Mac's Service Shop: Designing a Receiver for Cable TV
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In the early 1970s when this story was written, the television industry was just getting started with building out infrastructure for cable delivery to homes. All signals were analog of course, and there was very little digitization of receiver circuitry in TV sets; i.e., synthesized tuners. A large percentage of televisions used vacuum tubes and relied on twin lead transmission cable between a rooftop antenna and the back of the set. The plug-in connection of vacuum tubes often caused problems due to high resistance contacts (or no contact) between the tube pins and the sockets, and could also suffer from passive intermodulation (PIM) signals generated by corroded contacts. Unshielded twin lead, which has the advantage of much lower signal loss than coax, was a prime source of interference pick-up, especially if a long run was used. Cable TV used a 72-Ω coaxial cable between the street connection and the receiver, so it was pretty much immune to interference. The cable distribution system could provide the required signal level by using amplifiers, so the relatively high loss of coax was not an issue. Mac's trusty sidekick, Barney, solved an interference issue caused by a combination of a loose tube in on neighbor's TV set and a long twin lead cable used by the guy across the street. Mac's Service Shop: Designing a Receiver for Cable TV By John T. Frye
Perhaps TV manufacturers should recognize cable-TV is here to stay and start building receivers for use with it. Barney came in out of the hot and humid June day, parked his tube caddy on the service bench, and stood appreciatively with arms akimbo in front of the cool output of the shop air conditioner. "Man, summer is here!" he announced to Mac, his employer; "but I licked that interference problem," he added with satisfaction. "What was it?" Mac asked. "You'd never guess. Remember we were getting complaints of a vertical bar that kept sliding across the screen of a black-and-white set. Sometimes the interference was present; other times it wasn't. But it was very much in evidence during the popular evening shows. I only got a glimpse of it one time a couple of weeks ago, and then it looked like a Barkhausen oscillation that moved! It consisted of two narrow vertical lines that were about a half-inch apart, but instead of standing still at the left side of the screen as proper Barkhausen lines should, these kept moving across the screen from left to right. Before I had a chance to touch the set, the lines disappeared. I noted, though, that this customer was using an outside antenna, while most of his neighbors were on cable. None of them was having any interference problems at all. "The difficulty was really pinned down by another customer of ours, one to whom we sold a new color set last Christmas. A couple of days ago he and the black-and-white customer, who lives directly across the street, were talking about the interference; and it came out the interference began just about the time the new color set was installed. The two neighbors did a little experimenting on their own, and sure enough the bars appeared on the black-and-white set every time the color set was turned on and disappeared when it was turned off. To add to the confusion, the color set was on the cable and so had no antenna to radiate any signal it was producing." "Hm-m-m-m, that is strange. When you first started talking, I thought vertical sync bars from the cable signal were being radiated from a long run of twin-lead used on an unauthorized installation. We've both run into that one several times." "That was my thought, too, but I'm glad to have company in my ignorance. Anyway, I started checking out the color set and found a damper tube that was not seated well in its socket. I pushed it down, and that cleared up the trouble immediately. I suppose other receivers in the vicinity were not bothered because they were on the cable. The combination of strong signal from the cable plus reduced pickup of external signals made these sets relatively immune to the interference. In fact, the color set itself did not pick up the interference it was producing; yet the receiver on an outside antenna, at least two hundred feet away, suffered bad interference." "And the lines moved because of the slight difference in the sync frequencies of a color set and those of a black-and-white receiver," Mac suggested. "Ordinarily we only see interference from a short-ranging Barkhausen oscillation on the screen of the set producing the oscillation and timing it with the set's own sweep circuits; in which case, of course, the bars stand still." "Yeah, I was mulling all that over while I was driving back, and I concluded I was darned lucky those neighboring customers of ours were on speaking terms. That situation had all the ingredients of a really nasty service problem. But it would probably never have happened if our black-and-white customer had been on the cable." "True, and that reminds me of something I've been thinking about the last few days. I read an article put out by the National Cable Television Association in which it was stated that about three million of the TV sets in this country, or roughly 6%, are operating on cable now. Some 2200 CATV systems are in operation, and about the same number are franchised or are under actual construction. It is estimated that by the next presidential election in 1972 some twenty-two million more people will be watching TV via the cable and that within ten years 85% of all sets in the country will be cable-connected." "So what?" "So it seems to me it is not a bit too early for TV manufacturers to take cognizance of this fact and start redesigning their receivers for optimum performance on cable." "I think I see what you're driving at. Up until CATV came along, the major requirements of a good TV set designed to operate in either a fringe area or near a transmitter have been such things as" - and Barney ticked off on his fingers - "a low-noise front end and good sensitivity; a stable sync as immune as possible to ignition noise, airplane flutter, and other types of interference that would cause a weak-signal picture to jump and jitter; an a.g.c. system that will permit the set to handle a very wide range of signal strength so as to provide maximum sensitivity on weak signals without overloading on strong signals; and, of course, sufficient bandwidth to provide good picture detail." "So far so good," Mac said approvingly. "Now let's consider what is needed for a set operating off the cable. To start, a low-noise front end and good sensitivity are no longer of paramount importance. The signal delivered by the cable is usually around a thousand microvolts across 72 ohms on all channels provided by the cable. The hot front end is required in the cable front-end receivers, not in the TV set." "The same thing goes for the a.g.c. system," Barney suggested. "The a.g.c. in those front-end receivers and in the line amplifiers is designed to hold the signal level delivered to cable-connected sets within comparatively narrow limits under all receiving conditions. As a result, the TV set's a.g.c. system has very little to do. What's more, the sync signal going along the cable is relatively noise-free. If the receiver's input is protected from interference emanating outside the cable, much less emphasis need be placed on sync-stabilizing circuitry." "That brings up an important point," Mac interrupted. "I was thinking that the conventional length of 300-ohm line running from antenna terminals on the back cover of the TV receiver to the tuner should be replaced with coax that could be connected directly to the cable and so shield the input of the receiver completely from outside interference. Under the present arrangement, in which an impedance-matching transformer must be used between the single-ended 72-ohm output of the cable and the 300-ohm balanced input of the receiver, that length of twin-lead between the antenna terminals and the tuner provides plenty of pickup for strong signals or interference. What's more, in some cases we have both seen, it permits radiation of the cable signal into nearby receivers working off their own built-in antennas or off outside antennas." "I know you're right," Barney said with a nod. "We don't have the problem here because there is no nearby TV transmitter; but boys working on cable installations in a city where there is a powerful transmitter tell me one of their most annoying problems comes from the direct reception of a strong signal by that length of twin-lead interfering with the same signal delivered by the cable. They say you can get some dan-dan-dandy ghosts out of that arrangement. Complete shielding of the front end, as you suggest, would eliminate this." "One requirement for both cable-connected sets and for those operating off an antenna remains the same," Mac continued. "I refer to the adequate bandwidth requirement. And that brings us squarely up against a serious problem. Cable operation of TV receivers presents those receivers with a problem they were never designed to handle. I refer to the reception of equally strong signals on adjacent channels. In the beginning, the FCC went to great lengths to avoid assigning adjacent channels to TV stations operating in the same area so receivers would not have to contend with this problem; and, in general, they were successful. Only receivers operating in fringe-areas where overlapping signals from equally distant stations had trouble with this particular kind of interference. "But it is a different story on cable. Here, on many CATV systems, the receiver has to receive a station on one channel while equally strong signals are present on both sides of it. Receivers for use on cable, therefore, should have provision for better adjacent-channel rejection. We both know cable systems try to help this situation by considerably reducing the audio level of a channel below its respective video level, but this is still not enough to prevent interference with many receivers. The i.f. response curves of the receiver must be made to coincide more nearly with the perfect curve shown in the textbooks - and never seen on our scopes. I realize this is not going to be easy to do because greater reduction of adjacent channel signals tends to narrow the bandwidth of the i.f. system; but I am sure our TV design engineers have whipped much tougher problems than this." "Perhaps the manufacturers can use some of the money they now have to spend on other sections of the receiver-sections that will not be so important in cable reception - on improving this adjacent-channel rejection," Barney suggested. "That's my thought, too. But it seems to me the most important thing light now is for the CATV people and the TV set manufacturers to get together and work out their mutual problems - and they are mutual problems. It certainly will be to the advantage of both that good reception be had off the cable in the future when the majority of sets are going to be working off the cable." "They can use a little cooperation," Barney offered. "As things stand now, the CATV people are inclined to blame less-than-perfect reception on the receivers, and the service technicians for the dealers blame the cable for poor reception. The poor customer is caught in the middle. Perhaps both factions should remember how the TV transmitter manufacturers and the receiver manufacturers had to work together in the beginning when TV was getting off the ground. If they hadn't, most of us would probably still be gathered around the console radio in the living room instead of the color-TV set."
Posted June 6, 2017 Mac's Radio Service Shop Episodes on RF Cafe This series of instructive technodrama™ stories was the brainchild of none other than John T. Frye, creator of the Carl and Jerry series that ran in Popular Electronics for many years. "Mac's Radio Service Shop" began life in April 1948 in Radio News magazine (which later became Radio & Television News, then Electronics World), and changed its name to simply "Mac's Service Shop" until the final episode was published in a 1977 Popular Electronics magazine. "Mac" is electronics repair shop owner Mac McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.
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