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Which - AM or FM? England Ponders
October 1951 Radio-Electronics

October 1951 Radio-Electronics

October 1951 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

It might be hard to believe that at one time there was cabinet-level debate in government offices regarding whether FM radio should be permitted to encroach on AM radio's well-established presence in the commercial broadcast radio domain. When this 1951 Radio-Electronics magazine article appeared, stereo was not even part of the contest since neither AM nor FM had implemented it. The primary points of contention were cost to purchase and maintain a receiver (AM wins) and consistency and quality of the sound over a broad range of frequencies and amplitudes (FM wins). Some argued frequency drift would cause FM to need constant re-tuning even with crystal control. Four months later the February 1952 issue of Radio-Electronics reported on the winner in an article entitled "xM Wins in British Tests" (don't want to kill the suspense) for you.

Best system in doubt after year's test; winner to be the standard v.h.f. system; government may still enter a dark horse.

V.h.f. slot antenna at Wrotham - RF Cafe 

Fig. 1 - V.h.f. slot antenna at Wrotham.

By Ralph W. Hallows

Some time ago the British Broadcasting Corporation realized that it would have to provide a nationwide v.h.f. broadcasting service soon. The reason is simple: the medium-wave band is so overcrowded on this side of the Atlantic that interference-free reception can't be guaranteed even from high-powered transmitters at short range. For example, I have not been able for months to rely on receiving programs from the 100-kw London station which transmits on 247 meters (1214.5 kc), though it is barely 16 miles from my home.

If a full-sized v.h.f. network is planned, the first thing to decide is whether it will be AM or FM. No reliable or completely unbiased data was available, at any rate, not for high-powered services. The BBC concluded that the only recourse was:

  • Build AM and FM transmitters of approximately equal output power, each to be the best of its kind that could be designed and each to maintain the same standards of high fidelity.
  • Install both at a site which could serve the whole London area; when it would be decided which system was the best, the equipment not required would be dismantled.
  • Broadcast the same program simultaneously on AM and FM with separate carrier frequencies, of course -from the same antenna.
  • Provide a large number of observers with a standard, specially designed, high-fidelity receiver, so arranged that the listener could switch instantly from FM to AM or vice -versa.
  • Conduct the tests for at least a year and arrive at no final decision until the mass of listeners' reports - and the economic aspects OCTOBER, 1951 Foreign News of the FM-AM problem - had been fully considered.

The tests have now been going on for more than 12 months with carrier frequencies of 91.4 mc for FM and 93.8 mc for AM. It is expected that the report will appear shortly. I believe that report will be of worldwide importance. Completely free from any kind of political or commercial bias, it will show conclusively which of the rival systems has proved itself the better.

The Transmitters

V.h.f. transmitter hall, with 25 kw FM unit - RF Cafe

Fig. 2 - V.h.f. transmitter hall, with 25 kw FM unit, foreground, 18 kw AM, rear.

The dual transmitting station is at Wrotham (pronounced something like Rootum) in the county of Kent. The site is 730 feet above sea level, so that the 470-foot mast gives a total antenna height of 1,200 feet. Fig. 1 shows the v.h.f. aerial and part of the triangular support mast. The aerial, which is shared by both transmitters, consists of 32 slots in the wall of a cylinder 110 feet long and 6-1/2 feet in diameter. The slots are arranged in eight tiers, four slots in each tier.

The FM transmitter has a power output of 25 kw, and the AM is rated at 18 kw (unmodulated), which comes to just about the same thing. Both transmitters can deal faithfully with audio frequencies up to 15,000 cycles, but the normal range covered is 30 to 13,000 cycles, with a linearity better than ±1 db. The maximum available deviation for the. FM transmitter is ±100 kc; ±75 kc is generally used.

Fig. 2 shows the transmitter hall. In the foreground is the 25-kw FM transmitter, and beyond it is the 18-kw AM transmitter. The kiosks, from which the transmitters are controlled, are behind the windows in the wall on the right. The two doors between the transmitters give access to the air ducts forming part of the tube cooling system.

The first thing that strikes you on going into the transmitter hall is the much smaller size of the FM assembly. The modulator of the AM transmitter has to supply a pretty large part of the total output power and is therefore rather large. The FM gear is thus more compact, since it requires no such giant modulator.

FM transmitter uses a new, frequency modulated quartz drive system - RF Cafe

Fig. 3 - The FM transmitter uses a new, frequency modulated quartz drive system.

The AM transmitter needs no special comment; but its FM partner incorporates a new drive system, developed by the Marconi Company and known as F.M.Q. (Frequency Modulated Quartz). Fig. 3, another view of the v.h.f. transmitting station at Wrotham shows: Top, the F.M.Q. drive for the FM transmitter, consisting of a directly modulated crystal oscillator and a series of frequency multiplying stages; bottom, first r.f. amplifier stage, consisting of one C144 double tetrode, and (bottom) second r.f. stage, consisting of two TT16 tetrodes. 

The oscillating crystal is directly modulated and the carrier frequency is at all times crystal controlled. With 100 kc deviation, the center frequency change is less than ± 10 parts in one million. The crystal is cut so that it produces no harmonics inside the operating range of frequencies.

The modulating audio signal, after passing through a low-pass filter and an attenuator, is fed to a balanced pair of push -pull modulator tubes, the output of which goes to the crystal oscillator by way of an amplifier. Part of the r.f. output of the crystal is fed through a phase-splitter to the modulator tubes. The susceptance of the balanced modulator is varied by the applied audio signal and the frequency generated by the crystal is correspondingly varied. In other words, the oscillations generated by the crystal, which controls the carrier frequency, are frequency-modulated by the a.f. signal; hence there is no instant at which the carrier and its deviation are not completely taken care of by the crystal.

The Standard Receiver

This receiver (see Fig. 4) is made to BBC specifications by R. N. Fitton and Company and was designed by F. H. Beaumont. It represents the finest FM/AM high-fidelity table radio (range 87.5 - 95 mc) that can be made, with little regard to cost. There are five controls in addition to the on-off switch on the right side of the cabinet. Reading from left to right these controls are: FM volume; AM/FM change-over (a 3-position switch; the middle position gives AM plus the noise limiter); AM volume; signal muting (this enables a dead quiet background to be obtained with the set on); tuning. The maximum undistorted output is 5 watts. A high-grade loudspeaker is fitted with a corrector network designed by the BBC.

The circuit uses 18 tubes (including two rectifiers and two stabilizers). The r.f. and i.f. stages are common to both systems, the intermediate frequency being 14 mc. The first FM limiter acts also as AM detector. The output of the AM detector is fed to one half of a double-triode a.f. amplifier, the output of the discriminator being fed to the other half. The noise limiter can be used, as we have seen, with both AM and FM. The oscillator is kept dead steady by temperature compensation and by the voltage-stabilizing circuit; a.f.c. is also provided to make doubly sure, the grid biasing voltage being taken from the discriminator.

AM /FM receiver designed for tests - RF Cafe 

Fig. 4 - AM /FM receiver designed for tests is linear, ±2 db, 30- 12,000 c.p.s.

The extended bass response of the receiver was an interesting problem, for it was found that with conventional circuit arrangements it tended to swing the whole high-voltage line stability. The answer was to isolate the output tube's plate circuit from this line and to provide it with its own plate-voltage supply by means of an auxiliary rectifier and smoothing circuit. The fidelity of the receiver for both FM and AM is ±2 db from 30 to 12,000 cycles. For both, the sensitivity is 2 watts into the speaker for less than 150 µv input (at ±75 kc deviation in the one case and 40% modulation in the other) and the over-all distortion 1.8%.

Which system is going to be chosen? From my own experiences and those of friends who live at various distances from the transmitting station I'd be inclined to back FM so far as performance is concerned. Points are: the FM range is rather better - the carrier amplitude doesn't vary and you get good reception in any place where the signal is strong enough to operate the limiter; much less volume compression is needed - no fear of fading very soft passages right out, or of unsatisfactory signal-to-noise ratio in view of the dead quiet background; FM certainly seems to deal better with automobile ignition interference than AM, a serious problem in the region of 90 mc.

Performance, though, isn't the whole story. There are other considerations; the questions of initial cost and upkeep expenses. Some feel that if a popular priced FM receiver is produced it will need realigning at short intervals by the serviceman. This will cost the listener money. Those who hold this view claim that if AM reception is nearly as good as FM that it should be chosen out of regard for the pockets of the listener.

It will be a close thing between the two. All things considered, I back FM to scrape home by a narrow margin. But at present the problem still remains to be decided.

(From latest reports received from Mr. Hallows as we go to press, it seems that the issue is more in doubt than ever. He says, in part:

"The debate in the House of Commons has done no more than provide us with a new Radio Mystery ... There is no possible doubt that FM proved itself the victor in the tests. The BBC has come out strongly in favor of it.

To everyone's surprise, the Postmaster General announced during the debate that a recent development made it inadvisable to decide in favor of either AM or FM.

Though he would give nothing away, the impression left was that he had another sort of modulation (possibly pulse) in mind.

Top BBC engineers are as mystified as the rest of us and know nothing of any such development. If there is a new development likely to compete with AM and FM, it is something produced by the engineering department of the post office. There are some brilliant people there, and it is possible that they really have got something. " -Editor)



Posted January 12, 2022

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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 tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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