthe'invention., Y v l l characteristic clone grid providing a gamma l What I claim is: greater than unity, the input voltage-anode currentvcharacteristic of another grid providing a gamma less than unity," a first kinput poten- ,tiometer coupled between said channel'and said onegrid, a second input potentiometer coupled between-saidchannel and said other grid and control means. l common l to saidy .potentiometers n the television signal-insaid channel mayfbe divided and impressed on eachl of said grids to impart. afgammato the resulting signal which may be either greater or4 less .than unity.,V

5. I'I'he methodof translating atelevision signal comprising the steps ofi-dividing a kvideo signal intoJat least two components, imparting agamma,

greater` than unity to one of said components. .imparting a gamma lessthan unity tothe other vof; said components and combining said com-po- Ynents to obtain a signal having a gamma either sum of which is equalto said given voltage, im-

putpotentio'meter coupled between said ,channel andsaidV one electrode, a 'seond inputpotentiorneter'coupledbetween'said channel and said `other electrode and control meanssforfoperatingsaid potentiometers toincreasevthe output of one potentiometer and decreaseftheoutput Vof the. 'othery potentiometer vwhereby the television "signal may be divided and. impressed v[on `eachol said yelectrodes to. impart a gamma to the resulting signal which vthan unity.

prlslnga television channel, a multi-grid 'elecparting a gamma greater than unity to one of said components, imparting a gamma less than unity to -the other ofl said components and combining-saidlcomponents to obtain a signal having aV gammaeither greater or less than unity and an amplitude substantially equivalent. to the amplitude of said video signal.

'7. The method of translating a television signal comprising the steps of dividing'a' video signal of a given voltage into voltagecomponents the sumr of which ls equal to said given voltage,

may begveither greater or less v4. Ateievisin signal trailsiatingsystm omlimpressing each component on `a different gridof anzzreletron tube, imparting, a lgamma greater than-,unity to one oi." said components, impartinga gamma'less-than unity to the other ofsaid components and';controlling,the output o! said electron ,tube 'by said components to obtain a ,signa-l having a gamma either greater or less than unity and a magnitude substantially equivaient -to the magnitude of the input signal.

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