| 摘要 |
A filter for use in a cable television system for filtering out a given channel comprising a two pole filter, said filter including one pole which is fixed at the center frequency to be tuned out and a second variable pole responsive to an input voltage, the second pole adapted to vary in frequency about the first pole. The filter is employed in addressable control system which includes an address generator at a head end, generating address control signals for each of a plurality of remote subscribers, a modulator for modulating a carrier with the address and control signal and coupling it onto cables, circuits at receiving terminals for receiving and demodulating the modulated carrier to recover the address signals, address decoding means for each subscriber to decode the address and control signals and circuits responsive to the decoding means to select the channels which each subscriber will receive. The system also has an encoder for encoding the status of the receive signals and a modulator for modulating a return carrier with the encoded states and putting this on the cable so that the signals may be demodulated at the head end. To carry out the demodulating and also to moduate the return signals, a local oscillator and mixer is used which generates both an intermediate frequency for the mixer and a carrier frequency for the return signal.
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