From: utzoo!decvax!duke!harpo!npoiv!alice!mhtsa!eagle!karn
Newsgroups: net.video
Title: Re: Ripoff features of T.V. sets
Article-I.D.: eagle.723
Posted: Tue Jan 11 20:44:30 1983
Received: Thu Jan 13 04:52:43 1983
References: eagle.719 cbosgd.<2970

Someone mentioned that cable TV companies offset their channels; this is
actually done in order to improve video quality, not to intentionally
make it harder for people to fine-tune their sets.

This scheme is called Harmonically Related Carrier (HRC).  As its name
implies, each video carrier is a harmonic of a base 6 mhz oscillator at
the cable company.  Since standard TV channels have their video
carriers 1.25 mhz up from a frequency divisible by 6 (except VHF channels
5 & 6), the cable companies have to shift everything down by 1.25 mhz
from the standard on-air frequencies in order to make HRC work.  For
example, on-air channel 2 has its video carrier at 55.25 mhz, while an
HRC cable system has it at 54.0 mhz.

The idea behind HRC is that intermodulation products between signals
that are generated by the line amplifiers are less objectionable because
they tend to fall directly on top of other video carriers.  They can
often get several more video channels on a system in this way, where the
distortion would otherwise be intolerable.

Phil Karn