AM AMPLIFICATIONS

Of course AM radio is not limited to a top of-to use old radio terminology--5,000 cycles! Our infelicitous usage of the English language for technical matters is still something else. Whoever invented that inaccurate term "limited"? Unlike a yardstick or its modern equivalent, the meter stick, a "limit" in broadcasting is a technical term that is far from simple. Our units of measurement are always being refined to ever more incredible accuracies (did you notice that our figure for the speed of sound in air has been revised again?), but a limit, and notably in broadcasting, is emphatically not a unit of measurement. We do not chop off our signals the way a butcher whacks through a hunk of beef. Most audio-signal qualifications are, shall I say, negotiable.

They operate much as does the all-too-human flesh when a buxom lady gets herself into a not-so-buxom evening gown. You give a little push here and something pops out there. This is not to say that limits are unprecise! Eventually either the gown splits or the flesh abrades. Similarly, the broadcast signal can go just so far, in various alter native ways, before it simply goes too far. I hope that I hear at least a hundred radio and TV engineers solemnly intoning "amen." True, we don't seem to have any limits anymore, FM or AM or what have you. The expectation, even so, is that not too many evening gowns will split so it shows. If that happens, the government will be back to help with a refitting.

These somewhat fulsome remarks (one of my favorite words, fulsome-it is so very positively negative) are occasioned by a mini-storm of correspondence concerning John K. Mitchell's letter to this mag printed in my department in the July 1986 issue. Yes, Mr. Mitchell made a linguistic boo-boo, no two ways about it. No, AM broadcast, again, is not "limited" to 5 kHz.

But an awful lot of people think so and have been thinking so for more than a half-century, which means there is sense to the thought, if not accuracy.

I haven't communicated with Mr. Mitchell and would not, in any case, have "edited" and corrected what was, I must remind you, a letter, not a commissioned column. The importance and interest of his letter had to do with his personal experience, notably in early Bell Labs hi-fi demos and in the equalization of telephone lines back in the '30s for improved frequency response and noise characteristics--including the frantic attempts to meet Major Armstrong's requirements for FM. You will note that Mr. Mitchell was a telephone man, not a broadcaster.

Presumably he remained so, even though he is broad-minded enough to read our honored publication.

I can't help thinking of what is popularly called a Freudian slip, though that term has a sexy context when rightly used. Mr. Mitchell unintentionally re versed the two halves of AM, broad casting and reception. Just like mil lions of us. Such slips, you see, reveal more than they say in literal terms. And more truthfully. Freudian slips can be embarrassing! They're not merely revealing, but more true than accurate, if you see what I mean.

I do like the engineering mind be cause it is so unlike my own. I have letters on AM broadcasting from a number of chief engineers in AM stations now actively on the AM air. Admirable precision! I am given the exact and precise FCC rules, chapter and verse, specifically Part 73.44, section (a), (1), that govern, or did govern, AM emanation in the U.S. My inclination, needless to say, is not to quote them. (Indeed, I will not! If you are a good engineer, you can find them for your self, especially if you work in an AM station.) Break them at your own risk; it is now a matter of cooperation among competitors, which is to say, the rules are more important than ever.

But the AM engineers are unanimous, and rightly, I am sure, in ascribing the base problem with AM fidelity to poor receiving sets, over these many years. I might suggest privately that some of the broadcast negotiables over the half-century-those involving amplitude modulation-have been stretched to the splitting point by quite a few broadcasters. But that tends to be changing as plain old hi-fi-clean, noise-free, wide-range-becomes not only more and more appreciated by the large public, but more and more available in every medium we know.

In a word, before we go onward, AM is now cleaning up its act. This, of course, in spite of the enormous dead weight of pop music and its freight of semi-deliberate distortion and "limits," both in the recording/broadcasting and in the reception. I do think that AM is on the move and perhaps would be even without the challenge of AM stereo, which must compete directly with all the highly perfected forms of stereo we now can provide.

There is a ring of truth to these engineering claims that the fault in AM quality has always been in the receivers we have had. Without a doubt, the "limits" do allow a very creditable AM broadcast signal to go forth--if we will put aside the unfortunate matter of interference, which was one major reason, almost the basic reason, for FM in the first place. Interference arising from nature as well as from the jammed-up, overcrowded AM band we've inherited. Interference from a million thunderstorms and almost as many man-made noise sources. Interference from unwanted stations the world over. Increase the selectivity and you cut down the bandwidth--a trade off that was unavoidable until the genius of FM gave us our cake to eat and to have. AM was a mess even after the great reforms that launched broad casting back in the '20s. We inherit, rigidly, most of that mess today, and we can do no more than play around the edges when it comes to eliminating the more hideous parts. Unless, of course, we start all over again. Fat chance of that. But we can-and do send out a superior AM signal now and then. The limits remain negotiable.

They always have been.

Yes, I have an AM radio. Tubed, ancient, table-top. I leave it fixed to a New York news station 100 miles from my home. Most days that station is clear (with background hiss) but some times, instead of New York, I get Toronto, even louder. Vagaries of nature -- Toronto's news instead of Manhattan's. Either way, this old set (with miniature tubes and a coil antenna which responds nicely to a finger touch) is able to reproduce sibilants quite clearly; it is not limited to 5,000 cycles, or 5 kHz, and its dynamic speaker with the big electromagnet is surprisingly clean. Yet in the evening, and in the morning until an hour after sunup, it is useless, bringing in dozens of distant stations one on top of the other. And at the slightest thought of a thunderhead I am blasted by explosions. Indeed, I use this AM radio as a convenient lightning indicator when in doubt about my expensive hi-fi. It is sensitive up to 50 miles or so and always warns me loudly of a storm. I then pull the hi-fi plug in a hurry. Our rural power lines are all above ground.

One correspondent mentions an early Scott AM/FM tuner in which, he says, the AM component is of unusually fine quality. This would be H. H. Scott, Herman Hosmer. The reasoning behind it was good: It was designed to receive AM/FM simulcast stereo (left channel via one transmitter, right channel via the other) in the years before the present FM stereo system was launched, after unconscionable delay (and compromise). The AM section was built to come as near as possible to FM quality so that the two stereo channels would not be unduly different in sound. Interesting. This would ac count for quite a number of excellent AM receivers in the '50s and early '60s.

Since then, AM in our fancier home equipment has been pretty much unused, with inevitable deterioration. A few years ago I found that my old AM table-top was easily superior in its reception to that of a new, top-line AM/ FM tuner. The development money and the manufacturing cost in that new model, and in too many others, went into the FM segment.

If you have a very old AM/FM tuner and can fix it up with AM specifically in mind, you may find you had a bargain back then, well back into the tube era.

You might want to experiment with current AM, just to find out what these modern-day AM broadcasting engineers are actually doing. WDAF in Kansas City, Mo., for instance, is one of dozens of AM stations that now feel strongly about transmitting a quality AM signal.

Perhaps the most exciting correspondence on this general subject came from an amiable curmudgeon who was kind enough to make copies of a whole series of Scott factory news letters dating from as far back as March 1935. This was the other Scott, E. H., who built fabulous AM radio receivers (component and console) in the prewar years. My friend has a Scott of this vintage which astounds when heard today.

Now here was an AM receiver! With 23 tubes (different numbers in later versions), it was an all-wave model for reception the world over, as was then popular in expensive radios--my 1934 Midwest with a mere 16 tubes was a lesser example of the type. I used to listen to Hitler "live" on that one. This new Scott model (he had been in the business for a good many years) was announced in March 1935. It was evidently extraordinarily sensitive and had a continuous selectivity/bandwidth control from very sharp to broad. There is a remarkable account of an all-night DXing session apparently on the broadcast band which logged clear signals from Paris to New Zealand.

Wide-open, one Scott models overall response was essentially flat, radio and audio, to 16 kHz. All this, mind you, in early 1935.

What astonished me most, though, was the Scott newsletters' liberal use of the term high fidelity, back in 1935 and the specs definitely warranted it.

Scott even refers to "other" high-fidelity radios that weren't worthy of the term but used it anyway. So our history moves back still further.

No FM in the 1935 Scott. Not so much as a mention, though FM was already under demo in those years.

Scott probably knew, but he seems to have been an AM genius who knew what AM could do, in receiving as well as broadcast, and was out to prove it.

He did that, for sure.

(by: EDWARD TATNALL CANBY ; adapted from Audio magazine, Nov. 1986)

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