| 摘要 |
The signal is transmitted from a transmitting centre (10) to a number of receiver locations (11). Coding is effected by shifting the frequency of each frequency component in the transmitted signal, and decoding is effected by coordinately shifting the frequency of components in the received signal, at authorised locations. Both during coding and decoding, the frequency shifts are obtained by subjecting the signal to mixing with an unmodulated radio frequency signal, the frequency of which is shifted in accordance with a selected pattern, followed by a filtering stage. The original signal is thus retrieved following decoding. |
