Aperture compensation for an RCA type 6326 Vidicon camera tube

Item Type text; Thesis-Reproduction (electronic)

Authors Enloe, Louis Henry, 1933-

Publisher The University of Arizona.

Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.

Download date 01/10/2021 14:23:36

Link to Item http://hdl.handle.net/10150/319717 APERTURE COMPENSATION FOR AN RCA TYPE 6326 VIDICON CAMERA TUBE

by Louis H. Enloe

A Thesis submitted to the faculty of the Department of Electrical Engineering in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in the Graduate College, University of Arizona

1956

Approved ^ J Din^g£or of Thesis Date

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgement of source is made. Request for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major depart­ ment or the dean of the Graduate College when in their judgment the proposed use of the material is in the intere&s of scholarship. In all other instances, however, per­ mission must be obtained'-from: the author.

SIGNED file author wishes to thank Mro Harry Stewart j, professor In the Electrical Engineering Department3 for his technical guidance and adviee -in: the preparation of this thesis,. \ . - TABtS OF GOMTEMTS

Ghaptefe ' Page

IMTSODIJGTIOlif : .O O <> O' O O 6 O O O 0 0 0.0 0 0 0.0 0 0 0 © o b o o o o o o o X lo THEORY OF SCANNING © o © © o o © © o©o©o©© © ©©ooo©©©©. 3

- i:iX:oX;;'; Basic'- Elsaezits of S: TeXevisioa- 0.0 0 0 0 0 0 0 0 0 0 6 oooo oboooo 0 0 0 ©OOOOOO 3

102 Ra S O IXX*fc 2.0$^ o 0 0.0 ©OOOOOOOO © o o 0 0 boooooooo o 3

103 General Description of Scanning © © © © © © © 4

Xo4 Mathematical Analysis of Scanning © © © © © 4

2© RESPONSE OF A FINITE AP1RTGRE © © © © © © © © © © ©o© © 9

2© X Introduction © © © © © © © © © © © © © © © © © © © © © © © © © © 9 2©2 Mathematical Development of Aperture Response © © © © © © © © © © © © © © © © © © © © © © © © © © & © © © 9

3» ■ APERTURE RESPONSE LIMITS RESOLUTION. © © © © © © © © 13

3 ©1 Response Of RCA Type 63 26 Yidicon © © © © © 13 3©2 Aperture and Video Amplifier Response Comparisons © © © © © oo©©©©©©©©©©©©©©©©©©©© 13 4© PRINCIPLES OF APERTURE CORRECTION © © ©©©©©©©© 1?

4© 1 Intrcduction © © © .© © © © © © © © © © © © © © © © © © © © © © © 1Y 4©2 Phase Shift Distortion of Sharp Cutoff Filters ©©©©©©o©©©©©© © © © © © © © © © © © IS

4 ©3 Noise ©oo©©©oo©©©o©©oo©©0 0 ©oo©©©©©©©©©© IS

4© 4 Kinescope © © © © © © © © © © © © © © © © © © © © © © © © © © © © ©j. 25 4©5 Overall Resolution Improvement © © © © © © © © 26 5© GHOISE OF APERTURE COMPENS ATION NETWORK © © © © 2?

5 © X Introduction © © © © © © © © © © © © © © o © © © © © © © © © © © 2.7 Chapter • ' Pag€ 5o2 Parabolic Response Function , s 0 =. o»»o o»6 30 5»3 Aperture- Compensation Circuit ooeoo-oooo 32 6o EXPERIMENTAL PROCEDURE 0»=»=o=c=»o0=»oo==A»o 37 6ol Introduction oooooooobooaoooooooooooooo 37 6 o 2 Equipment ooooooopooooooooooooOoooooooo 37 6*3 Television System Operation *******,:*** - 39 o o A Dat a Taking 0*000*000 ooo****** *oo * * * o * * i$-0 7» EXPERIMENTAL .RESULTS * * * * * * * * * *.* *** ** ****=.* * * 42 7*1 Introduction 00*00*000 0000000* 00*0-0000* . 42 7 *2 Oscillograms o * o * * o * o o o o o © * o * * * o * o o o o o * 42■

7*3 Response Curves of the Compensated Aperture 0 *0 *000000*00**000*0*0*00©©000 44 7o4 Discussion of Errors ** s ,*.•«« * ** *»** *« 44

7* 5 Conclusion * * * o-o * * © * * o * o * * * © o o *. © * * © 0 © © o 45 ' APPENDIX Development of Transfer Function of. Output Stage of Compensation Circuit © ©^ 4$ 1

: INTRODUCTION

Closed loop television systems have become of in­ creasing importance .during the past ■ few; years; .-/Many.^ processes in industry, which cannot be directly observed because of dangerous environmental conditions, can be ob­ served by a television.system. . There are-also many cir­ cumstances.in which one person can simultaneously observe- multitude of scattered 'events by means of a. television net worko Television-proves to. be a very practical and useful solution to many such commercial and military'problems„ The purpose of television is to provide the viewer

with an accurate visual representation of some event! The desired quality of the image, of course, depends upon the individual circumstance. "The resolving powers of modern closed loop television systems are mainly limit­ ed by the finite size of .the electron beam in the camera tubeo The electron beam forms part of the: electrode-"u : optical transducer and has-rigid requirements placed" . upon the total current in its beam.- Practical consider-, -ations place a, limit upon the current densities .andh 1 thus the cross sectional area of the beam. The. effect of the finite size of the cross sectional area, iso to - reduce horizontal resolution. This is generally. referred to as the. wap@rturimg effe.ct”. of the electro® - beamo

B r o Pierr® Mertz and Dr0 Frank- Gray ^ have made a study ®f the theory of scanning from which it is possible to - show that the effect of a finite scanning aperture can b© replaced by a point aperture in series with a low pass electrical filter0 This suggests that an amplifier with a nonlinear frequency response might -be incorporated into the video channel which would . compensate for.the aperturing effecto A significant increase in resolution should: be possible providing the signal-=t@=n©ise .ratio, is sufficiently higho . The object of this thesis is threepfold: \ 'lo 'T© derive'an .expression for the response of the equivalent low.pass filter representing the aperturing' • effect of;the finite electron beam of an RCA ^ type

. 6326 'Vidieon;camera tubeo

2 o' To analyse a network that provides compensation for the aperturing effect* ■ • 3-o ;. To investigate experimentally the improvement - in horizontal resolution^

.■ 1 - Pierre- Herts ' and Frank Gray 3 ifA Theory of Scanning, and Its Relation to the Characteristics of the Transmitted Signal in Telephotography ' and Television w Bell.System . Technical Journal,, July.g 1934o - ■ . 2 Radio Corporation of America0 '• ’ . ■ THEORY OF SCANNING .(lol) Basie Elements of a Television . The basic function of a television system is to transmit a succession of images:by electrical means in, such a manner that they may be reproduced.for.view­ ing by a distant observer0 A television system consists ©f three basic elementsp -The - first element is an electro^optical transducer which transforms optical information obtained by-a lens arrangement into e!ee= ' trical information^ The second element consists of...the communication channel which is used for the trans" mission of the information to the receiving stationo : The third element is an el@etro=>optieal transducer which donverts the electrical information into an optical

image 0 ' . . £l02 l Resolution - The subject of resolution will arise many times in this thesis and a thorough understanding of it is necessaryo. The degree to which the. image at the receiving station resembles^the original image at the- sending station depends upon. the quality of the electron-optical

transducers and transmission channelo The defining - ability of a television system is measured by a quantity called "resolution”o Resolution in television is expressed in terms of a certain number of lines dis­ criminated on.a test chart = For a. number of lines' H ■ ' (.normally? . alternately black and white) the'-width of each line is 1/N times the picture height „ Rote tha.t the number, of- lines N is defined in terms of the image ■ : heighto Because the aspect ratio is 4/3 5 the width of each . line is also 3/4 N.. times the picture width» (io^) - General Description of Scanning ' 1 Television uses much the same' method for displaying .. - . ' the continuous motion of an image as does a moving ' . picture projector* Hamely, a succession of Vstill” images is displayed in rapid succession, which gives the illusion of continuous motion* The process consists of moving an exploring aperture over the image to be transmitted in a periodically repeated path covering the entire image area* The exploring aperture is associated with an'electro-optical transducer that . generates an electrical signal of magnitude proportional to the brightness of the picture element being explored* The electrical signal is then transmitted over a. transmission channel to an optical-electrical transducer which moves .in perfect synchronization with the first transducer and thus reproduces the original image*

(I#4) Mathematical Analysis of Scanning • Let the brightness variation of the original image 'be represented by the ,function .Bf:J '.where

X arid T are respectively, the horizontal vertical

..coordinateso 45ia@@ the image is seanned in a fashion3 the brightness yariation may b e'

by a Fourier' serieso ^ Thuss ' for a

line J. 1 .

oo •BrrrkX

k - o Ak'cos ( — * ek 1

^oo ' - . j2irkX V 2 1 1 ^ - \ exP ' n w . ■ 0 - X —

1 61 where $ K s= a partiemlar harmonic ©f the . =ooeffiieient of kth .harmonic ly m ,-npmber-of horizontal scanning lines . , 1 = particular scanning being investigated

- • W;. : = image width . . ' : - . r . ' 0^ =p M s e angle of kth harmonie

• ; V - 1 .George Eo ■ Anner9 '^Elements of Television Systems^ .PrentiSja=Ball.9: . Jnc»g:. Hew' Yorks 1551 ' • . ; ' woili' also be a Fourier series® This would'allow'the explicit representation of the two

dimensioEal brightness. variation=' However g in. this . investigationa only the variation in the horizontal direction need be .studiedo The image which will be ; analyzed consists of a ystripw one ’scanning- line wide

and 525'Sdanhing line'letigths Ibngd:. It is assumed ' that the aperture does not respond:to brightness' variations transverse to its direction of motion0, This greatly simplifies the mathematics and the results will

' be. perfectly'Valid o . . ' Equation’111 has more meaning when;it is expressed- in terms of electrical frequencyo Assuming the aperture to be a point9 Equation lol b®Somes

GO, E(tfc) s. c 2 ^ Ak cos + 0, ) 1 k=o . . - ‘ +00: “ l/2 C > " iL exp . (j2-irFnkt) t k=-co ^ :io2 v a'-

■ ■ - _w_ ' ■ - ' -

nP

2 f 0IC6 Zworykin and GeAo EortoB8 wTelevisions^ 2nd Edo 9 John Wiley and Sons^ Ine0 lew Yorkp loYo s 1954 and"6 = transfer funetion of th© electro=optieal 1 transducer T = horizontal scanning velocity T = horizontal period " F '” frame frequency / -n = number of horizontal scanning lines Z , t =" time ; ':Z. ; \

in interesting fact is immediately apparents the lowest possible frequency occurs when -k ■= lo ■ For k = ls

: f s Fnk - C30)C525)(1) = 15g750 cycles per second which is'the horizontal scanning frequency» All frequency components ard harmonics of this frequency 1 c@ = $ k = s ± $ 2 9 0

Equation 1*2 allows the determination of the frequencies of the harmonics of the electrical signal for a given 'television resolution number® Gonsider the following examples n ^ 525 scanning lines F = 30 frames per second M = horizontal resolution = 6$0 television lines

Aspect ratio - 4/3 The number of vertical black or white lines in the image would be 4/3 x l/20 This would also be the value of k for the fundamental frequency of the electrical square wave0 per seeoBdU The hamonies would be of this frequenejo Their amplitmde may he easily @al«

The eosimmsieatioh ©hansel, bandwidth imposes-:a mpper limit t© horizontal resolutioBc Th®' in the above example must have a per see©ad if it is to pass-

of television will he fundamental than the cutoff

•It is component as.: am of a resolution chart@ This 9

- - - . Chapter 2 . ■ ■ ;

RESPONSE OF A FINITE APERTURE - : ■ ' - (2.1) Introduction . , 1 ■ - ' • ■ In Chapter 1 the expression'for the electrical signal was derived upon the assumption that, the aperture was a. point aperture = •. In physical cases the.aperture is of finite size. The effect is to y V cause the response of the transducer to fall off rapidly at the higher'frequencies3 thus decreasing

horizontal resolution. • ■ (2.2) Mathematical Development of Aperture Response . Let the response of the aperture be represented by the function .S (@) 1 where S is measured from the center of the. aperture as 'shown in Figure 2.1. The aperture is assumed to by symmetrical about the center, which 'is essentially true for physical apertures. Now • because the electro-optical transducer responds to the total integrated brightness over the aperture, the ' .signal voltage will depend upon the brightness at x - and the aperture response 3(6). . The brightness' of the image at x+6 is given by. 10

FIGURE 2.1 The above £raph of B(X) represents the imane brightness as a function of the horizontal position. The response of the aperture is not constant over its sampling, area but rather a function of its dimensions. S(&) represents the response of a symmetrical aperture as a function of the distance § from the center of the sampling aperture. The instantaneous horizontal position of the center of the sampling aperture is represented by X. 11

oo Sirk, B(X + 5) = 22 Ai, cos ( n w (X + 5)) k=o k

amd the eloetrie&l sigaal become

E(X) = /S (5)B(X -f 6) dd •. aperiur© .

oo : • 2 irk , - /S ( 6 ) 2 L Ai cos ( nw (X + 6) d8 . ape " k=o • « ' ■ ; . •foo jZirk ==. 1 / 2 / S (5) A._ exp ( nw (X + 6)) ap» -do K 4-00 JgrrkX S= 1/2 y Y(k) A^ exp ( nw )

...... ■ ■ : 2 pi wherl ' -

j2irk8 . • Y(k) - /S(8) exp ( n w : “. ) ap» .. ; . : ';

5 Pierre Herts- and Frank Gray9 n A .Theory of- Scanning and Its Relation to the Characteristics of the Transmitted Signal in Telephotography and Television/8 Bell System Technical Journal a ?ol o 13 s 1934. 12 Expressing Ifx! as a fraction o f .time

0] . JKX) ' ‘ , : J E(t) = -— Y(k) &_ exp ( j2rrPnkt) . . 2 , %oo . ' k

2<>2 fhiS..:expression is identical with that'.of eqnatioa lo2 exOept that the coefficient of each term of the infinite series is.multiplied by Bfk)o 1 |k) is a . function of the aperture sig@ and response0 fhe same ■ effect' pduld be obtained electrically by cascading a linear' network9 which has a transfer "admittance Yfkls with 'the cbmmraicatlorn ekamnelo ffk) is often referred

t© as ^apeftmf© admittanc©,?o ' : . If S|S1 ih symmetrical about the origin Cie@o 9

an even ftin-ctioh) s Equation 201' may -be r®Witten in

the f#ll©wiigi' f6r®§ '.- _

' .. 2tr.k5.■ , : Y^k) = f s ( 6 ) cos. ( nw ) db V. . . ape ' - : ; ■

: . r ;V ; ' ■ ■ V ■ 2o3 ■ An .interesting observation is that although the %

amplitude of T(k)‘is frequency dependent3 it is a ■ real number' and there is n© 'associated phase shift® 13

Chapter 3;

. APERTURE RESPONSE LIMITS RESOLUTION

at© the theory dissussed in of the RCA type 6j d8Bera'v.tub®» ' RCA has determined that the electron :;; : . ' ' ; , 2 ' . v-.r . ' .'varies as : ® €S^ .ami has a diameter of o003 incheso This diameter corresponds to the'poimt where the momalised^imtems'ity .is — ©oOISIh Simc< k. & the expression - for .the ■apertmr® response may : be calculated and plotted wrsms frequency0 . Curve A of Figure' 3ol is a plot 'of- the frs

of the 6 3 2 6 iridiooa. as:\ ■ £ ' . " .: . 1 ■. . - '' . data.?, Cury© l .of the sai$e. fi, respomse of - the.sase tube as actually measured

It will be well t© i n .mind that . all.of -y presented so far. .e.ea from a .Fourier analysis .. it ; ©f viewo The ploi tkj is then, the response of the apertmr® to am:© sis© wave -aad mot a res@#in:..i o lutiom test:©hart o f © square waveso RCA

■ # "' foSo & and G0 Ac Mortem^ wT©levisi©ia»1:r Edo 8 John Wiley SohSs Ineo 9 lew Torka 1934 $

l-ZE

^l-. |T{ ;Hln m m m

.4- i . i \ . • 15 measured the data for Curve B of Figure 3.1 by scanning an image consisting of sinusoidal brightness variations. Curves A and B of Figure 3«2 show respectively the theoretical and experimental responses of the 6326 vidicon as functions of television lines of reso­ lution' . However, they are plotted against the number of television lines of resolution N whose- ■fundamental '• is the frequency used to compute the response (see page 5, Chapter 1}, ' Thus these curves are not actually - the response to be expected from the scanning of the black and white optical square wave of a. resolution -charto . They are similar however, and beyond the rnid-

- - . . ' ‘ ■ , bandwidth point they are identical because only the fundamental is, passed by the amplifiers. ; It should be kept in-mind that all response curves in this thesis are for sinusoidal variations. (.3.2) Aperture and Video Amplifier Response Comparisons In Chapter 1 it was shown that the resolution of a system depended greatly upon,the bandwidth of the electrical communication channel. The bandwidth, should be. made as wide as is practical. Video amplifiers . are commercially available, which have bandwidths -in"-: / excess of 8 -megacycles. . Since the type 6326 vidicon has a. bandwidth of only 2 megacycles, it would greatly limit the resolution of a system using such video ampli­ fiers. , • : / ■ 1

a

S4 17

Chapter 4

PRIMCIPEES OF APERTURECORRECTIOW (4ol) Imtroduetiom ' . _ - - , The response of the aperture of the RCA type '

6 3 2 6 vidieon to a repetitive horizontal pattern of optical sine wares was studied; in Chapter 3». It was shown in Chapter 1 that such a repetitire pattern is converted into an eleetfieal frequency by a scanning processo The attenuation of the higher line numbers by the aperture produces an attenuation of the higher electrical signal frequeneieso The response of a finite aperture can be considered analogous to that of a low pass electrical .filter,® It is thus possible to correct for the attenuation of the higher line numbers by the use of properly designed electrical filterso This process is termed, ^aperture compensation^ or ^aperture equalisation^ =, The: aperture compensating filter ideally would hare a response which: is the exact reciprocal of the relative aperture response o Obviously the compensating filter would hare a response which increases with frequencyTheoretically it would be possible to compensate the aperture in such a manner that the corrected response would be -a horizontal line out to the line number where the aperture response \ : ' . ■; ■ ' - - . v . • ■ ' ... ± $

becomes ' zero,- There are certain practical limitations to such a theory which will be discussed'in the follow­ ing sections = " . . • ■- " .

(4c2) Phase Shift Distortion of Sharp Cutoff Filters . Considerable phase shift occurs at the upper end of the pass .band of a sharp cutoff electrical filter. ' - The phase shift causes distortion which appears in the ' form of edge transients when a unit step function is appliedf Because the overall television system is analogous to an electrical filter, sharp cutoff must be avoided to prevent image distortion. Care must be exercised in the design of the aperture compensating network to ■ insure that the compensated response does not exhibit .sharp cutoff characteristics. A very similar- situation occurs when the bandwidth or rise time of a video amplifier is improved by: series peaking. A.point is reached where any further improve­ ment will introduce overshoot® (4.3) : Noise. : - .Noise has greatly limited the usefulness of aperture correction. The- increased response at the - higher frequencies is obtained at the expense of the signal-to-noise ratio. A thorough;,knowledge of the . 'origin and nature of noise in television circuits is therefore necessary to the application of aperture 19

T3 •H a peak-to-peak ripple

Time

FIGURE 4.1 The effect of a sharp cutoff filter on a unit step function. ' : / -V / . , ' ' 20 correction* Camera, tubes such as the' image orthicon which use electron multiplier structures produce noise primarily ' : ' ' ' " T , ' , ' - of the "flat channel" .variety* . That is to say, the amplitude of the noise is uniform over the entire frequency spectrum under consideration* ' This noise is primarily shot noise produced by the. scanning beam. Tubes such as the vidicon and iconoscope, which have no electron multipliers^ are limited primarily by the noise due to the first video amplifier* This noise consists of the thermal noise of the input re­ sistor and the shot noise of the amplifier tube* The effect of the amplifier noise becomes quite different . from the camera, tube, noise. This difference is caused by the large, stray capacity at the output of the camera.. tube which shunts the amplifier input resistor* The stray capacity and associated input resistor introduce a. "pole" in the. transfer' function of the video channel *. See'Figure 4°2* To eliminate this■pole a "high peaking" stage is incorporated into the video channel *

1 0*T* Shelton, "Study of High Definition Tele­ vision Techniques", Study of Army Television Problems, Vol*' 2, Engineering Products Division, RCA, 1954 21

Pickup tube I i n • c Video amplifier - 1------T R I I I C I T

Output coupling circuit of a pickup tube.

I ° S + a > c

internal source resistance ~ r!c R t— parallel combination of n R Rs R ~ load resistor

X = signal current

G - stray capacity

FIGURE 4.2 The stray capacity at the output of a pickup tube introduces a pole into the transfer function of the video channel. 22 C,

Ru f?= R Ci r J S + ( i + Jtti R k

i f S = i b i ? « (i -h^Rk)

Tp fft where: Fir — $1v R Ff> + Rk

fir f?r = l?K i + J m R x

J^-tube transconductance tube resistance

FIGURE 4.3 Hif^h peaking circuit introduces a zero into the trans­ fer function of the video channel.

4 T.L. Martin, "Electronic Circuits," Prentice-Hall Inc., New York, N .Y., 1955, p.#3 , which has an exeeediiagly SBialX cathode bypass capacitoro In its raid^frequency range the high peaker introduces a !?zerow which exactly cancels the pole introduced hy the stray capacityo See Figure 4»3o The high peaker restores the higher frequencies to their original amplitudeo The noise of the first amplifier is generated between the input resistor and the high peakero Therefore it is unaffected, by the stray capacity but has its higher frequencies amplified by the high peaker* The noise increases with increasing frequency and generally its amplitude is proportional t© the frequencyo- This type of noise is termed.' "peaked channele? noiseo "■ Signal=to=noise: ratios of systems with ..peaked. . ^ channel noise areC.baSe&lon a ^visual equivalenti? noise - letelo- \ The eye.attenuates the higher frequency noise components and therefor© increases the.signal-to^noise ratioo Sine©a in peaked channel noise most of

authorities on television fail to differentiate betweeri the functions of the high p< correction circuits0 They are both correct- ey '• eompensate for entirely different

. ,3 EoGo .Neuhauserj, _ "Vidicon for Film Pickup®, Journal of SMPTEt: Febo . noise is at the higher frequencies? the eye reduces the 'total noise level more than it would flat channel noise

of ah equal • signal-to-nois'evratio,. In practice,' a' flat channel signal-to-noise ratio is given as the peak-to- • .peak signal divided by the p.m.s. noise. Peaked channel •noise is assumed to be only one third as visible and so the r.m.s„ noise level measured •is .divided by three. 1 . The aperture compensation network boosts the gain • of the higher frequency, components. . Because the noise level of peaked channel noise increases with frequency3 aperture compensation decreases the signal-to-noise ratio e The pickup tubes such as the image orthicon which .have multipliers.are in general not suitable for aperture'compensation^ They have both flat and peaked channel noise, the flat channel noise swamping out the

peaked channel noise. The decrease in signal-to-noise ratio caused by aperture correction results in an image which in- general is inferior to the uncompensated image. ' - : ' ■ . \ ' .- .

_ - 5 . - - - • ' ■ - ... The vidicon, however3 when combined with a cascode preamplifier has an extremely large signal-td-

. 5 RoBo JameSj BoH0. Yine5 FoS, ¥eiths "Performance . of the Vidicon, A Small Developmental Gamera; Tube", EGA Review, March, 1952 , , " ; _ ' j '\ . 25 ' noises .ratio (approximately 300:1) o - It has been , : : ■ ■■ 6 - ’■ ■ ■ ' : - ’ ■ observed that for am acceptable pietmr® the signal =* t©=moise ratio may be as loir, as 10,: lo ' The, large margin by which the, vidieon exceeds minimum requirements is '

saffieient to allow the use of aperture eompensation0 '

The noise9 however8 imposes a definite limit to the _: ambiant of compensation which can be msedo ■

(4»4) Kinescope ^ : . . , ' ’ ■ . , . ' Aperture eompensation has been discussed only from the point of -view- ;Of - correcting apertures which were. : : '

ahead :of :the electrical7 channelo; It is. howev@rg ■possible to correct for apertures which followthe .' ■.

' .electrical channel0 1 The electrical signal would b©

over emphasized oh-.thf ;,high frequency end of. the / pass band, going into the kinescopeo The: amount..of over . emphasis will be limited by the dynamic range of the kinescope as the kinescope.will cut off large, amplitude , ; v - ; ■ -v r 1 hr rr-/’ff f ' : - ■ : '7 ^ . Wave shapes, containing'.:high frequency components,, , further enhancement can. be achieved, after the .range;, of

. the kinescope, has- been .exceededo Im practical' cases? ■-.

:: .. 6 ? 0lto Zworykin 3 60A0 Morton a and' Ld o FI dry s ■' ^Theory and Performance of the Iconoscope'5. Proco IRE, . 25g# CMgmst3: 1937) ;,:;^ - : f . . : . - ^ ^ . „

7 : 0bTe Shelton3 "Study of High Definition t&® kinescope should be considered in any attempt at aperture eompensation0 Th@ kinescope aperture is often the weakest link in a television system0 Since this thesis is limited in scope to the study of aperture compensation for the camera, tube only3 the kinescope will he megleetedo ' - : . (4o5) Overall Resolution Improvement . . Aperture eompensation iBcreases horizontal re solution omlyo ^ Vertical resolution is .not affectedo' The defining quality of a television image .is".'a function of both the. horizontal and the vertical ,resolmtio.no In; view of this fact a. subjective quantity termed ^apparent overall - resolmtlom^ is defined0 The apparent overall resolution is'found by taking the square root of. the product of .- the horizontal and vertical resolutions0 Therefore„/

resolution by a factor ka. it improved the overall - Chapter 5

' ' 'CHOICE OF APERTUBE COMPENSATION NETWORK \ (Sol) Introduction There are many possible circuits which might be used for aperture correction, 'In general, however, there are two philosophies of aperture compensation. One of these uses an. open-circuited transmission line in.

' ' ; . " 1 ' . . - ' ' ■ " 2. such a manner that a linear phase response is obtained. It will be recalled that the aperture admittance intro­ duced no phase distortion. However, a compensating ' network- with a linear phase response will operate - satisfactorily because it will, shift the phase of all frequencies by an equal, amount,. The circuit has a frequency response which has the shape of a. cosine ■

function as shown in Figure 5,1, The. gain is'unity at zero frequency and rises to a maximum value., Bm, at a frequency fp. It then falls to unity again at a frequency: . .. ..•■ ■■ - : ■ . . . ' ; -1' ''1;.4 - of 2 fp. The period of the cosine..function: is -— -r— and ; q/:- - ./i; . - ; r 2qfp; ; ' its peak magnitude i s'- • ■ ^ , The c.irou it is constructed in such a. manner that only Bm can be easily varied. The low frequency gain remains constant. Its main disadvantage

1 R,C, Dennison, "Aperture Compensation for •Television Cameras", EGA Review, December, 1953 28

B m

I p

Frequency

FIGURE 5.1

Frequency response characteristic of an aperture correction circuit using open circuited transmission li n e . is that the shape, of the response curve'is fixed. It is not flexible and tends to.. over correct some ' frequencies, giving a system response greater •than unity at the.se . ■■ . frequencies^ n l ' i ; ; , ' f ' _ _ : The other type of aperture correction circuit . . .. ''involves the /employment1 of simple networks' in the plates :■ / • or c at ho des of. t he video; amp 1 i f ie f s»® By using several • , . such networks in successive stages a. wide variety of response curves can be obtained. Because of the phase shifts introduced'by these networksthey must be follow- - ed by phase correcting networks." The primary disadvant- . ages are■that the compensating networks and associated - phase correctors are for the most part cumbersome and re­ quire a great deal of adjusting= • The particular circuit chosen for the purposes of this thesis falls into the second categoryIts use was suggested by Oliver, Chief Engineer of Hewlett- Packard Co*, Palo Alto, California. The circuit has been, used quite successfully by RCA and Kay Lab to provide simultaneous compensation for both the output capacitance and finite aperture of the vidicon camera, tube® In this thesis, however, the circuit was used •

2 Dr, Otto Hi Shade, /■"Electro Optical Chara.cter- istics of Television Systemsn» BCA Heview^ Part I , March, 19#:h il: : V • " ■'' : '..V. 3 : . EgDe Goodale, and B.Co . Kennedy, "'Phase and Ampli- ; tude Equalizer for Television Use", RCA Review, March, 1949 ■ • • . ■ so solely for aperture correction» The high peaker describe

in Section 4o3 was used for compensating the camera tube output capacitanceo In this manner the two phenomena were divorced c ; ' C5o2) Parabolic Response Function An examination of the reciprocal of the response curve of the vidicon reveals that it could be quite closely approximated by a parabola» See Figure 5o3© , • The equation for such,a parabola iss

■ : : - " ■ ' - , 2 ■ 2. - ' ' ' • ■■ • ’ ' ' : ' ' & = k(f f %): : . / . . . •

: ' , ■ , ■ " 5ol

A = amplitude of response f =s; frequency , ; .. f ■' =,constant • . k . . . k = constant It will be noticed that the expression is entirely real and will produce no phase shift0 This is an important requirement of the aperture compensation eir= euito See Section 4o3® Equation 5ol may be synthesized by multiplying a number of particular transfer functions together11m 4

Hi eh neaker Aperture Correction Circuit

_ J

FIGURE 5.2

Transfer functions of the video amplifiers used to provide correction for the output capacitance and for the finite aperture of the vidicon. the following manner s

S' - W%. . A ' g (s + o^) (s' + m^) ( ^ (s^ ^ Z , ' : - ' • : : ' .s + Mfc : , ' " • ■ 2 2 ? 9 ' ■ - -(u + %) = k(f" + ffc) ,. . if s = je

: : ' ' ‘ / . '■ ' 5*2 Physically this may be achieved, by. cascading isolation amplifiers which have the required transfer functions® See Figure 5°2® Notice that the factors which cancel in Equation 5°2 could be made unequal and thus varying amounts of phase lead or lag could be introduced® This is precisely what is done when this circuit is used to perform.the additional task of compensating for the output capacitance of the vidieon when a high peaker stage is not employedo The compensation obtained in this manner is not perfect;but it is very acceptable® (5*3) Aperture Compensation Circuit An .ordinary video amplifier with sufficient cathode degeneration can be operated to produce the poles of the transfer function required for the first two stages of the aperture compensating network as shown in Figures

5*2 and 4o3o The transfer function of the last stage requires a special circuit® The circuit required is the output stage of the aperture compensating circuit shown in Figure ,5°4° For this circuit the following equalities must hold: " R^,,

g10b 15b16

Rl4 ^u+1 )+Rl6+rp c10 ^ 14^16 )+Rl5Rll|. (u+1) +r15 (

These two equations, are derived in the Appendixo The only optimistic feature in the equations is that varies directly as the inverse of capacitor C^qo This allows for easy adjustment of the various degrees of Cqmpensation« The entire aperture Compensation circuit is shown'in; Figure 5o4o ' ' The correction circuit descrihed above has a res= pen.se curve whose shape is restricted to .a parabolae H©w«= ever 3 the $?bell?? may be varied in order to provide optimum compensation Simply by adjusting capacitors C^s and

■ An important feature is that any parabola which would be used to match the aperture.response would have a gain less than that.required for perfect compensation at the higher.frequencieso See Figure 5o3o This feature prevents the eorreeted aperture response developing the sharp cutoff characteristics mentioned in Section 4°2° Figure 5 o 3' also shows a comparison between the.uh correeted and;a'typically corrected aperture response0 The w r r e tu

EeHtfi retol P_3s: retspomie sjnrec

paraboj a m * ® rPt

co Qii&qiiMfil: Isa&i&hHa aslurea

. - y;:.coe ptaE-e afber-'ppemt? e[ qotnpenisatji^apifesB Ot OOjffl ‘ on - fieqwork: he.pojriigiiii.iarm m correcting parabola was made to match the reciprocal of the aperture response at sero and at 3 megacycle® fthere are approximately 100 lines to the - megacycle)o Not© that the bandwidth of the system has been more than doubledc The actual compensation to be used must be determined by an experimental studyo The over amplification of the lower frequencies can easily be tolerated as the eye is - not sensitive to amplitude distortibn0 The limiting factors will be noise and sharp cutoff effects both of which are best analysed by an experimental procedure0. 11 E out 15

FIGURE 5.4 Aperture Compensation Network R,-68 Rg-3.3k Ria-8.2k C^-.l uf C^2™30 uf

R?-15k R^q -1 .5k L.-30 uh C c-120 uuf

Rj—1 * 5k R j ^—ICk L2-7 uh C^-300 uuf

R4-510k Rl^-560k L -30 uh C7-10 uf

R^-7.5k R13-3k Li -7 uh Cg-10 uf

C, -i*00 uuf C — # 1 uf P,-10k Rlt-100 R^-150 R, ^-680 C,-10 uf C^q -IOO uuf VJ o\ Rg-10k Rl6-39° C (-10 uf C11-.25 uf 37

Chapter 6

EXPERIMENTAL PROCEDURE

(6ol j Introduction ...

The effect of the aperture compensating circuit was studied by use of oscillograms ef the video signalo The camera was focused on a calibrated resolution chart and. the resulting video wave forms2 before and after a.perture correct ion 3 were recorded in the form of am oscillogram* This method of analysis which proved very successful was 3 however3 affected by many factors0 A description of the equipment and measurement techniques is: included in the next section* (6o2 ) Equipment . , . The television system used for the experimental purposes of this thesis was am Industrial Television

Camera Systems model ■ manufactured by Kay Lab : of San Biegos California* . ' ;; Camera Unitg - .% i . ,1} , 6326 1CA vidieon camera tube .

21' Double easeode video preamplif ier (8 mega=> cycle bandwidth) . r - 3) Electro-magnetic focusing (focusing field strength apprbximately 40 gauss) with parabolic correction ' ;v .. , : . ; . ■; 38 \ 4) 4 Wollemsak 16 millimeter Glme Raptar Telephoto Iens.es of foGal lengths, and 6 inches*

Bamera'/Gemtrel Hnlt: '

1) Six stage broadcast video amplifier with combined aperttire correction and high peaking circmit C8 .megacycle bandwidth)

22) Sweep generators for camera Monitor Unitg

" 1 ) 10FP4 EGA kinescope

2 ) 2 stage video amplifier (Smegacycle bandwidth) .3) Sync generators;^ sweep generators^ etc*

The system operated as a normal 525 linej 3© frame system with interlaced seanning0 The combined aperture compensating and high peaking circuit was modified to operate solely as an aperture compensating eircuito ,A high peaking circuit such as described in Section 4»3 ® was installed in place, of ;the 5th'Video amplifier0; : . The oseilloscop^ used for all wave form measuree , . ments was a Tektronix model 535 oscilloscope 0 Since.this oscilloscope had a delaying sweep circuits it was possible to examine any single scanning line9 The verti=- cal amplifier has arise time of 0 o020 microseconds0 The camera used for photographing oscillograms was a, Du Mont Polaroid Oscillograph Camera^ type 297o The test chart used for all resolution measurements ' - , - x ; . - - ' ' ' - - '. " ■ . . - ' ‘ was a RBTEA Television Resolution Test Chart«

1 Radiog Electronics and Television Manufacturers* Association ' . '• :. v't ■ ; ^\^ -; ;:-; <: - ^i;;:: - :.^ 2 9 ',:, -: (6«3) Television Sy.stem Operation The RETMA resolution chart was imaged by the camera* In many cases it was difficult to determine the exact ' resolution, line number for.the specified image distance . of 4S inches for a 1 inch lens* It proved much easier to observe the . 200 line'center bar pattern and vary the image distance and focal length of the camera, lens to obtain the various line numbers required. The reflected light in all cases, was 400-800 as measured by a Weston Master Meter, model 715 * The lens focus arid stop, vidicon target, beam and focus controls were adjusted to give the sharp- ■ est visual image on the kinescope. Finer adjustments of the lens stop and focus and vidicon magnetic focus were then made by observing the video waveforms on the oscilloscope. The vidicon was greatly overscanned vertically and only slightly horizontally to,eliminate . the overlapping of successive beam scans and to obtain maximum horizontal resolution. . The high peaking circuit had a variable correction control. It was important that this circuit provide ' nearly perfect comp ens at ion because, over or under compen-. ■ sation by the high peaker would destroy the data in regard to aperture compensation. A black-white junction was imaged by the camera. The resulting video waveform was then examined on the kinescope side of the high peakero The junction appeared as a unit step function with a definite associated rise timeo Adjustment of the high peaker was at an optimum when the rise time was reduced to a minimum? without intrpduping.any over­ shoot <, . - :

(6.4) Data Taking / - The data obtained consisted of photographing the v j waveforms,(see Figure 7»2) of the video signal of a * representative scanning line. It was necessary that the white-1o=b1ack.signal ratio displayed on the oscilloscope he of the•order of several centimeters to provide sufficient differential at the high resolution line numbers for accurate readings. Unfortunately, the video level before/the aperture.correcting.network was insufficient to obtain this condition. An attempt was made to shunt , the compensating network and use the following video - amplifiers to provide additional gaipo ' Stray capacity and noise degenerated the signal. Therefore, the measure­ ment s of the video signal before and after aperture correction had to be taken separately. The aperture correction circuit had to be removed and ordinary video amplifier stages replaced to provide additional gain before the "before correction" data could be obtained. A black-white junction was used to obtain the same displacernent on the oscilloscope for "before" and . "after" aperture compensation comparison purposes. Great care was exercised in duplicating the exact conditions0 However, error although negligible was undoubtedly intro­ duced* The optimum aperture .compensation was determined by simultaneously observing the image on the kinescope and the video waveform on the oscilloscope0 The video waveform was observed in order to determine the amplitude of the corrected video signalThe kinescope image was observed in order to determine the point where excessive, noise or overshoot were introduced* Chapter 7

. EXPERIMENTAL R1SUL7S : C7o 1) Introduction - . , - ' - :; - ; - . The aperture compensation circuit was adjusted

to gire optimum compensation by the method described , ih Section 6a4o The measured response curve of the aperture compensation network after such adjustment is shown by Curve C of Figure 7oio■ Curve A of Figure.

7 ol-is a plot of the reciprocal of the actual aperture

response of the vidicone (7o2) Oscillograms . The wave forms of the corrected and unconnected video signals for 100=600 lines are shown in Figure 772o The black-white junction displacement was four centimeters in both caseso The results of the aperture correction circuit are very well illustrated by the oscillogramso

In particular5 note the effect of the aperture correction

circuit at 600 lineso Without aperture correction, the video signal amplitude at 600 lines is about one-teath of the amplitude at 0 linesa With aperture correction

the video signal,Btrehgth^at 600 lines is still greater

than one-half of the amplitude at 0 lines0 . C&rS

fiZ'L'e

U-: m m

- 3r-^e#W

;;t ;Cpt Yt- comK €i': zlx luWi.fi

L 44 C7o3) Response Curve of the Compensated The data presented by means of the oscillograms of Figure 7o2 were used to compute the compensated response curves shown in Figure 7°3° -The Individual points on the curves are the average of the peak-to™ peak deviations for the line number measured from the oscillogramso Mote that the o?07 bandwidth of the aperture has been more than doubled by the use of

the compensation networko This is a decided improvemento

(7o4) Discussion of Errors . Errors in the theory of aperture correction due to approximations and the like were discussed as they were introduced» .However3 experimental errors which were introduced due to the methods of measurement;have not been discussede The oscillograms of the video signal before and after aperture correction, were taken on separate days

See Section 604o The quality of the video waveforms was. a function of many variables voltage s, beam voltage focus current, incident light intensity and, etCo)0 In spite of every precaution in duplicating exact conditions,

errors were undoubtedly introduced0 The errors are judged to be small, but assigning a definite value to them would b© difficulto The oscillograms for different line numbers were taken using different lenses, image distances and focal UNC

m m

a i i m i

iii.i

5 00 L i n e s

a ^ i a a a g i TTTT 300 Lines 6 00 L me s

FIGURE 7.2 lengths o. The lenses were of a high quality and were operated with a small aperture= Any error introduced would be common to both ^before" and "after" aperture

correction measurements 0 Again assigning a definite value to the. error introduced would, be difficulto The

measurement errors are, therefore3 deemed small enough

to be neglected o The camera tube used to obtain the data for this thesis had previously been used for approximately one thousand hourso The characteristics of the tube may have deterioated j but not visibly s.o:0' The relative improve­ ment due to aperture compensation would be more or less independent of agin^ at any rate<,

(7o5) Conclusions

The aperture response of an RCA type 6326 vidieon was considerably improved by the compensating networko The upper limit was found to be determined by the depreciation in the signal=to=noise ratio which in­ herently accompanies aperture compensation0 It was found that the bandwidth of the aperture response curve could be slightly more than doubled and still have a satisfactory signal-to-noise ratioo ■ i ira_:

Glrpve

gl

— .■APPENDIX-

Development of Transfer Function of Output Stage of Compensation Circuito

The development of the transfer function is a

straight forward problem in circuit analysis0 The iaid=frequency equivalent. circuit is shown in Figure lo The loop equations arei

«*g - M ' p 1 * V p and

ueg = V p + 12 (^£:+ r L- + ""p)

Now since ueg - u (ei ~ the loop equations become

n©! = i^(rp + S1 ■- $ $ * i2

uel = ^p + HL + rP )

Solving these equations for i^ and ig we obtain Output staple of the aperture compensation network

ue g

FIGURE 1 Equivalent circuit of the output stage of the aperture compensation network if is effectively an ooen circuit.

Univ. of Arizona Library ■Rk+RL i3_s=ue1 ( — ■--- —

snd

2 1 ^ rp( Rk+RL ) + ( ) (uRk 4-RL -i-Rk4.r )

Solving for the output voltage and substituting for i- and i0 1 2.' - .

eo=i2Rfc-iiEl : .

\ - scbibl e0=ue1 ( s€»_(rpJ^+rpRjj+EjBjj.+R^I^'U.+R^I^+R^r ) $Rk,(u*l.)-t-r^+E^

eo solving for — • and simplifying

eo: s"tok e.£ r. s+cok where A r

• 5 .' Rk • „ to^ = ■■ ■ ■ ■■■■■. ■ ■; and * h h

Rk (u+l)+RL^rp

% G(rp (■%+%) +R]Rk (u+l)+R£(RL+r ) BIBLIOGRAPHY

- Books . . ,Anner, George $.Elements of Televisionc Prentice- ; HallInCoj, New lorkj l951o Hartins Thomas L. Jr., Electronic Circuitsa Prentice- : HaI;l,?;I&CoV lew Yorks 1955o - Zworykinj foKo and Horton5 GoA.8 Television, 2nd Ed.? '■John Wiley'and Sons s Inc 12 Hew Yorks T9 $4*

.i ■ / Periodicals

Dennisons RoG» 3 Aperture Compensation for Television Cameras”, RCA Review, December* 1953

; Goodalej EoDo and Kennedya R 0G0 s wPhase and Amplitude Equalizer for Television Use?, -'RCA Review, March« 1 9 4 9 o ; 1 ^ James * RoBo* ¥ines. BpHo and Velth* FoSo*' ^Performance of the Tidicon.* A Small Developmental Camera Tub©”* ; RCA Review* March* 1952o Hertz * Pierre and Gray* Frank* ”A Theory of Scanning and Its Relation to the Characteristics of the Transmitted Signal in Telephotography and Tele­ vision”, Bell System Technical Journal* Vole . H U * July* 1953a. : . Heuhauser* RoGo* ”?idicon.for Film Pickup”* Journal . of SMPTE, February* 1954a Shade* Otto Ho* ”Electro~optical Characteristics of Television Systems”* RCA Review* March* 1 9 4 $ = - Shelton* GoTo * "Study of High Definition Television Techniques”* Study of Army Television Problems» ¥olo 2s Engineering Products Division* BOA* Camden* New Jersey* 1954o Zworykin* . ¥oEo * Morton* Go A, and Flory* L d o * "Theory and Performance of the Iconoscope”* Proceedings of the IRE, August, 1937°