🚀 go-pugleaf

In article <111777@sun.Eng.Sun.COM> rom@xor.Sun.COM (Achyutram Bhamidipaty) writes:
>The sound is fine, when I tune to a scrambled station I can hear all the audio.
>I have hooked my scope up to the monitor output of my TV and this is what the
>signal looks like:
[picture of the h-sync pulse moved up some, by a dc offset.]

It looks like all they are doing is hiding the sync pulse by adding in
a dc offset during the H-sync interval.  This makes it hard for the TV
to lock onto H-sync (since it is expecting a more negative [blacker]
signal than the normal black).  This will also make it hard for the TV
to lock onto the color-burst which sets the phase for the color
demodulation.

What you have to do is subtract this unwanted DC back out.  On the
system I've seen, the stable reference that you needed to lock into
was actually carried on the audio!  Just get a simple phase-lock-loop
(PLL) such as the kind for FM stereo decoding and tap it into the
demodulated audio before any <15khz filter.  Set the PLL up to have a
center frequency of 15.75 Khz.  Now take two one-shots (eg. 555's).
Hook them up so that the first one is a variable delay for 0-64us
triggered from the PLL.  Set the second one up to trigger off of the
first one-shot and have a pulse width of slightly less than the
"scrambling pedestal".  Now using the scope in dual-trace mode, set the
delay so that your pulse is contained within the scrambling pedestal.
Resistively mix the two signals such that the H-sync pulse now goes
negative relative the the rest of the signal again. You only want the
*real* H-sync to drive the signal negative.

If you can't find a reference sync signal on the audio, you can always
*try* to sync onto the video signal directly....

-wolfgang

Wolfgang Rupprecht	ARPA:  wolfgang@mgm.mit.edu (IP 18.82.0.114)
TEL: (703) 768-2640	UUCP:  mit-eddie!mgm.mit.edu!wolfgang