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It is a matter of record that the visibility of co-channel interference has maxima and minima at carrier offset frequencies which are a multiple of frame frequency. Subjective tests were made in the laboratory to determine the possible reduction in the visibility of co-channel interference by precise carrier frequency control and the stability requirements on the carrier frequency. The tests show an improvement of 12 db in the tolerable ratio of desired carrier to undesired carrier for an offset of 10,010 cycles and 18 db for an offset of 20,020 cycles; and that an offset frequency stability of plus or minus five cycles would be adequate. The above offset frequencies are the optimum offsets for a frame frequency of 29.97 cycles; they are even multiples (334 and 668) of frame frequency. For transmissions using a frame frequency of 30 cycles the optimum offsets are 10,020 cycles and 20,040 cycles. The stability requirements on the offset frequencies impose a relative stability on the picture carriers of plus or minus one part per hundred million for the VHF channels.
Broadcast Transmission Systems, IRE Transactions on (Volume:PGBTS-7 , Issue: 1 )
Date of Publication: Feb. 1957