btc@hp-pcd.UUCP (btc) (02/02/85)
The following has been extracted from HUMAN-NETS Digest V8 #3 (1/30/85). > Date: Tue, 22 Jan 85 13:12 EST > From: Kahin@MIT-MULTICS.ARPA > Subject: MIT Communications Forum Seminars > To: Telecom@USC-ECLC.ARPA, > To: *bboard@MIT-MC.ARPA, DEPhillips@MIT-MULTICS.ARPA > > MIT Communications Forum seminars are held on Thursdays from 4:00 to > 6:00 in the Marlar Lounge (Bldg. 37-252, MIT, 70 Vassar St., > Cambridge) > > > Unrecordable Video > > March 14, 1985 > > Andrew Lippman, MIT > John Woodbury, National Cable Television Association > Speaker to be announced > > > > Although motion picture producers depend increasingly on > revenue from television and home video, the spectacular growth of > videocassette recorders has provoked fears that much potential > revenue will be lost. The Electronic Publishing group of MIT's > Media Laboratory has developed a way of generating television > transmissions that can be viewed but not taped. This seminar > will present the technology, and industry representatives will > discuss the possible effects on distribution practices. Anyone have any info on how this can be done? Bob Clark Hewlett-Packard PCD Corvallis, OR {ucbvax!hplabs, harpo, ogcvax}!hp-pcd!btc
lauren@vortex.UUCP (Lauren Weinstein) (02/04/85)
There are a variety of procedures, mostly involving screwing around with the sync. Unfortunately, these also render the picture unviewable on some televisions. It is difficult to conceive of a "copyguard" system that wouldn't be vulnerable to fairly simplistic processing (such as by a proc amp in the simplest case). Clearly, since the picture has to be viewable on a standard television, it would always be possible to process the video to allow taping. --Lauren--
karn@petrus.UUCP (02/04/85)
> Anyone have any info on how this can be done?
In the past, this has been done by modifying the sync pulses such
that they're still good enough for a TV, but not good enough for the
servos in a VCR. This is the "Copyguard" technique. Such a system
can be easily defeated with a common device in the TV station called
a "processing amplifier" or "proc amp" for short. It regenerates the
sync signals back to their standard form. I suspect that the scheme
described here will be similar.
Attempting to make signals displayable on TVs but not recordable seems
doomed to failure. Even without proc amps, one could always record
after a fashion by simply pointing a camera at the screen!
Philmikey@trsvax.UUCP (02/06/85)
I suspect that broadcasting a signal that was out of spec to try to control recording would be doomed to failure. First off, I wouldn't record it for "time shift viewing" so I couldn't even see the commercials at a later time. I'm sure that the sponsors will care about this. Late night movies broadcast this way might loose some of there value to the advertisers. Secondly, what about the legal implications of broadcasting a signal that was deliberately out of spec? mikey at trsvax
karn@petrus.UUCP (02/09/85)
Certain broadcasters have been transmitting TV signals that are "out of spec" for many years now -- in the form of scrambled subscription television (STV) services. There are some implications involved in running TV transmitters under these conditions, though. The typical UHF TV klystron is very nonlinear. A lot of signal processing and pre-distortion goes on before the signal is fed to the transmitter so that it comes out looking "right". In particular, the power gain of the tubes falls off rapidly as the transmitter is driven to peak power, so the sync signals have to be greatly "stretched" to make the proper sync levels at the output. This doesn't hurt much in a "normal" signal, since the nonlinearity is in a "digital" portion of the signal where it won't do much damage. With the scrambling schemes, however, you have to be careful about several things. The first is that power peaks no longer occur during horizontal sync, but during the active picture interval (this is why the picture tears on a set without a decoder -- it sees peak picture information as false sync.) This means that you've got to work extra hard at linearizing the transmitter near its peak power output. The second factor is that if the coding sinewave is allowed to combine with the equalizing pulses (the "hammerhead" in the center of the vertical interval) the transmitter would be overdriven. That's why they turn off the coding signal during the vertical interval, and why the picture generally doesn't tear vertically on a set without a decoder. On cable TV systems these high power linearity considerations don't apply, so they are free to encode the vertical interval. They can also hide the decoding signal somewhere else in the cable spectrum; it doesn't have to be within the same 6 MHz TV channel. Phil