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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