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From: fiesta@cbnewsi.ATT.COM (eric.c.beck)
Newsgroups: sci.electronics
Subject: Re: More need advice on descrambler
Summary: I think I've seen this scheme before
Keywords: cable descrambler
Message-ID: <331@cbnewsi.ATT.COM>
Date: 23 Jun 89 21:49:03 GMT
References: <111777@sun.Eng.Sun.COM>
Organization: AT&T Bell Laboratories
Lines: 44

From the "oscillographs" it appears that the scrambling involves adding a
positive offset to the composite video signal during the horizontal
retrace time.  However, depending where the DC reference is, it could also
be interpreted as a large attenuation during the horizontal retrace time.
[This would be the case if the white level in your "oscillographs" (most
positive excursions) is at zero volts.]

The latter scheme is one that was once used by a TV station in
the NY metro area.  This scheme involved attenuating the composite signal
during the retrace time so that the horizontal sync pulse amplitude was
well below the peak of the video information (e.g. the black level).  A 15750 Hz
tone was sent in the audio channel as the new horizontal timing reference.
They also placed the program audio on a 31500 Hz subcarrier (DSSC) so that
it couldn't heard without the decoder.

Schemes for decoding the signal are straightforward.  The 15750 Hz tone is
recovered from the audio output -  a PLL is handy.  For the above scrambling
scheme, a multiplying PLL (VCO running at 31500 and then divided by two) is
employed:  this provides a 31500 Hz tone with which the program audio can
recovered, and a 15750 Hz square wave to control an attenuator/gain block
of some sort.  Since your audio program is already available, you could run
the PLL VCO at 15750 (this assumes that the 15750 is available in your
system).  The recovered 15750 Hz square wave would be used to trigger a
one shot, which in turn would trigger a second one shot.  The two one
shots are adjusted to generate a pulse (pulse position and pulse width) which
overlaps the attenuation interval. The output of the second
one shot would then be used to drive a variable attenuator/modulator through
which the scrambled video is sent.  Viola!  The origonal composite signal
is recovered.

I've seen descramber schematics using PIN diodes as variable attenuators.
I believe you can also use a CMOS bilateral switch (aka 4066) to switch
a simple resistive attenuator.   A balanced mixer might also work.

First things first though - is there a 15750 Hz tone on your audio out?
You may have to look right after the FM demodulator in case the output
is low-pass filtered.  If you have an FM multiplex output on your
receiver the 15750 Hz tone would be available there.

Of course, this is for your *own* viewing pleasure, right?(!)


Eric Beck                        |                #include <std_disclaimer>
fiesta@homxb.att.com             |