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From: brown@nicmad.UUCP
Newsgroups: net.tv
Subject: Re: Stereo TV
Message-ID: <312@nicmad.UUCP>
Date: Wed, 14-Aug-85 11:52:54 EDT
Article-I.D.: nicmad.312
Posted: Wed Aug 14 11:52:54 1985
Date-Received: Sun, 18-Aug-85 01:04:45 EDT
References: <309@nicmad.UUCP> <1249@pitt.UUCP>
Reply-To: brown@nicmad.UUCP (Mr. Video)
Organization: Nicolet Instrument Corp. Madison WI
Lines: 53

In article <1249@pitt.UUCP> hoffman@pitt.UUCP (Bob Hoffman) writes:
>In article <309@nicmad.UUCP> brown@nicmad.UUCP (Mr. Video) writes:
>>...
>>Some TVs, including one of them that I have, have a MPX jack on the back
>>of the set.  This is the tap point in the audio circuitry before the FM
>>de-modulator.  This is the baseband audio before any decoding takes place.
>>
>>We should all be a little careful on how we describe these things.
>
>I agree... we should be careful.  Baseband audio is what comes out of
>an FM demodulator, not what goes into it.

Yes, but, by definition the information coming out of the MPX jack is still
baseband, ie, it does not require further mixing with RF/IF frequencies in
order to get what you want.  True, you do have to but it thru a de-modulator
in order to hear the audio, but no IF frequencies are mixed with it.  I know,
the 19 KHz is still used, but that is after the demodulation.  Think of the
MPX output jack being somewhat like the video output of a VCR or TV that is
tuned to a cable channel that is scrambled.  The video signal is still
baseband, but you can't watch it.  But, bu doing a 'de-modulation',
en-encoding, you can watch it.  Yes, I know, most cable converters do it
at IF frequencies, but baseband decoders do exist.

So, just because we can't hear the information coming out of the MPX jack,
it is still baseband information.

>And now, a question of my own:
>What is the modulation scheme used by NBC (RCA) for their stereo
>broadcasts?

I am not sure what you are asking.  So, I will try and give you a guess.
NBC doesn't do the broadcasting.  Their affiliate stations do it.  I include
the stations that they own as affiliates.  The program goes from the New York
network control room to their contracted satellite uplink site.  I talked with
our local NBC affiliate engineer and he didn't know what the uplink method was,
as NBC provided the satellite receiver.  If it goes wrong, NBC fixes it.  I
suspect that it is the L+R and L-R method, ie, one audio sub-carrier for each
of the two audio signals.  From there, the local station gets separate L and R
audio signals to run thru their sound board and then to the audio transmitter,
which has the MTS encoding equipment.

So, NBC tapes are L and R, sent to uplink site, encoded for transmission,
received by affiliate, broken into L and R, sent the TV audio transmitter,
encoded into MTS format.

Basically the MTS format is like FM, ie, L+R (mono) and L-R (difference).
The added thing is the dbx encoding of the L-R signal and the addition of the
SAP (Secondary Audio Program) sub-carrier.

I hope this helps.  If not, let me know.
-- 

Mr. Video   {seismo!uwvax!|!decvax|!ihnp4}!nicmad!brown