Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 11/08/85; site unccvax.unccvax.UUCP
Path: utzoo!watmath!clyde!bonnie!akgua!mcnc!unccvax!dsi
From: dsi@unccvax.UUCP
Newsgroups: net.video
Subject: Re: MTS and Cable T.V. questions, comments
Message-ID: <417@unccvax.unccvax.UUCP>
Date: Sat, 11-Jan-86 10:04:45 EST
Article-I.D.: unccvax.417
Posted: Sat Jan 11 10:04:45 1986
Date-Received: Tue, 14-Jan-86 04:45:53 EST
References: <19@valid.UUCP>
Distribution: net
Organization: UNC-Charlotte
Lines: 71

> According to everything I have read, cable does not have the bandwidth for
> carrying MTS signals.  However, when I tune in to channels 4 and 20 (which
> both broadcast in stereo, 4 is network, 20 is independent) the "stereo"
> light on my Sony SL-HF400 VCR comes on, the LEDs show a difference between
> the left and right channel, and I can hear the stereo effect.  Channel
> 20, which is broadcast on cable on one of the cable channels, is quite
> pleasant and noise free, whereas channel 4, which is broadcast on channel
> 4 on the cable, is quite noisy and irritating and I usually switch out the
> MTS decoder.  I haven't yet talked to channel 4's technical people to find
> out what gives, but the cable company person was quite surprised to find
> that I received stereo at all.  She also said that they do not alter the
> signal in any way, they just pass it along.

     I am suprised that you have a problem with channel 4 - if your cable
system is "normal" (i.e. channel 5 starting at 76 mc) then channel 4 should
be one of the cleanest channels around. Some cable systems, in order to
reduce cochannel disturbances and to maintain precise separation between
channels, lock the low band channels to a multiple of 6.0 mHz, in which
case there might be an "adjacent" channel to 4, a channel 5 which would
be shifted 4 mHz negative.

     More likely, though, is that the BTSC signal is getting munged from
two causes:

     1) Multipath distortion.  Cable TV systems do receive finite amounts
of signal out of the air through the plant facilities (interconnecting
wiring, taps, and so on) which are amplified by the line amplifiers and
delivered to your home.  This can be especially troublesome at low
channel frequencies. If you are located in the "grade A" contour of the
channel 4 signal, the chances for multipath distortion are very high.

     2) Incorrect bandwidth inside the headend processing equipment. I
doubt seriously that modern cable plants "pass the signal unattenuated."
The Scientific-Atlanta series of channel processing amplifiers perform
a complete downconversion to the TV I.F., do the processing at the IF,
and then upconvert back to the desired channel. This is even true when
the same channel is to be passed. The advantage of this is that "offsets"
can be applied to the cable signal to reduce cochannel intermodulation
effects. (Offsets are + 10 kc, - 10 kc, and 0 kc of the nominal pix
carrier frequency). The filters inside these channel processors are very
sharp and are implemented in SAW technology; some SAW filters do not have
the proper slope characteristics for BTSC.

     If your cable company was stupid enough to use Catel or B-T channel
processors in the headend, there is no telling what is happening to
channel 4.  Often times, when there is no upper adjacent channel on
the basic-12, the cheapskate headend will install cheaper channel
processors because the spillage doesn't affect anyone.

     3) (Just thought of a third one) Modification of a television transmitter
for BTSC is a * NONTRIVIAL TASK *. You just don't go out and get a BTSC
exciter and plug it into the aural transmitter.  Very extensive mods have
to be made to the diplexer, output traps, and interstage coupling inside
the aural transmitter, particularly if it was designed in the old days.
IF diplexed transmitters don't have as many problems (check with your
TV station on this....but most stereo generator people won't let you
get away with just purchasing and installing it...) A marginal BTSC
installation will "false" and so on in fringe conditions.

     4) (A fourth cause) Heavy intermodulation distortion - where the
line amplifier is no longer operating in the linear region - will destroy
BTSC. 

     OK, analog engineers, you wanna make some money? Design some channel
processors and other goodies, as well as test equipment, so that the
cable system will pass BTSC unmunged. (Note that if you have Zenith ZTAC
boxes, you won't get BTSC at all...the signal is downconverted to baseband
and remodulated.) I've solved this by erecting a - gasp - antenna! 

David Anthony
DataSpan, Inc