| 摘要 |
<p>In a television system in which a signal of given bandwidth is to be transmitted over a channel of narrower bandwidth, the signal is subject to pre-filtering and subsampling at the transmitter and supersampling and post-filtering at the receiver. The post-filtering means (14) comprises three filters A, B and C in parallel which are selectable in response to the characteristics of the signal to be transmitted. In order to avoid anomalies caused by switching at the transmitter, the pre-filter (10) is a single filter with a fixed pass band equivalent to the union of the pass bands of the filters A, B and C of the post-filtering means (14). The pre-filter (10) thus effectively rejects signals of frequency, such that they would not be passed by any of the filters A, B and C.</p> |
