[sci.electronics] Roll your own MTS decoder

hansley@cscosl.ncsu.edu (Bill Hansley) (08/22/89)

Can anyone direct me to plans on how to build my own MTS decoder?

Thanks,
Bill Hansley

wtm@neoucom.UUCP (Bill Mayhew) (08/23/89)

MTS (multichannel television sound) is transmitted in much the same
fashion as a broadcast FM stereo multiplex signal.  The composite
TV signal is demodulated to an unclamped basband signal with a
bandwith of about 6 MHz.  The sound is an FM subcarrier at about
4.5 MHz  the exact frequncy is an integral submutiple of the
3.579545 MHz color busrt signal to help prvent getting buzz in the
audio due to the sync pulses and to help prevent getting audio hum
bars in the video.

I don't have a very new refenece book, but there are monolithic TV
audio subsystemns such as the LM2808 that require little more than
a simple and circit to provide fully demodulated audio.

The MTS system works by using a pilot tone at 4.5xxx MHx + Fh (the
horizontal line frequency of ~15.57 KHz).  The nornal L+R signal is
encoded in the audio baseband, and filtered to a cut-off of ~15
KHz.  This provided compatibility with old receivers.  When the
pilot tone is present, the L-R signal is a DSBSC (double side-band
supressed carrier) signal present at 4.5 MHz + 2*Fh. (~31.5 KHz).
Due to the fact that a very small amount of energy is present in
the DSBSC signal, the demodulated signal is very noisy.  The MTS
system Dbx encodes the L-R before modulating the sound carrier, so
you need to Dbx decoder chip before mixing with the L+R baseband in
your matrix output.  IF SAP (separate audio program) is present,
I'm pretty sure it is DSBSC modulated at 3*Fh.

Unless you really like rolling your own decoder, it is probably
easier to buy something like the Recoton MTS docoder.  I saw the
Recoton box at the local catalog store for a little more than $50.
The Recoton can use an inductive loop whcih you can glue on the
back of the set to passively tap the 4.5 MHz sound IF, but I'd
imagine the result is less than thrilling.

Some of the older sets that are "MTS ready" have have either a 4.5
MHz IF output or an unfiltered demodulated sound output.  Both
types come out on pin-style "RCA" jack.  I have a Zenith set that
has the 4.5 MHz output, while my NEC set has a baseband output.  I
believe Sony also used a baseband output.  The 4.5 MHz output is
preferable for use with an outboard decoder becasue then you don't
have to worry about level matching of the baseband with the
subcarrier going into the L+R/L-R matrix.

Last of all, safety is a problem if you are thinking of tapping
into your set's IF strip.  There are still quite a few sets that
use a nonisolated chassis that may have 120 vac line voltage
present if the wall outlet is wired backwards.  You could kill
yourself if you make a mistake.  Extreme caution is advised.  In
sets that don't have an MTS jack, the bandwith of the 4.5 MHz
output might be band limited to about 15 KHz, so you might have to
do some modifications to the point where the audio is picked off.

Bill    wtm@impulse.UUCP   or   wtm@neoucom.edu

royf@attctc.Dallas.TX.US (Roy Frederick) (08/24/89)

In article <3722@ncsuvx.ncsu.edu>, Bill Hansley writes:
> Can anyone direct me to plans on how to build my own MTS decoder?

Two different articles in Radio-Electronics: March 1986, a simple
decoder that does not make any attempt to handle the decompression used
in MTS.  Jan-Feb 1987 describes a more  complicated one that handles
the decompression, but alas, not in an exactly correct manner.  You see,
a special DBX IC is required.  It is not available to those of us that
have not signed a license aggreement with DBX.  Still, this one looks
pretty good.  I have not built it, however.

Roy Frederick (royf at attctc)
Dallas County Data Services
504 Records Bldg
Dallas, TX 75202  (214) 653-6340