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