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Abstract

Proposals have been made to use a digitally modulated signal in a UHF television channel to feed radio programme sound to the Channel Islands. It is clearly necessary to demonstrate that the proposed link will not cause unacceptable interference to existing UHF television services, including those of France. However, calculations concerning the problem have been hindered because the protection ratio needed when an interferer is digitally modulated was not known; in contrast, values for continuous wave (CW) or television interferers are provided by the CCIR and are well established.

Subjective tests were therefore conducted to establish the protection ratios for SECAM and PAL television signals suffering interference of the type produced by the proposed digital link. The results confirm that the energy dispersal provided by digital modulation of an interfering signal reduces the visibility of worst case interference so that the protection ratio can be relaxed accordingly, by an amount ranging from 3 to 12 dB for SECAM and 2 to 10 dB for PAL. If 'precision offset' of the carrier is used the visibility of CW interference is greatly reduced whereas the visibility of digitally modulated interference is unchanged; the protection ratio for digitally modulated interference is therefore the same with and without 'precision offset'.

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