Path: utzoo!attcan!uunet!cs.utexas.edu!uwm.edu!rutgers!mcnc!thorin!oscar!tell
From: tell@oscar.cs.unc.edu (Stephen Tell)
Newsgroups: sci.electronics
Subject: Re: Making a mono VCR into stereo?
Message-ID: <15619@thorin.cs.unc.edu>
Date: 13 Aug 90 18:34:51 GMT
References: <1679@oucsace.cs.OHIOU.EDU>
Sender: news@thorin.cs.unc.edu
Reply-To: tell@oscar.cs.unc.edu (Stephen Tell)
Organization: University Of North Carolina, Chapel Hill
Lines: 60

In article <1679@oucsace.cs.OHIOU.EDU> bwhite@oucsace.cs.ohiou.edu (Bill White ) writes:
>
>	Several years ago I bought a brand new four-head Panasonic PV-1535K
>VHS VCR.  It has lasted me without a hitch since then, barring occasional
>needed cleaning and such, and one annoying problem (which I'll get to in
>a moment).  However, it isn't stereo, which isn't the end of the world,
>but since I have it hooked up to a component audio system, it seems a
>waste.
>	If I understand things correctly, the second audio track is kept
>on the control track, and can be read by the control head.

Sorry, you have been misinformed.

Linear stereo machines (Hi-Fi stereo is a whole 'nother ball of wax) squeeze
the two audio tracks where your machine puts one track, in the exact same way
that stereo and mono audio cassette decks differ.  This gets us free mono
compatibility; if you play a mono tape on a stereo machine, each channel gets
the same signal.  If you play a stereo tape on a mono machine, the two
channels get summed to mono right in the head.

The control track is on the opposite edge of the tape from the audio track(s).
I suspect this may be to keep noise from the 60Hz control pulses away from
the audio.  (Or are they 30Hz - 1 per frame, not per field?)

>             If this is
>correct, wouldn't it be possible to tap off the control head,
Since your initial assumption is wrong, any reasoning based on it is null
and void...

Converting the machine to stereo would require:
1. Obtaining a stereo audio head and figuring out how to mount it where the
mono one was.  Note that there may be other heads (like the control head)
in the same physical package as the mono audio head; I don't know how
VHS arranges its heads.
2. Replicate the existing audio circuitry for the new channel.

It sounds like (2) may well be within your capabilities; As for (1)
it will probably be easy if there Panasonic made a linear-stereo version
of your machine with the head available as a spare part, and perhaps quite
hard if you have to hack up a way to mount a head that wasn't designed to
fit your machine.

>	I already know the modification to get the stereo signal from the
>RF input; that's exceedingly simple.
>
>	Oh yes, the annoying problem: the audio buzzes strongly on signals
>with a bright white or with saturated colors.  I don't see any obvious
>adjustments, and RF / IF electronics isn't my field of choice.  Any
>suggestions, besides scrapping the demodulator?

I think this is the classic problem of a poor or misadjusted filter in the
audio separation part of the detector.  Since RF isn't my field either,
I can't offer much advice ehre.

>|   Bill White			Internet: bwhite@oucsace.cs.ohiou.edu	|

-------------------------------------------------------------------
Steve Tell					tell@wsmail.cs.unc.edu
CS Grad Student, UNC Chapel Hill.
Former chief engineer, Duke Union Community Television, Durham, NC.