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From: jimb@tekcbi.UUCP (Jim Boland)
Newsgroups: net.video,net.analog
Subject: Re: MTS stereo decoder
Message-ID: <509@tekcbi.UUCP>
Date: Thu, 13-Feb-86 20:38:53 EST
Article-I.D.: tekcbi.509
Posted: Thu Feb 13 20:38:53 1986
Date-Received: Sat, 15-Feb-86 03:34:02 EST
References: <218@catnip.UUCP> <1543@videovax.UUCP>
Organization: Tektronix, Beaverton OR
Lines: 58
Xref: watmath net.video:2024 net.analog:683

> In article <218@catnip.UUCP> ben@catnip.UUCP (Bennett Broder) writes:
> >The current (March 1986) issue of Radio-Electronics magazine contains
> >plans for a stereo tv decoder.  
> >
> >  - Is it worth building one of these?
> 
> NO! NO! NO!
> 
> >  - How noisy will it be without the dbx chips?
> 
> About as noisy as FM Stereo, when compared against FM mono (e.g., S/N is
> degraded by 23dB on weak signals).
> 
> >  - 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.
> 
OK Folks.  I had all the parts and a few hours so, I built this thing.
Note:  the schematic does not show you that you must apply VCC to pin 1.  Also,
I put decoupling caps on the outputs.  I'm not to fond of sending 12V through
shielded cable to my amp.

I used the MPX output from my Sony 2700 and 2710.  Both had the same result.
Yes, the LED recognizes whether there is a stereo signal present there or not.
That is about the only positive thing I can say about this circuit.  
When on a station that is only mono, the sound is fairly normal.  The same is
true when on a stereo station and detuning the osc. frequency on the decoder.
However, when the decoder osc. is adjusted correctly, the output sound is
noticeably in stereo (through headphones) but EXTREMELY bad.  
How extreme you say??? Of course, it is all subjective, but how about this:
	a. Very noisy
	b. very "hissy"
        c. hum and noise levels varying as the picture varies in level, etc.
        d. signal to noise ratio as being close to 0 db. ie,  1:1 or worse
           yet, 1:2+
In a nutshell, those are my experiences.  someone else may have others.

> 
> In my opinion, you would be better off buying a real MTS decoder and not
> wasting your time building this circuit until the dbx chips are here.
> Otherwise you will be dissappointed.

I agree 100%.  The article did warn you that it would not be ideal but that
you need to start building circuits sometime.  I think that "sometime" is
still in the future.  Look at the simple schematic in RE.  Then go to your 
local radio shack and look at the schematic for their tuner.  Ignore the tuner
and amp part and just check out the decoding part.  Big difference and a lotta
lunch in that box.  I think we're still far from getting it as simple as in
the RE article.
> On the positive side, the article is educational.  It explains how the MTS
> system works.
Yes.

and of course, these opinions are only mine.
tektronix!tekcbi!jimb