PROCEEDINGS OF THIE I-R-E

PHYSICAL CHARACTERISTICS

Fig. 15 is a front view photograph of the colorplexer,showing the arrangement of the major components. Thecolorplexer is built on a standard bathtub chassis, 19'inches high. A 117-v ac input is required for the fila-ments and bias supply. A plate source of 280-v dc at 380ma is also necessary. Most of the 34 tubes used are pen-todes which provide a higher degree of circuit isolation.

CONCLUSION

Colorplexers of the design discussed in this paper havebeen installed at NBC's Colonial Theater color studiosin New York City where they have shown good per-formance under actual operating conditions.

ACKNOWLEDGMENT

The writers wish to express their appreciationJohn W. Wentworth for his capable assistance andrection in the preparation of this paper.

todi-

Fig. 15-Front view of colorplexer chassis.

Transients in Color Television*P. W. HOWELLSt, MEMBER, IRE

Summary-A color television system transmits three inde-pendent signals, each of which specifies one of the three co-ordinatesthat determine the location of the reproduced color in a three-dimensional color space. When a color transient occurs, each of thesesignals responds in a different manner determined by the charac-teristics of its own channel. The system response may be char-acterized by the resulting path along which the reproduced colorpoint moves through the color space from its initial to its final loca-tion. The shape of such color transient paths as determined by theindividual transient responses of the three channels is analyzed,and the subjective appearance of different transient-path shapesis discussed.

INTRODUCTION

T7M HE main difference between monochrome andcolor television lies in the amount of informa-tion needed to specify an element of the picture

being transmitted. In monochrome transmissions thepicture element is completely specified when we knowits luminance. In color transmissions the picture ele-

ment is completely specified when we know three quan-

tities, such as the amounts of three primary lights re-

* Decimal classification: R140XR583. Original manuscript re-ceived by the Institute, September 18, 1953. Presented, I.R.E. Con-vention Record, part 4; 1953.

t General Electric Co., Syracuse, N. Y.

quired to match its color. Thus, we may say that inmonochrome television the information transmitted isthe location of each picture element on a one-dimen-sional luminance scale while in color the informationtransmitted is the location of each picture element in athree-dimensional color space.The operation of the color system might be described

as follows: the camera, in scanning the scene, makes acolorimetric analysis of the picture element under scanand produces three output voltages. Let us say that thecamera is so designed that these three voltages repre-sent the amounts of the NTSC display primaries, (R),(G), and (B). These voltages determine a point in colorspace, the camera color point. In the process of codingthe color signals for transmission, the R, G, B voltagesundergo a linear transformation (9), (Appendix I)which produces three related voltages representing theamounts of the NTSC transmission primaries (W), (I),and (Q). (Let us ignore gamma precorrection for thetime being.) These transmission primary voltages stilldetermine the location of the camera color point, but interms of a different set of cartesian axes in the colorspace. Between the point at which they are formed andwhere they are recovered in the receiver, the trans-mission primary signals undergo the usual difficulties

January212

Howells: Transients in Color Television

with noise and interference, and the different band-width limitations necessary for simultaneous transmis-sion through the 6-mc channel.At the receiver, an inverse transformation is per-

formed on the transmission primary voltages to regainthe display primary voltages R, G, B for application tothe tricolor display. The transmission primary signals(or the R, G, B signals) at the receiver determine thelocation of the receiver color point in color space. Forperfect reproduction, this receiver color point shouldcoincide with the camera color point. In practice, thesystem should be so designed that the colorimetric de-viations of the receiver color point from the cameracolor point due to noise, interference, and band limit-ing are such as to produce the least perceptible effect inthe picture.

THE COLOR TRANSIENT

As the color camera scans the scene, the scanningaperture encounters areas of different color, and thecamera color point responds by moving about throughthe color space. As long as its motions are not too rapid,the receiver color point is able to follow them exactly.However, when the scanning aperture crosses a bound-ary between areas of different color, the camera colorpoint may change position too rapidly for the receivercolor point to follow. Since the transmission primarysignals have different bandwidths, the path taken bythe receiver color point in response to this shock is acomplex one. Being able to move more rapidly in somedirections than in others through the color space, ittraces out a curving path having several more or lessabrupt changes in direction. This three-dimensionalpath through color space, plotted as a function of time,may be called the transient response of the color system.It is analogous to the curve of luminance versus timewhich represents the over-all transient response of amonochrome system and, in a similar way, its shapeaffects the appearance of transitions in the color picture.

Given the transient responses of the individual signalchannels, it is the purpose of this paper to developmeans of determining the color transient response of thesystem. Such a four-dimensional figure is difficult todisplay as a whole, so it will be analyzed into its com-ponent luminance and chromaticity transients.

THE COLOR SPACE12For a study of the transient response of the color sys-

tem, it is most convenient to deal with a color spacewhose axes correspond to the three transmitted colorsignals EW, El, and EQ, since it is these signals whichare band limited in the process of transmission. Ifgamma is assumed to be unity, it can be shown3 that

I W. T. Wintringham, "Color television and colorimetry" PROC.I.R.E., vol. 39, pp. 1135-1172; October, 1951.

2 F. J. Bingley, "Colorimetry in Color Television," PROC. I.R.E.,pp. 51-57, this issue.

' P. W. Howells, "The concept of transmission primaries in colortelevision," PROC. I.R.E., pp. 134-138, this issue.

these signals represent the amounts of three primariesknown as the transmission primaries, (T), (I), and (Q).Fig. 1 shows such a linear color space. The vertical axisin this sketch corresponds to the NTSC luminanceprimary (WV), which has the chromaticity of IlluminantC white and which supplies all of the luminance of thecolor produced by the receiver. The two horizontal axescorrespond to two chrominance primaries. These arenonphysical zero-luminance primaries similar to the(X) and (Z) primaries of the CIE system of color speci-fication. The base plane of Fig. 1 is therefore a plane ofzero luminance, and higher horizontal planes are planesof constant luminance. If equal units on the twochrominance primary axes are made to represent anequal number of volts of the corresponding signals trans-mitted in quadrature on the color subcarrier, the baseplane may also be considered as a phasor diagram of thecolor subcarrier, in which the amplitude and phase ofthe color subcarrier may be measured directly.

Fig. 1-Color space in terms of transmission primaries.

It is somewhat simpler to construct this color spacein terms of the earlier chrominance primaries (R - )and (B - ), than (I) and (Q). [Equations (11-14), Ap-pendix I. ] The (I) and (Q) axes are then located 33 de-grees counterclockwise from the (R - g) and (B - H)axes, respectively. The units of equal size on the (R- W)and (B - N) axes

R-WSRIB= J

1.14

andB -W

SB = l2.03

corresponding to the weight given these signals in trans-mission.As with other sets of primaries, the chromaticity of

a color is determined by the ratio of the amounts of theprimaries required to match it. This means that alllights having the same chromaticity lie on a straightline which pierces the origin. The Illuminant C line,for example, is the vertical (T) axis, while the NTSCprimary (R, G, B) axes are oriented as shown in Fig. 1.

1954 213

PROCEEDINGS OF THE I-R-E

A constant luminance plane is shown in the sketch; theboundary of the plane is the RGB color triangle de-termined by these axes. We may transfer the chroma-ticity diagram to this plane, [(21) and (22), AppendixIII]. For this case, where gamma is unity, the lines ofconstant x transform to the set of lines radiating from apoint beyond the green primary, while the lines of con-stant y become the set of parallel lines shown.

THE EFFECTS OF GAMMA PRECORRECTION

In the preceding discussion, the assumption has beenmade that the electrical signals are linearly related tothe colorimetric quantities they represent. In a prac-tical system, this is not the case. The color display de-vice has an approximately exponential transfer char-acteristic with an exponent of -y. The correct primarysignals for this tube, then, are not R, G, and B, but

R' =RllY,

G' =Gll/y,

and

B' =B1-Y

(Note: Expressions involving electrical or colorimetricquantities in nonlinear relationships may not be di-mensionally correct unless supplied with the properconversion factors. These conversion factors are omittedin the interest of brevity.)

Precorrection of the R, G, and B primary signals forthe nonlinear characteristics of the cathode-ray tubeis performed at the transmitter. Subsequent linear op-erations to convert these gamma corrected signals tothe transmission signals and, at the receiver, to convertthem back, are performed exactly the same as in thelinear case. Where the transmission primary signals areformed in this way from gamma corrected primarysignals, they are primed to indicate this fact.

Because of this nonlinear relation between colori-metric quantities and electrical signals, the picture ofFig. 1 must be somewhat revised. The transmission sig-nals now establish a curvilinear co-ordinate system forthe color space. Alternatively, if we represent colorimetricquantities in terms of linear cartesian axes correspond-ing to the gamma-corrected transmission signals, W',I', Q', (or W', SR', SB'), the chromaticity diagram andthe constant luminance surfaces are warped from theiroriginal shapes.

Fig. 2 shows such a space. As far as the electricalquantities are concerned, the space is identical to thatof Fig. 1. The base plane still corresponds to the phasordiagram of the color subcarrier. It can be shown [(21)and (22), Appendix III], that a given ratio of thegamma corrected transmission signals still correspondsto a particular chromaticity, so straight lines from theorigin still locate all lights of the same chromaticity.However, in general, the direction of the line is differentthan in the linear case of Fig. 1. Exceptions to this rule

are the NTSC display primaries, their complements andIlluminant C. For these special cases, the ratios of theR, G, B signals after gamma correction are the sameas before, so these axes are located exactly as in thelinear case.

THE EFFECT OF THE SUBCARRIER ON LUMINANCE

Fig. 2 shows a subcarrier plane (or plane of constantW'), bounded by the NTSC color triangle. This planeis no longer a surface of constant luminance. For agamma of 2, these surfaces, as defined by (15), AppendixII, are a family of similar, concentric ellipsoids. Thecurved surface shown is the section of one of these con-stant luminance surfaces bounded by the NTSC colortriangle. Note that this surface is parallel to the sub-carrier surface at the (W') axis, that is, the subcarrierplane approximates a constant luminance surface verywell near Illuminant C, but less and less closely as thesaturation increases. The contours defining the constantluminance surface are its intersections with various sub-carrier planes. Similarly the contours shown on the sub-carrier plane are its intersections with the constantluminance surface shown, and several surfaces of higherluminance.

(si)

Fig. 2-Color space with gamma correction.

The effect of the subcarrier on luminance may beshown by a map of these intersections in the subcarrierplane, Fig. 3. The origin of the subcarrier plane (atIlluminant C) marks its point of tangency with a partic-ular. luminance surface, and the contours indicate itsintersections with surfaces of higher luminance. Theluminance factor, KS, on each contour is the ratio of thelIminance on the contour to that at Illuminant C. Asindicated by (16), Appendix II, the luminance factordepends only on the ratio of the transmission signals, soa normalized diagram, good forall levels of the luminance(W') signal, may be constructed in terms of the ratiosSR'/W and SB'/W. The factor Ks indicates the con-tribution of the subcarrier to the luminance of the re-ceiver color point according to the equation

Y = KsW'Y, [(16a), Appendix II]

where W'y is the luminance which will be produced by

JanUarY214

1954 Ho~~~~~~~iwells: Transients in Color Television21

luminance signal alone. For example, where Ks = 2 the"luminance" signal contributes only one-half of the totalluminance, while the color subcarrier contributes theremainder. Strictly speaking, then, we should not referto W' as the luminance signal, but as the monochromesignal.

R 2 -

I,

4.0

3.02.

T.0~~~~~~~~~~.

2.

4 k u N k) ,))Z-;ii1-1 IQO

3

I)

Fig. 3-Luminance factor (Ks) contours in

normalized subcarrier plane, -y = 2.2.

subcarrier itself. In its recent work on color television,the NTSC has investigated two different methods oftransmitting the color subcarrier, which result inradically different types of transient response:

(1) Prior to the end of 1952, both components ofthe chrominance signal were transmitted in a vestigialsideband fashion with the result that the quadraturecomponent of the SB' signal was detected in the SR'channel, and vice versa. The feature of Color PhaseAlternation (CPA) was incorporated in the system in aneffort to cancel the effects of this cross talk.

(2) In the final NTSC signal specification the chro-minance component requiring the least resolution forgood subjective equality of the image is restricted inbandwidth so that it may be transmitted as a double-sideband signal. Quadrature components detected withboth chrominance component signals may be eliminatedby this scheme, since both signals are double-sidebandover the frequency band they share.' (This system hasbeen called the acuity-matching system, since the band-widths of the three transmission signals are so propor-tioned as to match the acuity of the eye.)The color transient response of both of these systems

will be analyzed.

The luminance factor, Ks, is the inverse of the

chromaticity factor, K0, introduced by Applebaum.4

Since Ks is shown in terms of the transmission signals,

while Kc was evaluated in the chromaticity diagram,

the derivation of the expression used to evaluated Ksis given in Appendix 11.

THE EFFECT OF THE SUBCARRIER ON CHROMATICITY

Fig. 4 shows how the chromaticity grid- appears in a

normalized plot of the subcarrier plane. Gamma correc-

tion has warped the grid so that the lines of constant x

and constant y are no longer straight as they were in the

linear case illustrated by Fig. 1. In general, variations of

the SR' and SB' signals have a greater effect on chroma-

ticity (gamma times as great) near the Illuminant C

point than they did in the linear case, and less effect on

chromaticity near the edges of the color triangle. In

these areas, much of the effect of the subcarrier goes

into changing luminance rather than chromaticity.

The expressions relating SR'/W' and SB'/W' to chro-

maticity are derived in Appendix

SUBCARRIER TRANSIENTS','

An important part of the over-all transient response

of the color system is the transient response of the color

4S. Applebaum, "Gamma correction in constant luminance color

television systems," PROC. I.R.E., vol. 40, pp. 1185-1195; October,1952.

5J. S. S. Kerr, "Transient response in a color carrier channel

with vestigial side band transmission," I.R.E. Convention Record,part 4, pp. 18-23; 1953.

6 W. F. Bailey and C. J. Hirsch, "Quadrature Crosstalk in NTSC

Color Television," PROC. I.R.E., pp. 84 90, this issue.

~~0 I2

Fig. 4-Chromaticity grid in normalized subcarrier plane, y= 2.2.

SUBCARRIER TRANSIENT PATH-CASE 1

Let us assume that at a color transient the camera

color point changes its position in the subcarrier

(SR', SB') plane, moving H units along a line making the

angle 0 with the SB' axis. This change in position is so

rapid that for the subcarrier channel it amounts to a

perfect step input. The center of the step is located at

point (CR, CB).- A sketch of this transient is shown in

Fig. 5(a). The signals applied to the SR' and SB' chan-

nels are the components of this step resolved along the

SR' and SB' axes, i.e., H sin 0 and H cos 0. As has been

7J. S. S. Kerr and P. W. Howells, "A Proposal for a Modifica-

tion of the Chrominance Signal Specification," NTSC Report No.

NTSC-P13-289; August, 1952.

\." i.5I I I I I

.

2151954

2

PROCEEDINGS OF THE I-R-E

shown by Kerr,5 these signals are detected at the re-ceiver as though they had been passed through thenetwork shown in Fig. 5(b). Since the two channels areidentical in this case, we may call Fi(t) the in-phaseunit step response of either channel and Fj(t) thequadrature unit step response. The two signals de-tected at the receiver are therefore:

SR' = CR + H sin 6Fi(t) + H cos GFq(t) (1)

SB' = CB + H cos 6Fi(t) - H sin OFq(t). (2)

CAMERA COLOR POINT TRANSMISSION PATH

S., /^/ H SINe

5(a) 5 (b)

RECEIVER COLOR POINT

SR r.,

5(d)

Fig. 5 Subcarrier transient path: Case 1.

Equations (1) and (2) are the parametric equationsfor the transient path of the receiver color point in thesubcarrier plane. These equations are simplified if wetranslate the axes to the center of the transient and thenrotate them 0 degrees to line them up with the directionof the transient. Translation of the axes to the point(CR, CB) changes (1) and (2) to:

SR1, = H sin 6Fi(t) + H cos OFq(t)SB1/ = H cos 6Fi(t) - H sin OFq(t).

(3)(4)

To rotate the axes by 0 degrees, we use the transforma-tion equations:

SR2' = SR1 cos 0 - SBI sin e

SB2' = SR1 sin 0 + SB1 cos 0

which reduce equations (3) and (4) to

SR2' = HFq(t)

SB2' = HFi(t).(5)(6)

Equations (5) and (6) show that the shape of thissubcarrier transient path at the receiver is always thesame, regardless of its angle or the location of its centerpoint.8 The position of the receiver color point in thedirection of the transient is given by HFi(t) while itsexcursion to the side is given by HFq(t).

Pp. W. Howells, "Brightness Errors in the NTSC System," Re-port to Ceiling Performance Subcommittee of NTSC Panel 13; Feb-ruary, 1952.

Fig. 5 (c) shows a subcarrier transient for a caseanalyzed by Kerr.5 The conditions are 1-mc bandwidthfor each color difference signal, 3.89-mc subcarrier,step-type vestigial sideband filter, and linear phase. Ifthe transient occurs in the opposite direction, or if colorphase alternation is used, the sign of the quadraturecomponent is reversed and the transient path is re-versed about the line between its end points as shown bythe dotted lines in Fig. 5(d). The timing dots shown onthe transient path are 8 ,usec apart (approximately therise time of the luminance transient).

SUBCARRIER TRANSIENT PATH-CASE 2

In the acuity-matching system finally adopted by theNTSC, the color subcarrier frequency is reduced to 3.58mc and one of the color difference signals is restrictedto approximately 0.5-mc bandwidth so that it may betransmitted as a double sideband signal. The other colordifference signal is allowed a bandwidth of 1.5 mc andis transmitted as a vestigial sideband signal. With properdesign of the transmission path, quadrature com-ponents in both signals as detected may be eliminated.Good results may be obtained by making SR' the

vestigial sideband component of the subcarrier. How-ever, it has been found9 that greater subjective sharp-ness of some color transients is obtained when thevestigial sideband component (I') and the double side-band component (Q') are located 33 degrees in advanceof the phase of the SR' and SB' components of the sub-carrier, respectively.

CAMERA COLOR POINT

SS6 I.1

TRANSMISSION PATH RECEIVER COLOR POINT

H Cosa O \ /L-S..

6(b) 6(dl

2 31.

2 345

H SIN eF It) 0

Q0

6 Ic)

HCOS 0 F2 (t)

t 6Fig. 6-Subcarrier transient path: Case 2.

Fig. 6 shows a typical shape for the subcarrier tran-sient path of the acuity-matching system. The cameracolor point (Fig. 6(a)) jumps H units along a line mak-ing the angle 0 with the Q' axis, so the heights of thesteps applied to the I' and Q' channels are H sin 6 andH cos 6. Let the unit step responses of the I' and Q'channels (Fig. 6(b)) be F1(t) and F2(t) (Fig. 6(c)). Since

9 NTSC Report No. NTSC-P13-286, "Tests Relating to theChoice of Narrow and Wide Band Components for a Balanced ColorGamut System," (RCA Laboratories) October, 1952.

January216

Howells: Transients in Color Television

there is no quadrature response between the color dif-ference channels in this system, the two signals detectedat the receiver, neglecting the dc terms, are simply:

I' = H sin 6F1(t) (7)

and

Q'= H cos 6F2(t). (8)

Figs. 6(c) and 6(d) show the S-shaped transient pathparametrically determined by (7) and (8). The S-shapeis characteristic of a system having a higher speed ofresponse in one direction than in the other. Unlike thetransient path of Case 1, the shape of this transient doesdepend upon the angle it makes with the Q' axis. Forinstance, when 0=0 degrees the path is a straightline parallel to the Q' axis and when 0=90 degrees thepath is a straight line parallel to the I' axis. When 0becomes greater than 90 degrees, the curve again hasan S-shape, but the S is reversed. Reversal of the direc-tion of the transient, however, does not reverse theS-curve.

(W)

YELLOW

1 ~~~~COMPLETEl,! \ ~TRANSIENT. PATH

(So) ~ (Q

SUBCRETRANSENTPATK

Fig. 7-Complete color transient path.

COMPLETE COLOR TRANSIENT

The complete color transient is the result of combin-ing the monochrome signal transient with the subcar-rier transient response just described. Fig. 7 shows sucha transient path. The projection of this three dimen-sional path on the horizontal subcarrier plane is thesubcarrier transient response, while the height of thepath above the base plane is determined by the tran-sient response of the monochrome channel. Note the cen-ters of all three transients (I', Q', W') coincide in time.

EVALUATION OF TRANSIENTS

The factors governing the color transient response ofthe system are illustrated by Figs. 2 and 7. Fig. 7 showsthe three dimensional transient path in terms of co-ordinate axes representing the transmission primarysignals, while Fig. 2 illustrates the relation betweenthese signals and the colorimetric quantities produced atthe receiver display. Since we do our calculations on

two-dimensional paper, however, these sketches aremore useful for visualization of the system behaviorthan they are for computation.

For computation, the three-dimensional electrical re-sponse of the system (in terms of variables W', I', Q')should be changed to normalized form (SR'/W', SB'/W')so the resulting luminance and chromaticity transientsmay be determined with the aid of the normalizedluminance factor and chromaticity diagrams shown inFigs. 3 and 4.

Specifically, the method of determining a colortransient, given its end points, is as follows:

If end points are (x1y1Yj) and (x2y2 Y2):(a) Plot end points on chromaticity diagram of Fig. 4

and read off the normalized color difference valuesSR'/W', SB'/W', corresponding to these end points.

(b) Plot these end points on the luminance factordiagram of Fig. 3 and read off the subcarrier luminancefactors Ks, and KS2 for the end points.

(c) Compute the values of W' for the end points, bysubstituting the values of KS and Y into (16a). Usethese values, together with the values of SR'/WI, SB'/WI[step (a) ], to determine the end points of the transientin the SR', SBA, subcarrier plane.

(d) Using the subcarrier transient path shapes ofFigs. 5 or 6, plot the subcarrier transient between theend points determined in (c).

(e) Transfer the subcarrier transient to the normal-ized subcarrier plane by dividing the co-ordinates ofeach timing dot by the corresponding value of W'.Since the interval between timing dots shown on thesubcarrier transients of Figs. 5 and 6 is equal to therise time of the luminance transient, W' is near itsinitial value up to the time t= - 1/2 interval, and itsfinal value at any time later than t = + 1/2 interval. Att= 0, the value of W' is the mean of its initial and finalvalues.

(f) Plot the normalized subcarrier transient in Fig. 4,and read off the values of x and y corresponding to eachtime point. These determine the chromaticity transient.

(g) Plot the normalized subcarrier transient in Fig. 3and read off the values of Ks for each time point.These values, together with the corresponding values ofW' may be substituted into (16a) to determine theluminance transient.

RESULTS

Using the method outlined, specific color transient re-sponses may be evaluated for any case in which thetransient responses of the individual signal channels areknown. Several transients have been so evaluated forthe two systems previously described.

In Case 1, both color difference signals have a shortrise-time but each contributes a sluggish quadraturecomponent to the other. In Case 2, the system adoptedby NTSC, these quadrature components have been elimi-nated. The rise-time of one of the color difference signalshas been decreased; that of the other increased.

1954 217

PROCEEDINGS OF THE I-R-E

For comparison of the systems, two main types ofcolor transieilt are shown: transients from saturatedcolors to de-saturated colors, and transients betweensaturated colors. The first type cuts across the constantluminance contours of the subcarrier plane (Fig. 3)while the second type runs more or less along these con-tours. The effect of the quadrature component of Case 1is quite different in these two instances.

the path taken on odd fields when color phase alterna-tion is used).Note here that from the center of the transient to the

final chromaticity, three units of time have elapsed, butthat the color point overshoots by a perceptible amountand does not return until after six units of time. Thisovershoot is produced by the trailing negative peak ofthe quadrature component. Depending on the direction

yNOTE- UNIT OF TIME

=1/8 MICROSECOND

.6

z .4

z

=) .2

l If t

FLESH i=t

0 .1 .2 .3

GREEN-T--~~~~~~~~E

.6

.6 .7 z9X : .2

01 1-4 -3 -2 -I 0 2 3 4 TIME

Fig. 8-Color transient in luminance and chromaticity: Case 2.

Figs. 8 and 9 show the transient response with Case 2and Case 1 transmission, respectively, for the transi-tions from a saturated color to a flesh color. In thechromaticity transients, the timing dots are spaced atintervals of 8 ,usec. The crosses on the transient pathindicate the start and finish of the monochrome signaltransient.The chromaticity transients for Case 2 (Fig. 8) show

how this system takes advantage of the color percep-tion characteristics of the eye. McAdam's data'0 onequally noticeable chromaticity differences at constantluminance indicate that in the central region of thechromaticity diagram the direction of minimum per-ceptibility lies more nearly along the y than the x axis.Note how, for all three transients, the relatively slowapproach to (or departure from) flesh color is made inthis direction of low sensitivity. This feature enhancesthe subjective sharpness of these transients. If we as-sume that, for the small area of the color transition in-volved, a change in y of 0.03 is just perceptible, we seethat the elapsed time from the center of the transientto its end at a chromaticity not noticeably differentfrom flesh color is 2 units of time for the transient fromred, and four units each for green and blue.The chromaticity transient of Fig. 9 shows the re-

sponse of the Case 1 system to the transient from fleshcolor to red. The dotted curve shows the path takenwhen the transition occurs in the reverse direction (or

10 D. L. McAdam, "Quality of color reproduction," PROC. I.R.E.,vol. 39, pp. 468-485; May, 1951.

xNOTE- UNIT OF TIME

1/8 MICROSECOND

FLESH

l5l~~~iI==E 0 .1 .2

lb

IRED

-a -6 -4 -2 0 2 4 6 8 TIME

Fig. 9-Color tranisienit in lumiinance anid chromaticity: Case l.

of the line between the end points of the transient, theovershoot of Case 1 may occur in any direction; notnecessarily in the direction of least perceptibility.The luminance transients shown in Figs. 8 and 9

show a minor contribution from the subcarrier transient.A small leading white and trailing black may be seenin all four transients due to this effect.

NOTE- UNIT OF TIMEa1/8 MICROSECOND

BLUE-3 -2 -I 0\3--- TIM-

-3 -2 -i O 2 3 TIME

Fig. 10-Color transient in luminance and chromaticity: Case 2.

Fig. 10 shows, for Case 2, a transient between a fairlysaturated red and blue, in which the subcarrier transientexerts a greater effect on the luminance transient. Inactual color pictures, such a transition would rarelyoccur, but it is included to show an interesting differ-ence between the two systems. The chromaticity tran-

:rL

-

\ _ al I.-

0

218 Januarv

X

AU,

eZ

.3RED

.2

ly -t

14Howells: Transients in Color Television

sient in this case is very good; the elapsed time from thecenter of the transient to either end is only two units oftime. The luminance transient, however, shows an ap-preciable anticipatory drop contributed by the sub-carrier transient.

y

NOTE-I UNIT OF TIME1/8 MICROSECOND

.3

w

Z.2

Ja

- .2

1 '-I | |F- = BLUE Ix

_Lt5itE'''!--- .1Fig. 11 Color transient in luminance and chromaticity: Case 1.

Fig. 11 shows the response of the system of Case 1to the same input transient. Here the effect of the sub-carrier transient on luminance is pronounced. The rea-son is this: the direction of this transient is along theluminance contour lines of the subcarrier plane (Fig. 3),so the excursion to the side produced by the quadraturecomponent is across these contour lines in a directionaffecting luminance. The impress of the quadraturecomponent wave form on the luminance transient isclearly seen. Note that the two luminance transients(affected by quadrature components of opposite sign)cross at the instant at which the quadrature com-ponent is zero. When color phase alternation is used,these two luminance transients replace each other at a30-cps rate. For such transients between saturatedcolors, the result may be a quite visible 30-cps luminanceflicker occurring at the transition. For color transientsinvolving de-saturated colors, the flicker produced oc-curs mainly in chrominance and so is not noticeable, butthe presence of this luminance "edge flicker," even inrare cases, was one of the important reasons why thesystem was abandoned.

CONCLUSIONS

Comparison of the color transients resulting from thetwo methods of transmitting the chrominance informa-tion yields some interesting conclusions. In Case 2,the reduction in bandwidth of one color difference signalhas actually resulted in greater sharpness of the chroma-ticity transients as well as in a cleaner luminancetransient for transitions between saturated colors. Bothof these results are due to the elimination of the quad-rature components. When a strong quadrature com-ponent is present, its effect on chromaticity is such as

-8 -

to require its cancellation by means of color phase al-ternation. This feature is not necessary in a systemwhere quadrature components have been eliminated byproper filtering as in Case 2.

While tentative conclusions may be drawn directlyfrom a knowledge of the color transient path, its finalevaluation must be made subjectively. This may bedone either by direct experiment with a color system orby reference to data on color perception, such as havebeen published by McAdam.Y0 The final test is, of course,the appearance of color transitions in an actual colortelevision picture. The excellent results which have beenobtained with the relatively simple system of Case 2 arethe most convincing evidence so far of the quality of thecolor transient response of this system.

APPENDIX I

CO-ORDINATE TRANSFORMATIONS INNTSC SPECIFICATIONS

Note: The following transformations may be used foreither colorimetric quantities or electrical signals repre-sentative of them. The same transformations are usedin the linear case (gamma = unity) as in the gamma cor-rected case, for which all the variables are primed.New Transmission Primaries:

[I F .596 -.274 -.322- R 1

W = .299 .587 .114 G

LQI L .211 - .523 .3121 LB IFR - - .956 1 .621- I-

G --.272 1 -.647 W .

LB J _-1.106 1 1.703I Q _

(9)

(10)

Old Transmission Primaries:

[SR I .615 - .515 - .100- [RiW = l .299 .587 .114 G (11)

_SB_ L -.147 -.289 .436I B1

rR_- 1.14 1 0° FSR1[G j -.581 1 -.3941 W . (12)LB 0 1 2.03 ISB-

WhereR-W

SR = 11.14

B-WSB =

2.03

Old and New Chrominance Primaries:

ESR1 rs.839 [5451LSB L-.545 .8391 LQ

rI r .839 -545rSRlL Q L .545 .839I LSB

Wheresin 33 = .545cos 330 = .839.

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.,

1954 219

- --

RED --- ", --"-',

I

Z 0 2 4 6 a 10 TIMETiur

PROCEEDINGS OF THE I-R-E

APPENDIX I I

LUMINANCE FACTOR MAP OF SUBCARRIER PLANE

The subcarrier plane shown in Fig. 2 is the surfaceobtained by setting the luminance (W') signal equal tosome constant. The gamma precorrected primary sig-nals, R', G', B', which are applied to the display deviceof the receiver are then obtained from the transmissionprimary signals by means of the linear transformation of(10) or (12). Given these input voltages, the display de-vice produces Panel 7 primary lights R, G, and B pro-portional to the Pyth power of R', G' and B'. The totalluminance (Y) on the screen of the display device is ob-tained by adding the primary lights, each weighted bythe proper luminosity coefficient. That is:

Y = 0.299R + 0U.587G + 0.114B- 0. 299(R') 7 + 0. 587(G') Y + 0. 114(B')TY.

[R 1.91 -0.532 -0.288--X

G _=-0.985 1.999 -0.028 Y .

_B 0.058 -0.118 0.898L_Z(17)

Since the chromaticity coordinates are given by

xx =

x+ y+z

z =

where x+y+z=1, wevalues

xx= zZ=

y

obtaining

V- = - and

X + y +Zz

X+ Y+Z

e may substitute into (17) the

(1 - x -y)Z = ~~Y

y

z-yy

LR 1.91 - 0.532 - 0.288-

G = Y - 0.985 1.999 -0.028

B _ 0.058 -0.118 0.898_

Substituting the values of R', G' and B' obtained from(12), we obtain

Y = 0. 299(1.14SR' + W')T+ 0.587(- .581SR, + W' - .394SB )y+ .114(W' + 2.03SB')-. (15)

From (12), we may write the amounts of the displayprimary lights at the receiver as

R = W' (1 + 1. 14SR'/W'W) (19)

and

B = W'½(1 + 2.03SB'/W')z. (20)If we divide both sides of this equation by W'½, we

obtain the expression

Y/W z = .299(1.14SR'/W' + 1)y+ .587(- .581SR'/WI + 1 - .394SB'/W )

+ .114(1 + 2.03SB /WI). (16)

We may write this as

YIW'7- Ks, (16a)

where KS is a factor dependent only on the normalizedsubcarrier voltages SR'/W', and SB'/W'. KS is the ratioof the reproduced luminance to the yth power of theluminance signal, assuming that all constants of pro-portionality between electrical and colorimetric quanti-ties are unity. The luminance factor contours of Fig. 3were obtained by evaluating the right-hand side of (16)for a number of values of SR'/W' and SBI/W.

APPENDIX I1I

CHROMATICITY MAP OF SUBCARRIER SURFACE

The relation between the display primaries and theCIE nonphysical primaries is given by the lineartransformation4

Equating the values of R and B given by (19) and (20)to those given by (18), and solving for SR'I/W' andSB'/W' yields

-= 0.877 [ -0(2.198X-0.244y 0. 288)1

-0.877

SBI I 1/-/'= 0493 - W (-0.840x-1.016Y+0.898)W y W'7

-0.493.

(21)

(22)

Note that the factor (Y/IW') in the above expressionsis simply KS. Since contours of this factor in the chroma-ticity diagram have been evaluated by Applebaum4andLivingston,1" work may be saved by reference to theirdata. Fig. 4 was plotted using (21) and (22).

ACKNOWLEDGEMENT

The writer would like to acknowledge his debt to thework of Sidney Applebaum and J. S. S. Kerr, and tothe many contributions by H. A. Samulon.

11 D. C. Livingston, "Colorimetric properties of gamma-correctedcolor television systems," I.R.E. Convention Record, part 4, pp.51-56; 1953.

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220 January