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Practical Color TV for the Technician
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The main purpose for bothering to reprint articles like this one on analog color TV theory is to reveal the complexity and ingenuity that went into cramming a lot of information into a relatively (at the time) small bandwidth. Signals within signals and signals riding on top of and below other signals was the name of the game, and pulling it off successfully required many well-designed and well-aligned circuits. Anyone old enough to remember watching a show on analog television can appreciate the difference between a high quality set with self-adjusting capability and a cheap set that required constant fiddling with the tiny, fluted knobs on the back. I, by the way, always had (and still have) the cheap sets. A bad picture on today's digital displays consists of screwy color tones or a few missing pixels, but at least you can stand to watch your movie or ball game. If an analog set started acting up, the picture could creep to the top or bottom of the screen, the horizontal and/or vertical scan synchronizations could scramble the picture into an indiscernible mess, multipath combined with a poor receiver could cause ghost images, along with many other annoying phenomena. Proof of improvement is that instances of having a foot put through a TV screen nowadays is vastly more likely due to a poor performance on the part of a sports team than to a crappy picture. Practical Color TV for the Technician By Ken Kleidon National Color TV Manager Hycon Electronics Part 2. What service practitioners should know about the components of the color video signal.
Because of the compatibility requirement, a monochrome receiver must be capable of receiving a color transmission and of reproducing directly from it a picture in black-and-white without modifications or additions to that receiver. To facilitate this requirement, the same transmission standards imposed on monochrome signals apply equally to color signals. The latter must contain, at least, all the information provided by a black-and-white broadcast and the same specifications must apply, including the 6-mc. bandwidth for the channel, placement of the sound carrier at 4.5 mc. above the picture carrier, and so on. When the transmitted monochrome signal is analyzed from the standpoint of the service technician, it is found to consist of three component signals - one relating to video information, another to sound information, and a third to synchronizing information. A color transmission must carry each of these, but it also includes separate, additional information relating to color. Since this added intelligence must be contained within the same 6-mc. bandwidth that is allotted to the monochrome transmission, this color-signal content has been devised in such a way that it will not interact or interfere with the monochrome signal and that it will not affect operation of the circuits in a receiver designed for black-and-white reception only. As a result of this seemingly odd relationship between these separate but related monochrome and color signals, the manner in which a color TV picture is processed and reproduced in the receiver is quite distinctive. First the monochrome signals are processed by circuits similar to those in conventional monochrome receivers to produce a black-and-white picture. Then the color signals are separately processed by additional circuits especially designed to respond to them. The resultant color-producing information is then superimposed over the monochrome picture to produce an image in color.
That this manner of producing the end result is indeed used can be demonstrated in a practical way without going into technical details, if a properly adjusted color receiver is tuned to a color TV broadcast. If the color (or chroma) control is rotated to its minimum position, a black-and-white picture results. This is what has happened: turning down the chroma control has had the effect of discontinuing operation of the special color-processing circuits, or at least of preventing their output signals from reaching the picture tube. The separate monochrome circuits continue to operate, however, and a black-and-white picture results. A practical analysis of the transmitted color signal reveals that it includes five components. Three of these - video, sound, and sync - are identical to those found in monochrome transmissions. The other two are incorporated to permit the addition of color. Since the sound, signal is virtually a separate transmission on a separate, although related, frequency and since it is not affected by the fact that we are dealing with either a monochrome or color broadcast, we can put it aside. The video (or brightness, or luminance) information, which provides variations in light or dark, is interwoven with the sync signal in standard monochrome practice. The purpose of the latter signal is simply to make sure that the variations in light occur in the right places on the screen of the receiver. In dealing with color information, we have a somewhat similar situation: the chrominance signal, one of the two new components in the transmission, carries variations in color; while the color-burst or color-synchronizing information, the second of the two added signals, helps the receiver establish and separate the colors from the chrominance information provided, and makes certain that the right colors are being fed to the picture tube at the right time and in the right places. With the help of Fig. 1, we can see how the chrominance signal is squeezed into the limited bandwidth available Actually it co-exists with already present video information occurring at the same frequencies. Everything that appears in solid line pertains to the signals with which we are already familiar in the case of monochrome transmissions. A color subcarrier at 3.579545 mc., usually referred to as 3.58 mc. for convenience, is shown in broken line. The extent of its modulation sidebands are also shown in broken line. Actually, in order to describe a full range of color variations electronically, we need two signals, not one. If both of these can be varied over a wide range, and the final color produced is the result of the combination of these two, then we have an almost infinite range of possible combinations. This gives us a wide potential for representing different hues (red, green, blue, etc.) and different degrees of color intensity, or saturation. Since the limited bandwidth available for any channel makes it difficult enough to squeeze in even one additional carrier (at 3.58 mc.) , both of the signals required for chrominance information are ingeniously modulated onto this single carrier in such a way that they do not interfere with each other. It is as though two subcarriers at exactly 3.58 mc. were used. One, however, although it is at exactly the same frequency, is 90 degrees out-of-phase with the first. Hence, these two are said to be in quadrature. In this way, if we can adjust circuits in the receiver to be sensitive to the difference in phase between these two signals, we can have the effect of separate signals in the set. Since these chrominance signals are added in the form of amplitude modulation and since the 3.58-mc. frequency at which they occur falls within the 4-mc. bandwidth within which black-and-white video information also occurs, we have an additional problem. Because the receiver's video detector is designed to respond to amplitude modulation at this frequency, the color-carrying 3.58-mc. signal will show up as a rather fine-grained beat interference, marring the monochrome picture. To avoid this, the subcarrier that has been so carefully devised to provide us with desired additional information is filtered out and discarded at the transmitter! Its effect is not lost however: its modulation sidebands continue to be transmitted; and provision is made for reinserting the carrier in the receiver itself, safely away from the monochrome circuitry, so that it may once again be presented effectively with its sidebands. In a conventional black-and-white set, of course, no such reinsertion is made. The second new element added to the transmitted signal for use by color-receiver circuits is shown in Fig. 2. In solid line, we see the familiar horizontal blanking and synchronizing pulse, with video (luminance) signal visible just to either side of it. Inserted on the back porch of this pulse are 8 cycles of sine-wave signal at exactly 3.58 mc., as indicated by the broken lines. Although this color-burst signal, as it is known, has no noticeable effect on the operation of the sync and deflection circuits, it is picked up by certain added circuits in the color set that make important use of it.
It is principally used to synchronize a subcarrier reference oscillator built into color sets, tuned to 3.58 mc., in a manner that may be compared to that in which the 15,750-cps pulse is used to synchronize the horizontal oscillator in all TV receivers. In this way, the transmitter tightly controls the receiver's reference oscillator in phase as well as frequency. Thus the reference oscillator provides a reliable substitute for the sub carrier that has been filtered out at the transmitter and permits establishing the accurate phase relationship that is necessary to distinguish between the two quadrature signals that make up the chrominance information. At this point we would do well to summarize our knowledge of the signal. The monochrome transmission has three separate components, relating to video, sound, and sync. Two more are added, for a total of five, to make up the complete compatible color signal. One of these, the chrominance signal, can be regarded as the color video signal. The other, the color burst, is another sync solely for use by the special color circuits. It is used to synchronize a 3.58-mc. reference oscillator in much the same way as the horizontal sync pulse is used to control the horizontal oscillator. If we follow the course of these signals inside of a color receiver, we note that all five of them - the video (V), the sound (S), the sync or deflection (D), the chrominance (C), and the color burst (B) - enter the antenna and proceed through the tuner and i.f. amplifier stages together, as shown in Fig. 4. From this portion of the set, the 4.5-mc. sound i.f. carrier may be separated and sent directly on to the conventional sound circuits. The remaining signals go to the video circuits (detector and video amplifier). The sync or deflection signal is taken off for feeding to the sync circuits, which operate the horizontal and vertical oscillators. In addition, sync pulses are generally used to operate the keyed-a.g.c. circuits found in color sets. Video information is amplified and supplied to the picture tube. The color-burst and chrominance signals are applied to and processed by the color circuits. The resulting color video information is applied to the picture tube, where it is added to the existing monochrome image. The same system for processing color intelligence is not used in all receivers. However, as a starting point, the block marked "color circuits" in Fig. 4 has been separately expanded in Fig. 3 to correspond to one of the popularly used color systems. Since the color burst occurs during horizontal sync-pulse time, many circuits in the color-processing section take the pulse, in one form or another. It is applied, for various purposes, to the color killer, the burst keyer, and the bandpass amplifier. Also applied to the latter section are the chrominance signal and the color burst. After amplification, the burst is separated by the keyer, applied to the burst amplifier, and then fed to the 3.58-mc. color-reference sub carrier oscillator. Here it performs its important function of synchronizing that oscillator. The chrominance signal, after leaving the bandpass amplifier, is passed on to the two color-signal demodulators. In this receiver, they are the B-Y and G-Y demodulators. Y stands for the black-and-white (or luminance or brightness) component. B, G, and R stand for the three primary colors, blue, green, and red, used in color television, from which all other colors and color combinations are made. B-Y, then, would stand for all blue signal information minus the information concerning its brightness, or the amount of black or white with which it is mixed. (The latter, of course, is inserted separately by the monochrome that is supplied and which is then "painted over" with the appropriate colors.) Similarly, G-Y and R-Y stand for the green-only and red-only information. After the B-Y and G-Y (or blue and green) information has been removed from the total chrominance information found in the transmitted signal, it is possible to develop the R-Y signal from what remains without resort to a separate demodulator. These three color-difference signals, as they are called, are subsequently applied to the three guns in the picture tube. Much detailed information concerning the exact nature of the color signals has been left out deliberately. It is hoped that enough information has been covered, however, to give a broad understanding of what these signals are and to assist in understanding receiver function with respect to them. (To be continued) Color Television Articles
Posted September 23, 2014 |
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There are four areas of information, as
stated in the preceding article, with which the service technician must become
familiar if he is to service color receivers successfully. These areas cover
all aspects of the transmitted color signal, the special color circuits used
in the receiver, the new type of picture tube used at the receiving end, and
the special service techniques and procedures required. This article will be
primarily concerned with the signal. 
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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website: AirplanesAndRockets.com
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