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Path: utzoo!linus!decvax!decwrl!glacier!mips!hansen
From: hansen@mips.UUCP (Craig Hansen)
Newsgroups: net.video,net.analog
Subject: Re: MTS stereo decoder
Message-ID: <334@mips.UUCP>
Date: Wed, 12-Feb-86 13:34:24 EST
Article-I.D.: mips.334
Posted: Wed Feb 12 13:34:24 1986
Date-Received: Fri, 14-Feb-86 06:02:13 EST
References: <218@catnip.UUCP> <503@tekcbi.UUCP>
Organization: MIPS Computer Systems, Mountain View, CA
Lines: 26
Xref: linus net.video:2018 net.analog:681

> > The current (March 1986) issue of Radio-Electronics magazine contains
> > plans for a stereo tv decoder.  

>                                         But what they don't tell you 
> (directly), and here is the rub, is that in stereo TV, the dbx reduction is 
> only on the difference channel, not the main channel.  (It does show it in the
> diagram).  Obviously, they cannot modify the main channel without affecting all
> existing tv's.  Therefore, you would really need to add the noise reduction 
> on the difference signal prior to summing it with the main channel (L+R).  
> You cannot get to that point on that chip.  Therefore, you would not be able to
> do the noise reduction where it should be done.

Actually, it is very easy to get the original L+R and L-R channels back
from a monolithic multiplex stereo decoder chip.  Take the L and R
outputs and add them to get the L+R channel, and subtract them to
get the L-R channel.  A couple of Op-Amps can do it easily.
(This technique is widely used for cable & subscription
decoders, where the pay sound is in the L-R channel.)

You can then put the L-R through dbx compensation, and
add and subtract them again to get the real L and R channels.


Craig Hansen
MIPS Computer Systems
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