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From: bill@videovax.UUCP (William K. McFadden)
Newsgroups: net.video,net.analog
Subject: Re: MTS stereo decoder
Message-ID: <1555@videovax.UUCP>
Date: Tue, 18-Feb-86 13:03:40 EST
Article-I.D.: videovax.1555
Posted: Tue Feb 18 13:03:40 1986
Date-Received: Wed, 19-Feb-86 23:59:33 EST
References: <218@catnip.UUCP> <1543@videovax.UUCP> <772@ihu1g.UUCP>
Reply-To: bill@videovax.UUCP (William K. McFadden)
Organization: Tektronix, Comm Group, TV R&D
Lines: 46
Xref: linus net.video:2044 net.analog:688

In article <772@ihu1g.UUCP> fish@ihu1g.UUCP (Bob Fishell) writes:
>> >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:
>> 
>> >  - 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

You're on the right track, but there is a problem.  The dbx compression used
for MTS is not the same as their tape noise reduction system.
The compression on the tape NR system is linear 2:1, as you said, so it is
only amplitude dependent.  The MTS compression, on the other hand, is
amplitude *and* frequency dependent.  I've seen the curves, and they look
pretty strange.  Chances are, using the dbx type II NR system would eliminate
a lot of the hiss, but would not improve the separation much.  And since
separation is important in a stereo decoder, you are probably still better
off with mono until you can get a *real* MTS expander chip.

For those who are interested, I have found the following articles informative:

"TV Multichannel Sound - The BTSC System,"  C. G. Eilers, 
  _IEEE_Transactions_on_Consumer_Electronics_, vol. CE-30, pp. 236-240,
  Aug. 1984

"A Companding System for Multichannel TV Sound,"  L. B. Taylor, M. F. Davis,
  W. A. Allen, _IEEE_Transactions_on_Consumer_Electronics_, vol. CE-30,
  pp. 633-640, Nov. 1984

-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-594  Beaverton, OR  97077
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GTE:  (503) 627-6920        "How can I prove I am not crazy to people who are?"