whs70@bellcore.bellcore.com (W. H. Sohl) (03/16/90)
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
jas@proteon.com (John A. Shriver) (03/16/90)
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.