Conditional access system enabling partial viewing

摘要

A conditional access system includes an EMM (entitlement management message) analyser 31 for retrieving partial viewing authorisation. When partial viewing authorisation information permits partial viewing an ECM (entitlement control message) is decrypted, using work keys, to retrieve scrambling keys. This allows partial viewing to encourage a viewer to subscribe to a channel. Preferably the system includes means for intermittently retrieving the scrambling keys when only partial viewing is authorised. The system may operate so as to only decode I frames when partial viewing is permitted.