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Path: utzoo!utgpu!water!watmath!clyde!rutgers!topaz.rutgers.edu!ron
From: ron@topaz.rutgers.edu.UUCP
Newsgroups: sci.electronics
Subject: Re: Pay TV encoding scheme?
Message-ID: <13873@topaz.rutgers.edu>
Date: Tue, 11-Aug-87 11:09:10 EDT
Article-I.D.: topaz.13873
Posted: Tue Aug 11 11:09:10 1987
Date-Received: Thu, 13-Aug-87 01:18:39 EDT
References: <4030@utai.UUCP>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 31
Keywords: Pay-TV, encoding, video, code

Well, there are several methods.  From what you describe it sounds like the
most common (and for good reason, it is the cheapest) method of messing with
the sync pulse so TV's don't know when to start the trace.  They just sweep at
random or by other components of the signal that look enough like the sync
pulse they have been missing.  Also, sometimes the sound carrier center
frequency is shifted so that you can not hear it when the TV is locked on the
video signal.  Another method is to superimpose a signal that generates
effectively superfluous sync pulses.  Decoders to correct this are also cheap
and hence are fairly commonly available.  Frequently, your Cable TV box
already has the capability to do this and is programmed with jumpers.

For sattelite transmission, they now usually opt for better protection in
the way of the MA/COM Video Cipher.  The VC encrypts the audio portion
(the video being to much data for it's little mind) and screws with the
picture sync.  It's fairly easy to get the picture back, but the industry
gambled that not being able to restore the sound would deter people from
even wanting to just look at the pictures.  The enabling of the subscriber
boxes is done remotely.  Lauren Weinstein can probably give you the best

The only sure fire way, which is used for a number of cable channels in
my area and HBO only in the place I previously lived, is to block the
signal from appearing in the customers house at all.  This also has the
advantage (or disadvantage depeding on your view) of the subscriber not
having to have special equipment to decode the signal.  Hence I can use
my "cable-ready" VCR and TV set to the fullest of their capabilities
rather than having to use the piece of junk box the cable company supplies.
I don't have HBO right now, and the signal just plain isn't there.  The
AFC control in my VCR hunts on to the channel below it since it can't
find anything on the HBO channel.

-Ron