[sci.electronics] Making a mono VCR into stereo?

bwhite@oucsace.cs.ohiou.edu (Bill White ) (08/08/90)

	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.  If this is
correct, wouldn't it be possible to tap off the control head, feed a
second line into a decoder, and get stereo output?  I'm not asking if it
is _easy_, just possible (I have lots of free time these days to play
around with stuff like this).
	Also, how difficult would it be to reverse this process, i.e.,
set up some way to grab the second audio track, process it simultaneously
with the first, and stick it onto the control head without messing up the
entire VCR.
	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?


|   Bill White			Internet: bwhite@oucsace.cs.ohiou.edu	|
|	HILLYER'S PROGRESSIVE LAW OF MISREPRESENTATION:			|
|		Fool me once, shame on you; fool me twice, shame 	|
|		on me; fool me thrice and I'll blow your head off.	|

tell@oscar.cs.unc.edu (Stephen Tell) (08/14/90)

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.

brendan@otc.otca.oz (Brendan Jones) (08/14/90)

in article <1679@oucsace.cs.OHIOU.EDU>, 
bwhite@oucsace.cs.ohiou.edu (Bill White ) says:
> 
> 	If I understand things correctly, the second audio track is kept
> on the control track, and can be read by the control head.  If this is
> correct, wouldn't it be possible to tap off the control head, feed a
> second line into a decoder, and get stereo output?  I'm not asking if it
> is _easy_, just possible (I have lots of free time these days to play
> around with stuff like this).

'fraid not.  Lo-Fi stereo VCRs that record stereo on the linear track record 
two tracks very close to each other on the lower edge of the tape.  To be
compatible with mono machines, the upper track is L+R, whilst the lower
track is L-R. 

On playback, a stereo machine can read the lower track, and can add it to,
and subtract it from, the upper track to give you separated Left and Right 
audio signals.

A mono machine just reads the L+R signal as the mono signal.

The control track is in fact written on the upper edge of the tape and 
cannot be used as an audio track.

The only way to do what you want is to put in a new stereo audio head, plus a 
stereo modulator and demodulator, plus stereo audio output jacks.  Sounds
like too much trouble to me.

> 	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've had this problem when listening to the *TV* speaker, but not if I 
connect the audio output jacks of the VCR directly to my hi-fi.  Which is 
your problem?  If it occurs with the TV speaker, then don't worry as just
about any TV will do this from interference from the CRT circuits.

If it comes from your VCR, sounds like you got one sick VCR.

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