Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!uwm.edu!pyuxe!whs70
From: pyuxe!whs70@bellcore.bellcore.com (W. H. Sohl)
Newsgroups: rec.audio.high-end
Subject: Re: copy protected VHS tape
Summary: I think it's.....
Message-ID: <980@pyuxe.UUCP>
Date: 15 Mar 90 16:40:18 GMT
References: <9071@sun.acs.udel.edu>
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In article <9071@sun.acs.udel.edu>, mgray@sun.acs.udel.edu (Michael Gray) writes:
> 
> 	After reading the article on DAT tape and the format for copy
> protecting it, I was wondering if anyone knew what was used to copy 
> protect VHS tapes. Why can they be watched but when recorded onto 
> another machine they're altered or scrambled.

I believe the process works along the following lines:  A high level
signal spike is included on the commercially recorded tape (eg. of a 
movie).  Since all (at least all the ones I've seen) home VCRs
do not have a manual gain control for the record mode, the high
level spike causes the Automatic Gain Control of the VCR to lower
the gain to a point were the recording is at such a low level, the
recording does not produce a good picture (as mentioned, it
looks altered or scrambled).

I'm not sure, but a possible way around this might be to first have
the output go from the playing machine to the recording machine
and then again to another recorder.  I haven't tried that, but
it might "clip" the high level spike enough to not affect
the final recording process on the third VCR.

Bill Sohl