| 摘要 |
Before transmission via a cable network (23), the TV signal which represents the TV programme which is available to a restricted audience (i.e. it is subject to a tarriff) is coded by weakening the synchronisation pulses in it. Subscribers are given decoding circuits which amplify the weakened synchronisation pulses again (21, 22). Synchronisation information modulated onto a control carrier is fed to the decoding circuits via the cable network (23) for this purpose. The sound carrier of any other TV programme is used as the control carrier, and the synchronisation information is modulated onto it by amplitude modulation. Therefore, no additional carrier is necessary, and security against programme theft is improved by the free assignment between channels for charged TV programmes and sound carriers of other channels, which act as control carriers. For any chosen frequency for one or more charged TV programmes, a sound carrier in TV band I can be chosen as the control carrier, so that the decoding circuits are simple and cheap. <IMAGE>
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