Path: utzoo!attcan!uunet!deimos.cis.ksu.edu!ux1.cso.uiuc.edu!uwm.edu!proteon.com!jas
From: jas@proteon.com (John A. Shriver)
Newsgroups: rec.audio.high-end
Subject: Re: copy protected VHS tape
Message-ID: <9003151532.AA06533@monk.proteon.com>
Date: 15 Mar 90 16:41:10 GMT
Sender: news@uwm.edu
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Approved: tjk@csd4.csd.uwm.edu

premise of viedotape copy protection is to fool around with the number
of horizontal scan lines in a frame.  The television is not too picky
about this (although those which try and stabilize the horizontal scan
frequency for better interlace will probably get peeved), and the
dropped lines are all in the vertical retrace anyways.  However, a VCR
trying to copy this tape will be struggling to modulate the head drum
and/or tape speed servos, and will totally lose it's cookies.  (I
don't quite understand how one VCR can diddle its servos to play the
tape, but the second can't diddle them to record it.  Of course, the
errors multiply.)

The article on was a new device to do this copy protection on the fly
in real time to protect pay-per-view movies on CATV from copying.
Apparently there is resistance to early relase of movies via
pay-per-view for fear of illicit copies becoming available before
commercial VHS release.

Nonetheless, with a good television, my understanding is that copy
protection does show.  It also removes a margin for error, so that
flaws in the duplicating process, as well as (magnetic) tape wear, are
magnified.

It is probably not hard to build a simple (analog) black box to put
back in the missing horizontal scan pulses.  This may be what some of
the commercial "image stabilizers" do.  Of course, they can't
advertise this capability too blatantly -- they just claim to improve
you image quality on the screen.  (They might.)

Needless to say, this is all totally unrelated to the copy protection
for DAT, which is in the digital domain.