Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site nicmad.UUCP Path: utzoo!linus!decvax!genrad!panda!talcott!harvard!uwvax!astroatc!nicmad!brown From: brown@nicmad.UUCP Newsgroups: net.video Subject: Re: copying rented videocassettes Message-ID: <563@nicmad.UUCP> Date: Sun, 2-Mar-86 17:53:14 EST Article-I.D.: nicmad.563 Posted: Sun Mar 2 17:53:14 1986 Date-Received: Tue, 4-Mar-86 03:53:38 EST References: <161@ides.UUCP> Reply-To: brown@nicmad.UUCP (Mr. Video) Distribution: net Organization: Nicolet Instrument Corp. Madison WI Lines: 80 In article <161@ides.UUCP> rkj@ides.UUCP (Rick Janka) writes: >I have recently purchased a vcr (VHS) and have used it for playing rented >tapes and recording a few broadcast movies. I am aware that something >called Copyguard exists, but I know next to nothing about it. If I borrow >a friend's vcr and attempt to copy a rented tape, will I get garbage? One >dealer I spoke with said that he has not run into problems with recording >rented tapes, but that Copyguard typically prevents copying tapes borrowed >from a public library. I assume that a tape I record myself, from whatever >source, could be copied without problems, though maybe that is incorrect. > >I'd appreciate any enlightenment, but please don't waste your fingers on >flames regarding legalities; I am interested only in the technical aspects. 'Copyguard' hasn't been used on VHS tapes in a few years. The term implied the technique of changing the vertical sync, a little bit, so that the tv would still display it without rolling and the copying vcr would have trouble recording it. But, as the vcrs got better, most table top units would record correctly ANY tv signal that was even close in the vertical sync. I have been through two table top units where that is what would happen. A portable model of one of the table top units would still record the tape, but the head switch-over point could be seen wandering though the picture. Not exactly what the duplicators wanted. Plus, there are companies making devices that would replace part of the bad sync so that the recorders that wouldn't record the signal would. So, as time went on the duplicators dropped the 'copyguard' technique on VHS tapes. Beta machines, especially ones made by Sony, still wanted perfect vertical sync, so they tended to not record any picture if 'copyguard' was used. A couple of people I know ran into this problem. They have cheaper units, so I don't know what the top-of-the-line Beta decks do with damaged vertical sync. But, here is the newest killer to video tape copying: Macrovision. It is a technique whereby the AGC circuit of VHS decks is fooled. The vertical sync is not damaged. What is done is that during the vertical blanking area (the lines that following the vertical sync, up until the picture starts again) is changed. Right after the color burst pulse the video signal is sent to horzontal tip level and then to white level. About 7 of these transitions occur for about 5 lines worth. The sharp transition from horizontal sync tip to white level messes with the AGC circuit of a majority of machines. Also, about 7 lines before vertical sync and for 3 lines of the vertical sync, the area that is used for the color burst (know as the back porch), is sent to white level. This technique does not work on Beta machines, as the AGC circuit design is completely different. It also doesn't work on all VHS machines either. CBS Fox has recently announced that all product will be released with the 'Macrovision' anti-copy technique. Supposidly something like 5 million units. The next question is probably; how do the duplicators get around this signal. The company that makes the 'Macrovision' signal modifies all of the slave VHS machines, so that the AGC circuit is fixed and no longer operational. Obviously the cure for home machines is to modify the machines so that the AGC circuit is also fixed. The other way is to build a circuit, with a handfull of parts, that locks onto the sync and then replace the changed video area with good video signal. Since there are all kinds of sync separator chips out there, it would be an easy job. I personally feel that CBS Fox is wasting their money and ours, as will will ultimately pay the price in the cost of each tape. Because so many units will be shipped, the video enhancer manufacturers will build the above described circuit into their boxes. The above discussion doesn't indicate that I support copying of rented tapes. I am always interested in what the companies try and do to stop people from copying tapes. It is like a puzzle. I like to solve electronic puzzles. I have to laugh at the latest venture, as it won't be long before this one is beaten as well. There is nothing that somebody can design that someone else can un-design. When it somes to video, there isn't much that can be done to make it unrecordable. -- ihnp4------\ harvard-\ \ Mr. Video seismo!uwvax!nicmad!brown topaz-/ / decvax------/