Harmonically-related carriers

Harmonically-related carriers (also known as HRC) is a system for modulating video over a cable TV network. All vision carrier frequencies are on exact multiples of the channel bandwidth frequency (6MHz in the case of System-M used in the Americas, other TV systems worldwide use 7MHz or 8MHz). William Grant, in his book, Cable Television, 2nd edition, GWG Associates, Schoharie, NY, 1988, page 82, states:

"By harmonically-relating the carrier frequencies themselves it is ... possible to improve system performance. This does not reduce the beats produced, but positions them within the system transmission spectrum, such that they are more tolerable. In effect, all signal carriers are spaced precisely at 6 MHz apart, and thus, all beats generated are at 6 MHz increments.

Since the television signals are vestigial sideband modulation, if the beat products can be manipulated to fall on or near the RF carriers themselves, they are much less offensive."

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