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From: fish@ihu1g.UUCP (Bob Fishell)
Newsgroups: net.video,net.analog
Subject: Re: MTS stereo decoder
Message-ID: <772@ihu1g.UUCP>
Date: Fri, 14-Feb-86 18:02:05 EST
Article-I.D.: ihu1g.772
Posted: Fri Feb 14 18:02:05 1986
Date-Received: Sun, 16-Feb-86 03:58:25 EST
References: <218@catnip.UUCP> <1543@videovax.UUCP>
Organization: AT&T Bell Laboratories
Lines: 37
Xref: watmath net.video:2026 net.analog:685

> >Here is the rub.  The circuit does not contain any noise reduction.
> >Apparently, the compression technique used is proprietary to dbx, and
> >the chips necessary to do it are available only to licenced OEMs.
> >So, my question:
> >
> >  - Is it worth building one of these?
> 
> NO! NO! NO!
> 

> 
> >  - Will there be any other unwanted side effects (breathing noises
> >       or frequency response abberations) without the dbx?
> 
> YES!  Only the L-R channel is compressed.  If you try to rematrix it with
> the L+R without first expanding it, the separation, at best, could be
> described as terrible!  This is the primary problem with the R-E circuit.
> 
I haven't seen the article descibed, but I think the problem could be solved
(at least theoretically) by re-extracting, via a differential amplifier,
the L-R information, expanding it (I think NS has a single-chip expander),
and then rematrixing it with the L+R signal.

I don't know what compression dbx uses on the MTS system; it's 2:1 on their
type II noise reduction systems, so that might be a place to start.  In any
case, you can vary the expansion with a pot until it sounds right.  A problem
with [cheap] compansion systems, though, is that, even if you lick the breathing
problem, hum gets picked up by the expander and modulated with the
audio.  Very annoying.

Since I already have an MTS decoder, I probably won't try this myself, but
I thought I'd advance the idea anyway.
-- 
 __
/  \
\__/			Bob Fishell
			ihnp4!ihu1g!fish