Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site petrus.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!petrus!karn From: karn@petrus.UUCP Newsgroups: net.video Subject: Re: unrecordable video?? Message-ID: <282@petrus.UUCP> Date: Fri, 8-Feb-85 20:16:36 EST Article-I.D.: petrus.282 Posted: Fri Feb 8 20:16:36 1985 Date-Received: Sat, 9-Feb-85 08:49:40 EST References: <9700002@hp-pcd.UUCP> <58300025@trsvax.UUCP> Organization: Bell Communications Research, Inc Lines: 32 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