Reception of TV with sound in syncs at low carrier to noise ratios

Independent Television News have been using a low power Newshawk uplink. When operating through ECSI, using eurobeam aerials for uplink and downlink, a carrier to noise ratio of 11 dB is typically obtained. For a conventional limiter discriminator demodulator, the FM threshold effect will cause vision failure at about 10 dB C/N giving a fade margin of only 1 dB. Rain fades of a few dB are commonly encountered and so it is essential that some form of threshold improvement is employed at the receiver. The IBA have developed a threshold extension demodulator which provides a 4 dB static threshold extension for vision, giving a fade margin of about 5 dB. For 25 MHz/v signals, vision failure occurs at about 5-6 dB C/N (36 MHz). In order to provide low vision threshold, a special phase lock loop configuration is used. This makes use of the redundancy which exists in television signals