By I. Switzer, Switzer Engineering Services Limited
Television coverage plans in the United States originally used geographic spacing to provide protection against co-channel interference. Subsequent allocation plans improved the protection by use of offset channels. It was recognized at an early date that an offset of an odd multiple of the horizontal scanning frequency would be optimum, however consideration of the problem of providing protection where more than two channels are involved required the use of an offset which compromised the optimum. An offset of 10 KHz was chosen and stations were assigned "on channel", "+10 KHz offset" and -10 KHz" offset" in a pattern which generally assured that overlapping stations would be separated by either 10KHz or 20 KHz. Frequency tolerance was set at +- 1 KHz in recognition of the practical difficulties of operating television transmitters, particularly high band and UHF transmitters with lesser tolerance.