Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site decwrl.UUCP
Path: utzoo!linus!philabs!cmcl2!seismo!harvard!talcott!panda!genrad!decvax!decwrl!dec-rhea!dec-eiffel!lionel
From: lionel@eiffel.DEC (Steve Lionel)
Newsgroups: net.video
Subject: Confusion in stereo
Message-ID: <3581@decwrl.UUCP>
Date: Mon, 12-Aug-85 16:52:58 EDT
Article-I.D.: decwrl.3581
Posted: Mon Aug 12 16:52:58 1985
Date-Received: Thu, 15-Aug-85 01:29:08 EDT
Sender: daemon@decwrl.UUCP
Organization: Digital Equipment Corporation
Lines: 67


Jerry Askew's tale of the history of sound on videotape was, on the whole,
accurate, but omitted some key points and misrepresented a major
advance in the industry.

As Jerry said, the early VHS  format had a 1mm mono linear audio track.
This was later split into two .3mm audio tracks for stereo with a .4mm
guard band.  As you can imagine, .3mm of tape moving at the slow VHS
speeds is not conducive to high fidelity, so Dolby B noise reduction was
included with this new subformat.  All this did was to make the sound a bit
quieter.

However, Jerry misrepresented the merging of Hi-Fi sound with consumer video.
The revolution in video sound was when engineers at Sony (remember them?)
realized that if you could write the sound using the fast-moving video
heads that you could get a spectacular advance in audio performance.  
Sony's Beta format (at least in the NTSC version used in the US and Japan)
had enough extra bandwidth in the video signal that they could encode the
audio in much the same manner as FM stereo and mix it in with the video
signal.  This new technique, named Beta Hi-Fi, was forwards and backwards
compatible with the original, mono linear Beta audio track.  Beta Hi-Fi
debuted in the Sony SL-5200 (actually built by Aiwa) in the spring of
1983, to be followed by the SL-2700 in August 1983.

Now that Sony had, once again, shown innovation in consumer video, what
could JVC (the VHS format's keeper) do to catch up?  It was well known that
VHS did not have the bandwidth to adopt the same scheme that Beta Hi-Fi
used, so they came up with the clever method of writing the audio information
using separate heads at different frequencies.  What seems miraculous to
some, the VHS Hi-Fi VCR can pick up both signals with minimal interference
between them, and maintain compatibility with non-Hi-Fi VHS tapes.  The first
VHS Hi-Fi machine on the market was the RCA VKT-550 in the summer of 1984,
over a year after Beta Hi-Fi appeared.

Though the Beta and VHS Hi-Fi methods differ dramatically in their method
of recording the audio, both have approximately the same performance (Beta
Hi-Fi machines seem to do a bit better, but the difference is not usually
noticeable).  You get flat frequency response from 20Hz to 20kHz, negligible
wow and flutter, a signal-to-noise ratio of about 75db or better, dynamic
range better than 80db and distortion about .3%.  Note that since the
S/N ratio is so good (compare to a quality LP at 45%), there is no need
for noise reduction.  However, to get the desired dynamic range, some
"companding" is done.

Now, we get to the original question.  In Beta, there was never an official
stereo linear track, so the compatibility problem was not relevant.  Beta
Hi-Fi machines played and recorded the linear track just fine.  All was not
so well in VHS-land, however.  While there was no reason why a deck couldn't
include both the stereo Hi-Fi circuitry and also have the Dolby-encoded
stereo linear tracks, some manufacturers opted to economize by only
having mono linear capability in their Hi-Fi machines.  Thus, on these
machines, you can play VHS linear stereo tapes, but not in stereo and
without the Dolby processing.  Other makers kept the stereo heads, but
didn't let you record in stereo on the linear track.  Still others allowed
full functionality on the linear track.  So, you have to be very careful
when shopping for a VHS Hi-Fi VCR if you care about linear track
compatibility.

Well, that's our history lesson for today.  Perhaps in a year or so, we'll
see if JVC manages to come up with something approximating SuperBeta.  From
recent announcements, it doesn't seem that way.

			Steve Lionel

(Imagine drawings of boxes labelled Sony SL-2700, Sony STR-VX550 and
Proton 600M, with the pithy phrase "Beta Hi-Fi, REALLY the only way to
go" (I haven't yet updated that to "SuperBeta HiFi"...))