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From: sgcpal@watdcsu.UUCP (Paul Layman [EE-SiDIC])
Newsgroups: net.video,net.analog
Subject: Re: How does Beta HiFi audio avoid head switching transients
Message-ID: <1104@watdcsu.UUCP>
Date: Tue, 12-Mar-85 10:08:45 EST
Article-I.D.: watdcsu.1104
Posted: Tue Mar 12 10:08:45 1985
Date-Received: Wed, 13-Mar-85 00:07:48 EST
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Organization: U of Waterloo, Ontario
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> 
> 	Can anybody explain to me how the Beta HiFi system avoids
> corrupting the stereo hi fi sound signals when it switches from
> one spinning video head to the other (which should produce some sort
> of phase and amplitude transient every 60th of a second in the FM sound
> subcarriers). ?
> 
You might try looking at an article by Kono and Kamai et al. entitled
"VIDEO RECORDING AND PLAYBACK SYSTEMS -- BETA HI-FI VCR" in the IEEE
Transactions on Consumer Electronics, Vol. CE-29, No. 3, August 1983,
pp. 141-151.  This article gives the most complete description of Beta HI-Fi
that I've seen.  The most complete spec's as well ( would you believe
a 120dB S/N at 10 kHz !).

The most simple explanation I can give you from this article is that there
are actually 4 FM carriers -- 2 for each channel.  The signal for each
audio channel is overlapped on 2 of these channels. The two signals are 
mixed using a matrix and some select signals form the video heads.
The result is "a continuous signal with *no* switching noise." (:-)

By the way this article describes a system which was modified somewhat
when a new chip-set was introduced by Sony, I think.  It was described
in the November 1984 Transactions on Consumer elctronics, I think.
(My copy is missing).  If I recall correctly the performance was
different, possibly not as good, but I may be wrong.

While I'm at it all you VHS guru's might want to have a look at an
article entitled "HIFI VCR System" in the August 1984 issue which describes
the VHS HI-FI system in detail.  I hope you hope you can understand it.
It starts with "In applying this system to VHS VCR's, we have encountered
some compatibility problems, since there is no vacant frequency bands for
sound signals." And goes on to describe how they finally managed to get
it to work in both SP and SLP modes.  Unlike Beta they use only 2 audio
carriers and make no mention of how they avoide overlapp noise.

Paul A. Layman  [EE - SiDIC, University of Waterloo]