Television digest with electronics reports (Jan-Dec 1953)

subcarrier in a manner designed to prevent interaction.* * * * 5 The frequency of the subcarrier is carefully selected to reduce the visibility of interaction between the chroma information and the brightness signal on the received picture.5 * 10. The relative location of the subcarrier within the channel is also an important considei’ation. If the subcarrier is placed too near the picture carrier there may be intei'ference between the two. On the other hand, if the subcarrier is placed too near the edge of the channel it will restrict the width of the side-bands and limit the information which can be carried. The NTSC has comproxnised on a subcai'rier frequency which is 3.579545 megacycles above the video carrier. Since this frequency is approximately .6 me from the edge of the pass band (see Fig. 3, App. B), if the blue and red chi'oma signals were transmitted they would be limited to .6 me. Resolution being a function of bandwidth, this would limit the resolution of color to very coarse detail. This limitation to a .6 me bandwidth applies only when two sets of information such as the two color difference signals must be modulated on a single subcarrier. The reason for this is that in the quadrature method of modulating the subcarrier, both upper and lower side-bands of each color difference signal must be equal. It is, however, possible to send one set of infoi'mation by using a single sideband. Thus, it would be possible to send two sets of information up to .6 me and continue to a higher modulating fx-equency with a single set of infonnation, e.g., a single color difference signal, using only one sideband. This is the method used in the NTSC system with the result that coarse color detail in the scene being televised which produces signals of frequency less than .6 me is reproduced in approximately the original color. (The third color difference signal is reci’eated at the receiver.) Semi-fine color detail in the scene being televised which produces signals of frequencies greater than .6 me and less than 1.5 me is reproduced in hues which are contaminated. (With only one color diffierence signal being transmitted the primaries cannot propei'ly combine at the receiver.) Very fine color detail in the scene being televised which produces signals of frequency greater than 1.5 me is reproduced in monochrome. The NTSC has made a variation in the method of sending the red minus brightness and blue minus brightness signals. Instead of sending the blue minus brightness and the red minus brightness over the subcarrier, each of these signals is mixed with the other so that the blue minus brightness contains some red and the red minus brightness contains 6 A sine wave subcarrier can carry two sets of Information by splitting the sine wave Into two components In quadrature and amplitude modulating each component with one set of Information. c The line frequency Is chosen as 1/286 times the frequency difference between the sound and picture carriers (4.5 mc/sec) or 15,734.26 cycles/second. Since there are 525 lines per frame, the frame frequency becomes 29.97 cycles per second and the field frequency 59.94 cycles per second. The subcarrier frequency is chosen as an odd multiple of one half the line frequency which in this case was chosen as 455/2 of the line frequency or 3.579545 mc/sec. It will be noted that the line, field and frame frequencies are very close to the nominal values used for monochrome, namely. 15,750, 60 and 30 cycles/sec.; thus existing monochrome sets will be able to respond to such scanning rates. The above combination will result in the beat note between the quiescent sound carrier and the color subcarrier being an odd multiple of one half the line frequency. It has been determined that such a relationship results in a minimum visibility, on the received picture, of such interaction as well as a minimum visibility of the subcarrier Itself due to a similar relationship of its frequency to that of the line scanning rate. contains the original intelligence in one form or another. At the receiver the demodulation process results in recovery of the original intelligence and elimination of the carrier wave which has served its purpose. This is accomplished in a demodulator (detector) and there are a number of ways of performing the demodulation. One way is to generate in the receiver another frequency which is exactly equivalent to the transmitter carrier frequency. When these two identical carriers (frequencies) are placed in the same receiver circuit (demodulator) they have the effect of cancelling each other leaving the original intelligence. This is the system used in the recovery ol the color signals described above. (The system of modulation described above is known as amplitude modulation because the modulation varies the amplitude of the carrier.) Other systems based on the same broad principles are called frequency modulation and phase modulation. In the latter cases, the modulation varies the frequency or phase characteristics of the carrier rather than the amplitude: however, the objective of the modulation is the same. some blue.’ Corresponding changes are also made in the receiver circuits so that as long as both color mixture signals are received, i.e., in the modulation range 0-.6 me, the circuits could unscramble the mixture and deliver the red minus brightness, blue minus brightness, and green minus brightness to the viewing tube. Thus, there results no change in the coarse detail of the picture. However, when only one color mixture signal is transmitted, i.e., a .6 me to 1.5 me modulation, the receiver circuits unable to function as above produce a contaminated color varying from oxange to cyan depending upon the actual color being televised. This contaminated color produces less noticeable distoration in the semi-fine detail of the picture than when a single pure color is ti’ansmitted. The luminosity of the picture is approximately uniform throughout its range from orange to cyan, thus further inducing the apparency of its color distortion. This distortion in fine detail is sometimes called edge distortion, the reason being that fine detail only occurs at the edge of an object where it contrasts with the background, or, with other objects or with part of the same object. Thus, while the eye is relatively insensitive to the color in these edges, nevertheless, if the color is intense or sharply different from the adjoining area, some distortion will be apparent. The NTSC system overcomes this difficulty by using a blended color which does not call attention to the transition. (3) Synchronization 11. The NTSC color system requires no change in the black and white synchronizing standards except that additional synchronizing information, referred to as the “color burst,” is added. In order to demodulate the color subcarrier the receiver must generate a subcarrier of its own of exactly the same phase and frequency. It is, in fact, so important that the received subcarrier be identical with the transmitted subcarrier that it is necessary to send along a sample of the transmitted subcarrier which can be used as a reference by the receiver. It is rather a problem of just where to put this reference “burst” so that it won’t be in the way of the luminance signal, the chrominance signals or the other synchronizing pulses. The place selected was the so-called “back porch” (blanking interval) following the horizontal synchronizing pulse. This is the shoi-t period during which the picture is blanked out to prevent visible retrace while the scanning beam is returning across the picture to its starting point. Only a few cycles (9 cycles of 3.579545 megacycles) of the reference burst (derived from the color subcarrier) are ti’ansmitted. (4) The Combined Signal 12. Prior to ti’ansmission over the air the various signals mentioned above are combined into a composite signal. This signal includes the synchi’onizing signals, the brightness signal, and the chroma information on the subcarrier.8 C. The Receiver 13. The following description of receivers now known is included to indicate how the signal can be used to produce a color picture. 14. The composite color signal arriving at the receiver antenna consists of a brightness component and a chroma component. The amplitude of these two orthogonal components of the chrominance signals can be expressed in terms of color difference signals as follows: Eq' — .41 (Eb' — Ey') + .48 (Er' — Ey') Ei' = .27 (Eb' — Ey') + .74 (Er' — Ey') Where Eq' = narrow-band component of the color signal Ei' — wide-band component of the color signal 8 The complete color signal has the following composition : Em' = Ey' + Eq' sin (a)t + 33°) + Ei' cos (rot + 33°) Em' = Ey' + 0.493 (Eb' — Ey') sin wt + 0.877 (En' — Ey') cos (0t where the angular frequency 0) is 2 pi times the frequency of the chrominance subcarrier. The second equation above is only valid for color difference frequencies below 500 kilocycles since the Eq' signal is removed for frequencies above that range. 9