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From: edhall@randvax.ARPA (Ed Hall)
Newsgroups: net.audio
Subject: Re: AM stereo
Message-ID: <1645@randvax.ARPA>
Date: Mon, 23-Jan-84 21:10:11 EST
Article-I.D.: randvax.1645
Posted: Mon Jan 23 21:10:11 1984
Date-Received: Fri, 27-Jan-84 10:32:31 EST
References: <2224@allegra.UUCP> <161@ihu1g.UUCP>, <112@iwu1b.UUCP>
Organization: Rand Corp., Santa Monica
Lines: 35

----------------------------
An AM station just south of the border here broadcast in `AM Stereo'
for a few months about 12 years ago.  Although a Mexican station, the
signal was beamed Northward and was pretty easy to receive in the LA
area; it had an `easy listening' format and anouncements were in
English.

The technique used consisted of modulating the upper and lower
sideband of the signal with the left and right stereo channels.  By
tuning two AM radios slightly off-center, one above and the other
below the carrier, a moderate amount of stereo separation could be
obtained. (Actually, tuning an AM radio off-center often results in
better frequency response, as the bandwidth of the receiver is usually
rather small, and tuning so that the carrier is towards the top or
bottom of the passband allows higher-frequency information from one
sideband through at the expense of the other (usually redundant)
sideband.) This mis-tuning increased distortion and noise a bit, but
the spaciousness of the resulting sound was unmistakable.  A bit of a
pain to set up, though, as most households don't have two similar AM
radios easily placed in a stereo configuration.  This might be the
reason the broadcasts were discontinued; I never did find out just
what happened to the station.

The bandwidth required for this sideband technique is theoretically
considerably more than a normal AM signal, although passband filters
at the transmitter can reduce this at the expense of increased
distortion.

Several of the newer AM stereo methods use a similar technique, but
reduce the bandwidth in other ways by various (incompatable) encoding
schemes.

		-Ed Hall
		decvax!randvax!edhall
		edhall@rand-unix