Arny Krueger wrote:

"Terry Casey" wrote in message
...
In article ,
says...

"Geoffrey S. Mendelson" wrote in message
...

I don't know how well UK sets worked in the 1960's, but US TV sets were
not capable of receiving adjcent channels at one time, so they were not
used. For example, channel 2 was used in New York City, while the
nearest
channel 3 station was in Philadelphia, 90 miles away and too far to be
received without a large antenna.

US analog TV's improved greatly and were generally happy with adjacent
channels for maybe the last 20 years of their lives.

http://www.jneuhaus.com/fccindex/cablech.html

shows cable channels on 6-7 MHz intervals. Adjacent numbered channels
were
used all the time.

A couple of questions regarding that list:

Why is the HRC channel spacing offset[1] by 300Hz - 6.0003MHz instead of
6MHz?

I don't know.

Why are the IRC channels offset from broadcast channels (where they
exist) by 12.5kHz?

If memory sserves, two transmitters that are interferring just a little,
produce nasty herringbones if they are running at the same frequency, but
move them apart a tad, and the artifacts are far less objectionable.

[1] UK cable systems mostly use HRC at 8MHz spacing but this is
sometimes varied by a carefully calculated amount so that one block of
UHF channels coincides almost exactly with the broadcast frequencies.
This is done on systems with a by-pass facility to allow a few channels
- usually the local off-airs - to be fed directly to the TV giving the
subscriber direct access from the TV without needing an aerial.

Obviously this block of channels has to be chosen so as not to conflict
with local transmitters, so the offset will vary from system to system
and can't be fixed as in the US table

US cable systems ran on some of the same channels as local broadcasters.


Yes, but they would move the local channel to another frequency, if
direct pickup was expected to be a problem. United Video Cablevision in
Cincinnati moved the local channels up one channel,, and numbered the
channels from '1' instead of '2'. Then the original channels were used
for things like the program guide and community bulletin board channels
so that if there was ingression or direct pickup, it was less
noticeable. Do you know that the channel combiners in a CATV head end
were wired in odd and even banks, on separate groups to prevent IMD
caused in the passive mixing?


--
You can't have a sense of humor, if you have no sense.