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From: jimb@tekcbi.UUCP (Jim Boland)
Newsgroups: net.video,net.analog
Subject: Re: MTS stereo decoder
Message-ID: <503@tekcbi.UUCP>
Date: Tue, 11-Feb-86 17:55:12 EST
Article-I.D.: tekcbi.503
Posted: Tue Feb 11 17:55:12 1986
Date-Received: Thu, 13-Feb-86 19:03:31 EST
References: <218@catnip.UUCP>
Organization: Tektronix, Beaverton OR
Lines: 38
Xref: linus net.video:2012 net.analog:679

> The current (March 1986) issue of Radio-Electronics magazine contains
> plans for a stereo tv decoder.  
> ...... a cheap and convenient way of adding stereo decoding ...for about $40.
> 
> Here is the rub.  The circuit does not contain any noise reduction.
> Apparently, the compression technique used is proprietary to dbx
> So, my question:
> 
>   - Is it worth building one of these?
I read that article, also.  Actually, it can be built for about $10 depending
on what you have lying around.  About a dozen caps and resistors, an led,
a 2N2222 (or similar npn), and an LM1310 (or similar stereo decoder chip).
It's worth it as an experiment in the interest of science.  Actually, the
method used for mts stereo is similar to the one used in FM broadcasting.
The only difference is that the FM stereo uses a 19Khz pilot with a 38KHz
suppressed carrier and stereo TV uses a 15+ KHz (horizontal freq.) pilot
and a 31+ KHz suppressed carrier for the difference signal.  Therefore, all
the circuit in RE is doing is changing the oscillator frequency on that chip.
Nothing magic there.

>   - How noisy will it be without the dbx chips?
The article does mention that the circuit doesn't have the dbx noise reduction.
How noisy it will be, I don't know.  The article does say that you can add some
noise reduction to improve the system.  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.

>   - Will there be any other unwanted side effects (breathing noises
>        or frequency response abberations) without the dbx?
> 
I don't know enough about dbx to comment.  However, I do have all the parts
and a few National LM 1894 DNR (Dynamic Noise Reduction) chips so I think
I'll build this thing, try it out, and let you know.