COLOR TELEVISION - A PRIMER ON THE NTSC SYSTEM William Feingold Emerson Radio & Phonograph Corp. New York, N. Y. Introduction Although the technical aspects of color television are more complex than the monochrome version, this article will show, by a simplified step-by-step approach, that the fundamental make-up of the N'SC (National Television System Committee) color television signal is easily understood, as well as efficient and ingenious. The electronic engineer to whom color television is a new area of thought should find this elementary treatment an ideal introduction to the more detailed articles which have appeared in print. (See Bibliography.) Color When we speak of a color television system, it is obvious that we are talking about a complete entity consisting of a color camera, a transmitter, and a receiver, including a display device. We must also recognize that, before we may begin to attack the electronic problems of such a system, we must reduce our knowledge and understanding of color itself to a mathematical and electronic basis. What is color? Thorough studies have shown that a color is completely defined if we specify three characteristics: its hue (whether it is red, blue, green, etc.), its saturation (to what extent the Dure hue is free from mixture with white light - i.e., pastel shades are low-saturation colors), and its lumiinance (a measure of the intensity of the light). Althlough lay terminology has used the word color and hue interchangeably, we see that hue is considerably more specific. Spectrographic studies have permitted us to define each and every hue by a specific wavelength - for in- stance, red is 700 mx (millimicrons), green is 546 mn, and blue is 436 nl. Obviously, all other hues in the visible spectrun, even those devoid of ter- minology, can be similarly described. In the reference above to the characteristic called saturation, the statement was made that the pure hue may be mixed with white. Numerous in- vestigators have proven that there is no such thing as a universal, unique white. However, they have been successful in accurately specifying a par- ticular "white" which suited their purposes at the time. One such definition is the "equal-energy white." This "white" is the visual effect of the sum- mation of equal energies of each hue in the spectrum. It is obvious that any imbalance in this "equal" specification will cause a tinted "twhite." This brings us to another approach to this problem of defining "white." Experiments in the field of black-body radiation oroduced the interesting result that the colorimetric output of an incandescent black body was directly related to its temperature. As a consequence it has become quite common to define the colori- metric output of a nominally white light source as so many degrees Kelvin. Daylight has been measured as 4800 to 6500 degrees K. Tungsten projector lamps are rated at 3200° K. In comparison to each other, the "daylight white" is bluish and the "tungsten white" has a yellow-orange tint. From this gamut of 30
Transcript
COLOR TELEVISION - A PRIMER ON THE NTSC SYSTEM
William FeingoldEmerson Radio & Phonograph Corp.
New York, N. Y.
Introduction
Although the technical aspects of color television are more complex thanthe monochrome version, this article will show, by a simplified step-by-stepapproach, that the fundamental make-up of the N'SC (National Television SystemCommittee) color television signal is easily understood, as well as efficientand ingenious. The electronic engineer to whom color television is a new areaof thought should find this elementary treatment an ideal introduction to themore detailed articles which have appeared in print. (See Bibliography.)
Color
When we speak of a color television system, it is obvious that we aretalking about a complete entity consisting of a color camera, a transmitter,and a receiver, including a display device. We must also recognize that,before we may begin to attack the electronic problems of such a system, wemust reduce our knowledge and understanding of color itself to a mathematicaland electronic basis.
What is color? Thorough studies have shown that a color is completelydefined if we specify three characteristics: its hue (whether it is red, blue,green, etc.), its saturation (to what extent the Dure hue is free from mixturewith white light - i.e., pastel shades are low-saturation colors), and itslumiinance (a measure of the intensity of the light).
Althlough lay terminology has used the word color and hue interchangeably,we see that hue is considerably more specific. Spectrographic studies havepermitted us to define each and every hue by a specific wavelength - for in-stance, red is 700 mx (millimicrons), green is 546 mn, and blue is 436 nl.Obviously, all other hues in the visible spectrun, even those devoid of ter-minology, can be similarly described.
In the reference above to the characteristic called saturation, thestatement was made that the pure hue may be mixed with white. Numerous in-vestigators have proven that there is no such thing as a universal, uniquewhite. However, they have been successful in accurately specifying a par-ticular "white" which suited their purposes at the time. One such definitionis the "equal-energy white." This "white" is the visual effect of the sum-mation of equal energies of each hue in the spectrum. It is obvious that anyimbalance in this "equal" specification will cause a tinted "twhite." Thisbrings us to another approach to this problem of defining "white." Experimentsin the field of black-body radiation oroduced the interesting result that thecolorimetric output of an incandescent black body was directly related to itstemperature. As a consequence it has become quite common to define the colori-metric output of a nominally white light source as so many degrees Kelvin.Daylight has been measured as 4800 to 6500 degrees K. Tungsten projector lampsare rated at 3200° K. In comparison to each other, the "daylight white" isbluish and the "tungsten white" has a yellow-orange tint. From this gamut of
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"whites" the NTSC chose a "1white" called Standard Source C (aT)prox. 65000 K)which was standardized by the Commission Internationale Eclairage (CIE) in1931.
It must be remembered that any specification describing "white" is com-plete without reference to its luminance. The particular "white"imay have anydegree of luminance and still be entitled to be called "white." In a practicalcase, however, a black-and-white, picture is deemed to have "whitet' tones inonly the high-lighted areas, the remainder of the scene being considered to beshades of gray and black.
Physiological Aspects of Color
Investigations into the sensitivity of the eye to hue have come u withthe interesting and remarkable fact that the eye may be stimulated to make itapparently see a hue that is not actually there. It has been proven, for ex-ample, that by mixing various amounts of the three primary hues (red of 700 nyl,green of 546 rp, and blue of 436 y), the eye can be made to perceive any otherhue in the spectrum.1 A mixture of equal amounts of these three colors willproduce a "white." At this point, we observe another interesting characteristicof the eye. If the three beams of light which make up this "white" are sepa-rated, and each is observed separately, the eye notices an astonishing differencein the apparent luminance of the three colors. The green appears brightest, ap-proximately 6 times brighter than the blue. The red appears to be about 3 timesbrighter than the blue. (The NTSC is using the more exact figures of 0,59: 0.30:0.11 as the ratio of G : R : B.)
One must acknowledge this unique faculty of the eye in connection with thecompatibility requirements of our signal. Let us suppose that we are trans-mitting three saturated primary bars of color. On the monochrome receiver, thekinescope must present a picture of three gray oars of a luminosity range of6 : 3 : 1. The eye will receive this oicture in exactly this ratio of bright-ness, since the mecharism of the eye response to color is not brought into play.As a matter of fact, had the original 3-color bar display been picked up by aconventional monochrome camera the result would have been similar, since the-monochrome camera response is adjusted to make a luminosity correction iden-tical to that of the eye. On a color receiver, the presentation is entirelydifferent. Here the kinescope display must be a faithful reproduction of theluminosity of the original bar pattern so that the viewer's eye might make thenecessary 6: 3 : 1 correctioni. The important point here is that, whether theviewer is observing the bars in color or in monochrome, the luminosity of eachbar is the same even though the luminous output of the display devices is ra-dically different. A color signal which can achieve this dual effect is calleda "constant-luminance" signal by the NTSC. For a wde range of colors the pre-sent NTSC signal does a good job of approximating this "constant-luminance"requirement.
Bandwidth For Color
Up to this point nothing has been said about the bandwidth requirementsfor color. As a matter of fact, there has been the implication that if 4 mc
1. Certain exceptions to this statement are discussed in the papers listed inthe Bibliograpiy at the end of this paer.
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are necessary for monochrome, then 4 mc would be necessary in each of the threeprimary colors. This would certainly be true if there were not another limita-tion of the eye working in our favor to reduce the required bandwidth.
Experiments on this subject were conducted along the lines indicated inFig. 1. A three-color camera is used with each camera channel adjusted to passinformation to 4 mc. The low-pass and high-pass filters are designed to havea common 6 db cross-over frequency (fc), which is simultaneously variable onall four filters. If now we adjust fc to be zero, we find that none of the-signal passes through the low-pass filters. All three camera outputs are com-bined in the high-pass filter into one common signal which is simultaneouslyfed to all three kinescopes of the display. The result is a high-resolutionblack-and-white picture-. If we go to the other extreme and set fc at 4 mc,the high-pass circuit contributes nothing and each color camera is effectivelyconnected to its respective kinescope. We then see a high-resolution picturein full color. This would be the equivalent of a 12-mc transmission. If nowwe slowly reduce fc we would be amazed to realize that we can detect no visibledeterioration of the color pictu-re until fc reaches approximately 0.1 mc, atwhich point we begin to observe a slight loss of color in the fine detail area.Note the careful wording of this statement. We do not detect a loss of detail,only a loss of color in the detail. This loss of color in the detail is sounimportant that some lay observers find certain color pictures tolerable evenwith fc as low as 0.1 mc. Criti-cal studies in this area have led the NTSC toset the low limit between 0.5 mc and 1.3 mc. The fundamental physiologicalconclusion of this experiment is that the eye will accept a color picture ifthere is only large-area color information plus monochrolae detail. This isnot unlike the lithographic technique of printing in three colors plus blackto carry the detail. The electronic conclusion is that the 12-mc informationcan be compressed to 6 mc, namely 3 mc for the monochrome and 1 mc for eachof the three colors.
The Basic NTSC Signal
Recognizing the three visual characteristics of the eye discussed up tothis point, namely, the ability to stimulate a range of hues from the mixtureof three selected primaries, the unequal brightness versus color sensitivity,and the insensitivity to color detail, the NTSC has formulated a color signalwhich uses the monochrome signal as it is presently constituted for its lumi-nance component and has added to this a single color subearrier to convey thecolor information. All this has been accomplished witlhn the present channelallocations of 4 mc for video information by applying to the color subcarrierthe principles of double modulation and frequency interleaving.
Double Modulation of Color Subcarrier
It was mentioned earlier that the transmission of a color requires threepieces of information, namely, hue, saturation and luminance. We have thusfar assigned the role of carring the luminance data to the monochro-me channel.The hue and saturation are handled simnultaneously by one color subcarrier of3.58 mc according to a color phase spectrum which has been set up. as shown inFig. 2. The exact reason for the unique an4les indicated is not important,but it is essential to realize that the diagram indicates the phase the colorcarrier will take on as it represents the various color elements of the pic-ture. For example, if the transmitted picture consisted of three vertical
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color bars, one red, one blue, and one green, the color carrier would take onthe phase as indicated by the red vector for one-third of the line., then changephase to that of the blue vector for the second third of the line, and finallytake on the green phase for the last third of the line. Any other hue whichmight be present in the picture would of course require the carrier to set upits phase at some intermediate position. Yellow would fall between red andgreen, cyan between blue and green, magenta between red and blue, etc. In thisway, phase modulation of the subcarrier conveys the hue. In addition, the sub-carrier is amplitude-modulated at the same time to represent the saturation ofthe color. A full-color subearrier represents a saturated color. A half-ampli-tude or quarter-amplitude subcarrier is a pastel. The complete absence of thesubcarrier is obviously no color at all - just white or gray, depending on theinforation in the monochrome (luminance) channel.
That the subcarrier disappears in the absence of the saturation-signalmodulation (no color) certainly implies that we have not selected the conven-tional type of amplitude modulation where the carrier would take on a nominalamplitude without modulation. We are using a suppressed carrier type of mo-dulation for this color information. The concept of the suppressed carriermodulation is not new, but its application in communications has been rela-tively limited. One of the difficulties of this type of modulation is thefact that the carrier must be reinserted at the receiver to recover the signalinformation. In our case the requirement is not only that we reinsert a fre-quency of 3.58 mc, but, if we are to recognize the unique color phase of thetransmitted information, we must establish a local 3.58-mc signal frozen toa specific phase with respect to our phase spectrum of Fig. 2. To accomplishthis, it is necessary that a small burst oi subcarrier energy be transmittedalong with the color signal. For this purpose a 10-cycle burst of 3.58 mc,with phase set as indicated in Fig. 2, is added on the back porch behind thehorizontal sync pulse. This information is gated into the color sync cir-cuits at the receiver and used to freeze the phase of the local 3.58 mcoscilator.
Frequency InterleavingIf we made a Fourier analy3is of the video signal produced by a conven-
tional monochrome camera, a range of harmonics would be found running thegamut from dc to 4 mc. The fundamental frequency would be 15,750 cycles, andthe remainder of the spectrum would be made up of harmonics right up to 4 mc.We would further see that each harmonic (of frequency nH) would have a smallsprinkling of sideband energy around itself due to the 60-cycle informationin the signal. The spread of this latter energy is quite--small so that wemay conclude that there is preactically 14 kc of unused spectrun space betweenharmonics. It is in this area that the NTSC has placed the color information.Fig. 3 indicates in solid lines three typical harmonics of the luminance signalspaced H (or 15,750 cycles), apart. The color subearrier, Fcs, has been ghosento fall between the nth and the (n-l)th harmonic. This ma1kes Fcs = (2n-1)I("-.The sidebands of Fcs will spread out at multiples of H above and below Fcs andtherefore fall into the empty spectral spaces mentioned above. This is fre-quency interleaving.
Let us examine the practical benefits we derive from this mode of operation.Reduced to simple words, the formula above says that the color subcarrier wechoose should be spaced above the main video carrier by a frequency equal to anodd number times half the horizontal scanning frequency. The full impact ofthis specification will be seen if we examine the appearance of this color sub-
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carrier on a standard wideband monochrome receiver. As far as this receiveris concerned, this color subcarrier is a spurious frequency and the reader'sfirst presumption is that an annoying line pattern will appear. Let us ex-amine Fig. 4. We have assumed a uniform gray raster on our monochrome receiverwith our color subcarrier riding through the video amplifier to modulate thekinescope grid. The light rectangles roughly represent the brightening of theline during the positive peak, while the dark rectangles represent the darken-ing of the line during the negative peak.
Our sketch is numbered to show the 525 lines of one entire frame (twofields) stretched out vertically, completely ignoring the inherent feature ofinterlace. The lines are therefore numbered consecutively 1, 2, 3, 4, up to525. Note that due to our unique specification, all the odd-numbered lineswill start and end with a dark spot, and the even lines with a bright spot.This means that line 1 becomes line 526 on the next franme and for the next525 lines each bright spot will become a dark one, and vice versa. Due tothe persistence of vision the net visual effect is that the eye largely aver-ages out these changes in brightness due to the presence of the color sub-carrier.
Compatibility
Although the requirement of comatibility has already been alluded to,it is important enough to warrant a discussion of its own. Compatibility isthat characteristic of a color signal which enables an ordinary monochromereceiver to produce a black-and-white picture without the requirement of anadapter. Not only does this feature facilitate public acceptance of the colorsignal, but it guarantees the gradual growth of color transmissions becausethere is no loss of viewers during this transitional period. This is an im-portant consideration from the standpoint of the sponsor behind the program.The NTSC feels so strongly about compatibility that it has undertaken exten-sive studies to insure that the monochrome picture in this case shall sufferno deterioration just because it is transmitted as a color picture.
To this end we have seen that the color subcarrier is placed well up inthe video spectrum where most monochrome receivers have considerable attenu-ation. The color information is conveyed via suppressed-carrier modulation.This means that in white areas there will be no color carrier at all; thecolor carrier will be present only in colored areas. However, we have seenthat, due to our unique choice of color carrier, even in these areas the in-terference pattern due to the carrier will be almost invisible.
In our opening paragraphs we went to some length to show that the lumi-nance part of our transmission contains all the necessary monochrome informa-tion required by our monochrome receiver. Hence, the fact that our color datamay be completely attemnated in this receiver in no way degrades the picturewhich is received.
And last, but not least, in this compatibility picture is a recent im-provement in the sound department. Not too long ago it was noticed that inthe second detector of monochrome sets it was possible to produce a visible0.9-mc (approx.) sound beat between the color carrier of 3.58 mc, and thesound carrier of 4.50 mc. The cure for this was a simple and straightforwardapplication of the frequency-interleaving principle. The color subcarrier
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was given the added requirement of also having a frequency difference to 4.50 mcof an odd number multiplied by half the horizontal line frequency. Startingwith 4.50 mc, the NTSC has, for these reasons, selected the following values:
Color subcarrier (Fcs) 44.50 (.) 3.579545 mc
Horizontal frequency Fcs (i) 15,734.3 cycles
Field frequency H 52594 cycles,
Note that this double requirement of frequency interleaving has forced us todeviate slightly from the presently used horizontal scan of 15,750 cycles persecond. This deviation of 15.7 cycles is negligible and well within the pull-in range of even the poorest receiver. This new horizontal frequency is adependent parameter resulting from the fact that:
Fcs 455and
4.5 X 106 - 572
By setting 4h5 mc equal to an even number times half the. line rate, the inter-val from Fcs to the sound carrier becomes an odd half harmonic, giving thedesired reduction in visibility of the 0.9-mc beat.
The Signal
Let us sumnarize what has been said here by examining Fig. 5. This figureindicates the video information, entering the main transmitter modulator, ofone line of a picture containing four vertical bars, reading from left to right,green, red, blue, and white. The heavy line indicates the luminance signal,and is that signal which, when stripped of the 3.58 mc information, Will makethe picture on the monochrome receiver. It is exDressed as: Ey = 0.59 Eg + 0.30Er 4 0.11 Eb. When transmitting standard white, the values of Eg, Er, and Eb,which are the electrical outputs of the camera channels, are equal; i. e., Eg wEr = Eb. Note also that, because of the weighting ascribed to each of theseelectrical signals, Ey corresponding to white is equal to each of these threesignals.
The RF superimposed on each color bar in Fig. 5 is the 3.58-mc information.The phase of each area is different and, relative to the burst phase, is indi-cated in Fig. 2. This color signal is expressed as:
Ecolor = MlEb sin (wt - 12.90) + M2Egsin (wt - 119.30)+ M3Er sin (wt - 256.90)
The constants M1, M2, and M are based on considerations of ratio of coloramplitude to monochrome amplitude and for the record are: M1l 0.447, M2 = 0.590,13 . 0.632.
Bibliography1. Farr, K. E., "Compatible color TV receiver," Electronics, pp. 98-104.;
Jan., 1953.
2. Fink, D. G., "Color fundamentals for TV engineers," Electronics, pt. I,pp. 88-93, Dec., 1950; pt. II, pp. 78-83, Jan., 1951; pt. III, pp. 104-109,Feb., 1951; Corrections, pp. 336, 338, Mar., 1951.
3. Hirsch, C. J., Bailey, W. F., and Loughlin, B. D., "Principles of NTSCcompatible color television," Electronics, cover and pp. 88-95, Feb., 1952.
4. Proceedings of the I.R.E.; Oct., 1951; entire issue devoted to colortelevision. 0
