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From: fish@ihu1g.UUCP (Bob Fishell)
Newsgroups: net.audio
Subject: AM Stereo Methods (&Retractios)
Message-ID: <165@ihu1g.UUCP>
Date: Wed, 25-Jan-84 14:43:40 EST
Article-I.D.: ihu1g.165
Posted: Wed Jan 25 14:43:40 1984
Date-Received: Fri, 27-Jan-84 10:47:50 EST
Organization: AT&T Bell Labs, Naperville, IL
Lines: 41

Well, folks, seems the ol' fish has got a few eggs on his face, but it ain't
the first time it's happened...

In my earlier response to a question about AM stereo, I implied that the
FCC had ruled on more than one AM stereo format, and that this would result
in a gaggle of competing, mutually incompatible systems for the manufacturers
to contend with.  I was wrong.  I think I had the AM stereo ruling confused
with something else, stereo sound for TV, I think.  Anyway, I shall in the
future do a little research before shooting off my big, fat terminal.

The FCC did, in fact, approve only ONE of the AM stereo systems under
contention.  This was the AM/PM system developed by Magnavox.  The system
uses conventional amplitude modulation for the L+R (mono) matrix information.
The L-R component is broadcast by phase-modulating the carrier wave, with
a phase deflection of 1 radian (57.3*) corresponding to 100% modulation.
Amplitude modulation is kept to 95%, so that the phase information in the
CW is always detectable.  In addition, a 5-Hz trigger signal is imposed
on the audio as a way of telling the receiver that an AM stereo broadcast
is being received.  The system results in an overall signal degradation of
about 5dB, which is not too bad.

Other systems proposed to, and rejected by the FCC included Motorola's
C-quam "compatible quadrature" system, and an AM/FM system tested by RCA
as early as 1959.  That it only took the FCC a little over 20 years to
approve an AM stereo system inclines me NOT to retract my statement about
that august agency's hind quarters.

The Magnavox system got the nod early in 1980, but the FCC ruled to delay
introduction of AM stereo broadcasts until early in 1981, so as to allow
manufacturers and retailers of audio equipment to adjust.  Note, however, that
the arrival of AM stereo was not heralded with a blast of trumpets by 
that industry, receiving practically no notice in the audio mags. I
guess that, although Magnavox stated they would license the system to
any interested manufacturer, it just hasn't caught on.  Oh well, does the
nation NEED AM stereo? I doubt it.  Most AM stations play such incredible
garbage that it doesn't deserve to be played through anything bigger
than a 2" speaker anyway.  I may be wrong yet, but I think that AM stereo
will wind up on the ever-growing pile of audio Edsels that's been a
hallmark of the last 20 years or so in the industry.
 
                             Bob Fishell