rtc@masscomp.UUCP (Richard Carling) (04/29/85)
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I don't know if this is the technigue the Cotten Club is using
to stop TV taping but I know one method proposed for making cable
and broadcast video unrecordable on VCR's is as follows.
The vertical sync is run faster and slower during the broadcast instead of
remaining constant. A TV has no problem adjusting to the shift
but a VCR which has mechanical timings to deal with (head spinning
and tape speed) can't lock to a signal that varies significantly from the
expected frequency.
To record such a signal (yes, there is always a way) requires the
use of a Time Base Corrector (TBC). These things currently cost around
$2K (at least) but if such a technique was used, the market for TBC's would
drastically increase and the cost would come down.
Richard Carlingbrown@nic_vax.UUCP (05/01/85)
> I don't know if this is the technigue the Cotten Club is using > to stop TV taping but I know one method proposed for making cable > and broadcast video unrecordable on VCR's is as follows. Well, here it is, the method that is being used to 'spoil' dubs of video tapes. A couple of lead-in notes first. The information is for VHS. I don't have Beta, but from what I have read in 'Billboard' today, it doesn't sound as if a different method is being used for it. The 'Billboard' article indicated that it made copies hard to see and the sound noisy. That may be true if dubs were made via RF out to RF in, but I did a test by recording baseband signals. The video and sound from a JVC HR 2650U into a JVC HR D725U. The Hi-Fi soundtrack recorded didn't have any problem. The video recording was a little darker and tore at the top, for about 1/8 of the total picture. Not as bad as they claim. The sound will never be bothered in a direct recording, for obvious reasons. One other test I made was to record the video onto a Sony 3/4" U-Matic. Didn't even phase it. Treated the signal as if nothing had happened. So, until a black box comes along, and it will, if I really want a tape, I can always record it on 3/4". Now, on to what they do. I connected a scope to the output of the VCR and found the following signal. BTW, it can also be seen by turning the vertical hold to bring down the black bar. Starting right after the vertical sync pulses, during the vertical interval, 7 lines worth of square wave white to black signal is added. There are 5 pulses of this added square wave. Nothing special so far. Added square waves are there all the time, because of time-code and the caption signals. They, BTW, do not go completely to white level, not that it would make any difference. What does make the difference is that they also make the signal go to horizontal sync tip level. The following is a representation of what they do: ___ ___ ___ ___ ___ white | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | horiz | black | | | | | | | | | | ____sync ___v______ | |__ | |__ | |__ | |__ | |_______... | | color | | | | | | | | | | | | burst | | | | | | | | | | |___| area |_| |_| |_| |_| |_| |<---10 us---->|<------------------35.2 us----------------->| Now, the idea of the whole thing is to screw up the video AGC of the deck. I don't know exactly how, but since the video on home consumer units, does have AGC to guard against bad signal levels, the extra large signal would cause problems. More so on RF than on baseband video, which I saw. I did not do an RF check. Nobody dupes tapes using RF, not even the professional dupers. I suspect that the 3/4" deck didn't bat an eye because it is expecting NTSC standard signal levels, AGC is not required. Home decks cannot count on such perfect signal levels. Now, the article did elude to the fact that someone out there will either modify their decks (fiddle with the AGC, making it manual or automatic with a switch), as the duplicators had to do. They also indicated that someone may make a little black box (remember the anti-copy guard boxes?), that would fix the problem. A TBC (time-base corrector) would not work in this case, as it would pass the offending signal right thru. The easy fix, for you electronics buffs, is to wire up a vertical sync separator, lock onto the horizontal sync for 8 lines after vertical sync is done, electronically switch in a new black level signal and it is fixed. The level of the inserted signal is not real important. A variable dc level that can be adjusted with a pot and a scope would fix that. If the developer of this method thinks that it is going to last very long, they better think again. The developer's name is listed in the 'Billboard' article, but I left it at work (I am at home typing this). I personally think that the whole idea is a complete waste of time. It will be defeated in no time flat. > The vertical sync is run faster and slower during the broadcast instead of > remaining constant. A TV has no problem adjusting to the shift > but a VCR which has mechanical timings to deal with (head spinning > and tape speed) can't lock to a signal that varies significantly from the > expected frequency. > > > To record such a signal (yes, there is always a way) requires the > use of a Time Base Corrector (TBC). These things currently cost around > $2K (at least) but if such a technique was used, the market for TBC's would > drastically increase and the cost would come down. > > > Richard Carling I will have to disagree. Number one, I read that HBO and others were looking into the method that I described above. To vary the vertical rep rate on-line would not work with the HBO decks either. They are also mechanical and would have problems locking onto a varying time-base. BTW, all outputs of video decks, that are placed on air, are run thru TBC's. They have too, because the picture would tend to wiggle left to right on the screen. Why, because the sync pulses that are sent out over the air do not match what comes off of tape (mechanical tracking problems), so the horizontal synce from the tape can waddle as much as a micro second. I have seen it at home with the 3/4" deck on a scope. You don't see that problem at home, because the TV gets the sync from the video tape. In a studio it doesn't, hench the TBC. Number two, it would not be NTSC standard for broadcast and the FCC would have some say about that. Number three, to build a TBC made specifically to do the freq shift would not be cheap. The little box to do the above is much cheaper. Maybe you don't know this, but all the TBCs that I have seen require house sync or internal sync connected to the playing deck and the TBC. If internal sync from the TBC is used, it comes from the TBC and goes to the deck. Home VCRs do not have external sync inputs. TBCs must have the deck synced to it. What you are thinking of is a frame syncronizer. Now that device takes and stores a frame of information, and sends it back out based on house sync. That way the incoming signal can be out of sync but be corrected for house use. You see it done all the time with the satellite feeds (live) from all over various parts of the country. Nightline is a prime example. Frame syncronizers cost even more than TBCs. One final comment about 'The Cotton Club'. The VHS copy that I rented had many, and I mean many, Hi-Fi sound dropouts. There were even glitches in the video. Either the tape was really bad (my guess) or the video duplicator had problems. There are better movies (I don't mean tape quality, I mean story quality). I see plenty of other people write disclaimers, so I will do one. The above information is strickly my own, not my companies, and does not imply that you should make a little black box and copy video tapes. RIGHT!! -- o o \ / \ / (!ihnp4!uwvax!astroatc!nic_vax!brown) \/ |------------| Mr. Video | |-------| o| | | | o| JVC HRD725U | | | | |--------------| | |-------| O| | |----| o o o | |------------| |--------------| VHS Hi-Fi (the only way to go)
lauren@vortex.UUCP (Lauren Weinstein) (05/01/85)
No, The C. Club tape is not using the frame rate alteration system. In fact, the guy who invented the frame rate system (I spoke to him recently) doubts that his system will ever be widely used, since it doesn't display well on TV's with digital countdown circuitry and can still be taped by top of the line VCRs. --Lauren--
wapd@houxj.UUCP (Bill Dietrich) (05/01/85)
I don't know what the name of this one is, but an article I saw recently said something about an anti-piracy technique that would "fool automatic gain control" circuitry. No other details. Bill Dietrich houxj!wapd
david@tekig.UUCP (David Hayes) (05/06/85)
> > Now, the article did elude to the fact that someone out there will either > modify their decks (fiddle with the AGC, making it manual or automatic > with a switch), as the duplicators had to do. They also indicated that Hey, hey, My new RCA VKT 385 has an AGC switch on the front panel! Maybe someone had seen the light a while back. dave
brown@nic_vax.UUCP (05/09/85)
> > > > Now, the article did elude to the fact that someone out there will either > > modify their decks (fiddle with the AGC, making it manual or automatic > > with a switch), as the duplicators had to do. They also indicated that > > > Hey, hey, > > My new RCA VKT 385 has an AGC switch on the front panel! > > Maybe someone had seen the light a while back. > > > dave You sure that switch isn't for the tuner reception of the signal, frequency drift, etc. Normally it is labeled AFT (automatic fine tuning). Check your manual to be sure. If it really does turn of the AGC of the video gain, you are in luck. All you will need then is a second machine. -- |------------| | |-------| o| JVC HRD725U Mr. Video | | | o| |--------------| | | | | | |----| o o o | | |-------| O| |--------------| |------------| VHS Hi-Fi (the only way to go) (!ihnp4!uwvax!astroatc!nic_vax!brown)