Newsgroups: sci.electronics
Path: utzoo!utgpu!watserv1!sunee!dcschuur
From: dcschuur@sunee.waterloo.edu (DC Schuurman)
Subject: Re: VCR copy protection
Message-ID: <1990Nov27.210457.26202@sunee.waterloo.edu>
Organization: University of Waterloo
References: <1923@mts.ucs.UAlberta.CA> <746@pdxgate.UUCP>
Date: Tue, 27 Nov 90 21:04:57 GMT
Lines: 44

In article <746@pdxgate.UUCP> berggren@eecs.cs.pdx.edu (Eric Berggren) writes:
>userDANC@mts.ucs.UAlberta.CA (unknown) writes:
>
>>Is anyone aware if a scheme exists to 'copy protect' commercial
>>VHS tapes?  In running a recent tape through a second video recorder,
>>it looked as if the signal had become scrambled, though viewing it on
>>a normal television worked fine.  As a second year Electrical Engineering
>>student, I would be very curious in learning the details of the protection
>>scheme.  (looks like I have a bit of catching up to do).
>>Dan
>
>  There are several companies using a "copy protection" scheme called...

MACROVISION ENCODING !

I replied personally to the original sender assuming someone else would
identify the copy scheme but I'll post it here for anyone else who cares...

The scheme is simple yet effective - several large high frequency spikes
are inserted prior to and during the vertical blanking interval - 
Normally a television will ignore these high frequency pulses due to the
low pass characteristic of the deflection circuitry during the retrace -

BUT a VCR will adjust it's Automatic Gain Control (AGC) to these large   
incoming spikes and because of the finite time constant involved with the
AGC response the following picture field will appear dark and murky
if copied from another MacroVision protected tape.  These large spikes
are visible (as the genetleman above mentioned) as flashing squares
in the black line on your TV which appears during vertical retrace.

The scheme is effective yet simple - however there have been some problems
with very expensive multi-synch TV's which interpret these high frequency
noise bursts as a higher horizontal synch frequency and vainly try to 
adjust to these noise pulses mistaking them as a change in H synch.
The cheapest TV's seem to be least susceptible to MacroVision 
interference while viewing Macrovision encoded tapes - strange but true.

So there it is - I hope this helps


Derek C. Schuurman 
University of Waterloo 
Electrical Engineering
Hoping to Graduate in '91 - Got a job for me?