FM Wins in British
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Four months prior to the publishing of this article, Radio-Electronics magazine ran a piece by the same author, Ralph Hallows, asking the question Which - AM or FM? England Ponders. Here, Mr. Hallows lays out the process by which the British Broadcasting Corporation (BBC) and governmental entities decided on frequency modulation (FM) as the way forward in commercial broadcasting. You will not be surprised to learn that the primary drivers were wider bandwidth for greater fidelity and much high immunity to both manmade (QRM) and natural (QRN) electrical interference. Tests employing the help of non-technical radio listeners showed that the claim of off-tuning and frequency drift by small amounts did not appreciably degrade the quality of reception and listening satisfaction. FM Wins in British Tests
Graph of results of laboratory FM radio test. By Ralph W. Hallows Writing of the British tests of FM versus AM, in the October, 1951, issue of this magazine, I predicted that the BBC's report would be a very important document when it came. It is all that. It is a completely unbiased account of the results of the most thorough comparative tests ever made. And it settles once and for all the question behind the most heated controversy in the history of radio: For v.h.f. broadcasting FM is overwhelmingly superior to AM. FM deals much better with interference. For a given output power the service area with FM has a radius nearly twice as great. A high-fidelity FM receiving outfit need cost little if any more than an AM receiver. It is no harder to tune or maintain than the AM receiver. A word about the tests themselves. At a site about 25 miles southeast of London a specially built station was equipped with AM and FM transmitters, each with a carrier power of about 20 kilowatts. These worked simultaneously into a slot antenna array 1,100 feet above sea level. The AM frequency was 93.8 mc and the FM 91.4. In addition to laboratory and field tests by their own engineers, the BBC provided a large number of listeners - technical and nontechnical - with receiving equipment. Each listener sent in regular reports in the form of answers to questionnaires. The transmissions started in July, 1950, and the report is based on nearly a year's working. Laboratory tests had shown that there was very much less receiver hiss with FM than with AM. This was fully confirmed by listeners. With a simple dipole antenna a well designed receiver on the average required a field strength no greater than 50 microvolts per meter to make hiss barely noticeable with FM. To produce the same effect with AM called for 1,000 microvolts.
Subjective tests employing actual listeners at receivers, and motor cars to create interference slightly exaggerate the results of practical experience, FM is at all points much better than AM or AML. In terms of field strength it was found that where x microvolts per meter was needed with FM for auto ignition interference to become negligible, AM required 10x microvolts to produce a comparable effect. To enable the results to be adequately assessed and to be used for planning a nation-wide v.h.f. broadcasting service the BBC evolved a "standard listener" and worked out how he could be provided with a "standard service." The standard listener uses a simple type of antenna within 30-60 feet of a road carrying a full stream of traffic. He (or she) has good hearing and finds interfering noises objectionable if they are more than occasionally noticeable. Two grades of standard services are suggested: 1. First class. No perceptible interference from at least half of the automobiles that pass the house. No ignition system to produce more than slightly annoying interference, and such interference to be very occasional. 2. Second class. Interference from half of the passing vehicles to be no worse than perceptible. Only occasional cars to cause annoying interference. The table shows the field strengths required with FM, AM, and AML to give the standard listener a first-class or a second-class service. Not much doubt, I think, about which system is going to do the job best! FM needs a field strength no greater than 1 millivolt per meter to insure a first-class service or 1/4 millivolt for a second-class service - and even the latter is pretty good. To serve the standard listener as well AM must have field strengths from 10 to 12 times as great. With a given output power an FM transmitter has nearly double the range of an AM one. If you double the range of a transmitter, its service area becomes not twice but four times as great. Hence the number of FM stations needed for nation-wide coverage of Britain - or any other country - with a v.h.f. service is only about one-fourth of those that would be required in an AM system. Think of the saving in capital outlay! The report contains several surprises. One of these is that the bogey of increased cost raised by those who oppose FM becomes a myth. It's true that the receiving set may be somewhat (though not much) more expensive than its AM counterpart. But this is largely - if not entirely - offset by the much lower price of the antenna needed to make certain of good reception. In fact, it was found that a very large proportion of listeners living within 30 miles of the transmitter could obtain all they wanted from the simplest of indoor antennas. At greater distances the smaller field strength required for good FM reception always means that the antenna is simpler and less expensive than a comparable AM antenna. Surprise No. 2 really is a surprise! It has been stated again and again that the biggest point against the successful use of FM by the ordinary listener was that he would never be able - or would never bother - to do the accurate tuning necessary. We were assured that if the FM receiver were the smallest bit off tune, results would be horrible. Visiting the homes of non-technical listeners, BBC engineers have found that their slightly incorrect tuning of a v.h.f. FM receiver produces no worse effects than the same kind of inaccuracy does on the broadcast band. It has been impressed on us by the anti-FM-ites that oscillator frequency drift must be fatal to good reception, unless the receiver incorporates such refinements as crystal control or automatic frequency control. For all that, the tests have shown conclusively that simple and inexpensive sets, with no such luxury fitments, give no trouble once they have warmed up a few minutes after being switched on.
Posted January 12, 2022 |
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Receiver hiss is critical particularly as its
level is increased by the wide pass-band needed by high fidelity; but much more
so is the impulsive interference due to automobile ignition systems. The graph ·shows
the results obtained in the laboratory. (AML stands for AM receiver with limiter.)
The most important parts of the curves are those where the peak-impulse-to-peak-carrier
ratio is greater than unity, because it is there that ignition interference is most
annoying. Though there is a sharp fall in the effectiveness of FM from the point
at which the impulse is of equal strength with the carrier to the point at which
it has a little more than twice the strength, the curve levels out again. Over the
whole range of signal-to-noise ratios FM is much better than AM, though AM with
a feedback limiter runs about even with it for some distance.