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Path: utzoo!linus!philabs!cmcl2!seismo!uwvax!astroatc!nicmad!brown
From: brown@nicmad.UUCP
Newsgroups: net.video
Subject: Re: Confusion in stereo
Message-ID: <314@nicmad.UUCP>
Date: Wed, 14-Aug-85 21:50:05 EDT
Article-I.D.: nicmad.314
Posted: Wed Aug 14 21:50:05 1985
Date-Received: Mon, 19-Aug-85 04:50:25 EDT
References: <3581@decwrl.UUCP> <580@hou2a.UUCP>
Reply-To: brown@nicmad.UUCP (Mr. Video)
Organization: Nicolet Instrument Corp. Madison WI
Lines: 151

In article <580@hou2a.UUCP> pjk@hou2a.UUCP (P.KEMP) writes:
>> From: lionel@eiffel.DEC (Steve Lionel)
>> 
>> 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.
>
>Sorry Steve, but your article could use some clarifications also.
>I'm no expert in video myself, but I do read the various video rags.

Time for a little more clarification.

>> signal.  This new technique, named Beta Hi-Fi, was forwards and backwards
>> compatible with the original, mono linear Beta audio track.  Beta Hi-Fi
>
>I not sure what you mean by "forwards and backwards", but if
>you mean that BETA (AFM) HI-FI is upward and downward compatible with
>mono BETA machines, I must state that while some current BETA
>VCRs can be upgraded to HI-FI by use of an optional adapter,
>this is not the case for pre HI-FI models.  Therefore
>BETA HI-FI is not truly upwards compatible, except by
>expert modifications for older units (READ-> *expensive*).

Backward compatible because older VHS and Beta machine can play the tapes
and still have sound.  He was talking about the linear track.  If is therefore
correct in his statement.

>> 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
>
>Please read up more on VHS AFM recording!
>
>VHS AFM uses "depth multiplexing" and head azimuth offsets to
>minimize interference with the video signal.
>
>Also, BETA HI-FI for PAL (and SECAM?) systems uses the VHS technique.

Not only will I read up on VHS Hi-Fi, I will tell you what the VHS Hi-Fi
manual for the JVC HR-D725U Technical manual has to say about it.

Page 1-12  #T-8015  February 1984

In the VHS Hi-Fi system, the video and audio heads are independent of each
other.  First the audio signal is recorded and then the video signal is
recorded on top of the audio signal.  Because of this method, the frequency
band of the video signal is never affected by the audio signal and full VHS
compatibility is maintained.  This is a definite advantage over other Hi-Fi
video systems.  Generally, the recorded wavelength is determined by l=V/F.

l = lambda/wavelength  V = relative tape speed of 5.8 m/sec  F = Frequency

For a FM video signal of 4.4 MHz, the wave-length on the tape is approximately
1.3um.  For the carrier frequency of 3.3 MHz, the wavelength is approximately
1.7um.

As illustrated [in the book anyway], the recorded wavelength of the video
signal ranges from about 1.3 to 1.7um is recorded to a depth of about 0.3um
on the magnetic coating of the tape by the 0.3 u gap of the video head.

In contrast to this, the FM audio signal for Hi-Fi recording and reproduction
has a carrier frequency of 1.3 MHz for channel 1 and 1.7 MHz for channel 2.
Converted into th recorded wavelength, they are about 4um for channel 1
and about 3.4 um for channel 2.  [The drawing shows a depth of 4 u using
a head gap of 1 u.  Total tape depth is 20 u.]

When the video and FM audio signals are recorded on the same track of the
tape, the audio signal magnetizes the tape far deeper than the video signal,
as illustrated, and a portion on the deep layer remains even after the video
signal is recorded on the surface layer.  This means both signals remain
with no interference to each other.

>> 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.
>
>S/N ratio and dynamic range are closely related items.
>
>The raw S/N of VHS AFM playback is something like only
>40 dB, a noise reduction system called "HD" is used to
>increase the S/N to 75 dB - 80 dB.  I believe it is a
>companding form of NR.

The raw dynamic range of VHS Hi-Fi is about 60db, not 40.

>BETA AFM NR is similar, I think, but the "raw" S/N is
>greater.  [Someone correct ME on this if I am wrong!]

According to the VHS manual, Beta FM deviation is 75KHz max while VHS has
a max FM deviation of 150KHz.  This yields about 6db more in raw dynamic range.

>> 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.
>
>As far as I know, ALMOST ALL VHS HI-FI VCRs can playback the linear
>audio track in Dolby stereo.  Many however, do not RECORD the
>linear tracks in (Dolby?) stereo.  I myself cannot understand
>this, how much more expensive is it to include the extra
>circuitry to record in stereo?

True, most record linear stereo (with Dolby) if they are Hi-Fi stereo,
but be sure to ask before buying.  Don't assume.  The Panasonic PV-9600
in linear mono, with Hi-Fi stereo.  Because of its size (portable), there wasn't
room for both stereo systems.  There are even some table top units that
don't record linear stereo, with Hi-Fi stereo.  So, don't encourage the
video deck makers by buying one.

For a little extra information, here are the specs for HR-D725U:

Tape width			12.65 +/-0.01 mm
Tape Speed (SP)			33.35 +/-0.5% mm/sec
Drum Diameter			62 +/-0.01 mm (Upper drum)
Writing speed			5.80 m/sec
Video Track Pitch		0.058 mm
Total video width		10.60 mm
Video effective width		10.07 mm
Video track center		6.2 mm measured from bottom edge (reference)
Video track width		0.058 mm
Control track width		0.75 mm
Audio track width		1.0 mm single track
Audio track width		0.35 mm ch2 (R)  top edge
Audio track width		0.35 mm ch1 (L)
Audio track reference line	11.65 mm measured from reference line
Audio to audio guard width	0.3 mm
Video track angle		5 deg 56' 7.4" (Stopped)
Video track angle		5 deg 58' 9.9" (Running)
Video head gap azimuth angle	6 deg +/-10'
Positions of audio and
   control heads from video	79.244 mm
FM audio track width		Min 0.02 mm
FM audio head gap azimuth angle 30 deg

I hope all of this helps a little bit.

Now, what we really need is digital video recording, for the home market.
It is being worked on, but when will it be in the consumer price range.
Talk about making lots of generation dubs.
-- 

Mr. Video   {seismo!uwvax!|!decvax|!ihnp4}!nicmad!brown