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In article <4822@jhunix.UUCP> ins_aeas@jhunix.UUCP (Earle A. Sugar) writes:
>
>    Does anybody have the details of how MTS stereo is multiplexed and
>impressed on the VHF carrier?  What I am interested in in building my
>own decoder to connect to my stereo and TV.  Can I just put a normal
>FM stereo decoder chip in the normal audio
>signal from the TV tuner , and feed the output through the phono inputs
>on my stereo?  Or is there more to it (probably)?

You're right, it's not that simple.  To build a proper decoder requires a
special IC made by dbx that is not available to consumers.  In short, you're
better off buying a add-on decoder like F.R.E.D.  BTW, the audio subcarrier is
4.5 MHz above the video carrier and most add-on decoders pick up this sound
I.F. right at the video detector before it gets trapped out by the TV.

Quite some time ago I posted an article about MTS that might help explain how
it works.  It's a bit dated, but the technical information is accurate.

Subject: Stereo TV

-----------------------------------
From: net.ham-radio:

In article <1178@mhuxt.UUCP> rma@mhuxt.UUCP (ATKINS) writes:  
>Does anyone know where I can find out the scheme used for 
>stereo sound transmission on broadcast TV signals? Looking at 
>the signal it seems to be some kind of sub-carrier system, but 
>not (L+R) and (L-R) as in FM radio.

For those people unfamiliar with FM stereo broadcasting, it uses a 
system of subcarriers to encode the stereo information.  The mono (L+R)
information is broadcast as the normal baseband signal.  This is done to
remain compatible with mono FM receivers.

The difference (L-R) is transmitted as a 38 KHz subcarrier using double
sideband, suppressed carrier AM (DSB-SC).  A 19 KHz pilot tone is provided
to allow stereo receivers to decode the subcarrier (also provides an
indication that an incoming broadcast is in stereo).  To decode the left-
and right-channel information, the subcarriers are combined as follows:

          (L+R) + (L-R) = 2L
          (L+R) - (L-R) = 2R

Thus, a simple matrix is all that's needed to decode stereo FM.  Mono
receivers filter out the subcariers, which leaves the mono (L+R) component.
Now, on to stereo television.

Being an EE in the Television Products Division at Tektronix, I have
become familiar with the American stereo TV system, known as MTS or
BTSC.  MTS does transmit L+R and L-R, but not the same as FM radio.
First, the frequencies involved are different.  MTS uses 15.734 KHz for
its pilot and 31.468 KHz for the L-R instead of the 19 and 38 KHz of FM
radio.  15734 Hz is the horizontal scanning rate in US TV's, so it was
chosen as the basis for MTS to reduce interference between the
horizontal circuits and the stereo decoder.  TV people call this
frequency H.  Thus, the pilot is at H and the L-R is at 2H.

The principal difference between TV and FM is that the L-R subcarrier
in BTSC is compressed before transmission.  This was done to reduce
noisy reception in fringe areas.  Although designed by dbx, the
companding system used for stereo TV is not the simple linear 2:1
system used in their tape noise reduction systems.  In TV stereo, the
companding is both amplitude and frequency dependent.  This makes the
expander circuit fairly complex and I'm afraid no one has managed to
squeeze it into a single IC (yet).  (Actually, I have heard of one
company that it trying to develop a digital TV stereo chip, but that's
probably still a couple of years away).  The present decoders have lots
of discrete components and about 8 trimpots, so you can see there is a
lot of room for improvement.  [Now there is a single IC that does the
expanding, but is not available to consumers-- WKM 5/7/87]

For the sake of completeness, I should mention that BTSC provides for
two other subcarriers, SAP at 5H and PRO at 6.5H.  SAP stands for
Second Audio Program and is intended for bilingual broadcasts (e.g.,
SAP can broadcast a dubbed foreign language version of the audio).  SAP
is FM modulated and dbx compressed.  I believe the frequency response
is somewhere around 12 KHz.  The PRO (PROfessional) channel is for data
or low-grade voice transmission and is not intended for the general
public.

So far, there are several stations using SAP (to transmit in Spanish).  There
are about 250 TV stations broadcasting stereo now, with more on the way
soon.  Most of these stations carry NBC, the only commercial network
using stereo.  ABC and CBS plan to go stereo soon.

For additional information, see the following articles:

"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

I hope this helped!
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
UUCP: ...{hplabs,uw-beaver,decvax}!tektronix!videovax!bill
GTE: (503) 627-6920        "How can I prove I am not crazy to people who are?"