| 摘要 |
A transmitter, which remotely controls the demuting of a normally muted radio receiver, derives a command signal from a carrier wave, the command signal having a frequency or a duration which is precisely related to the frequency of the carrier wave. The receiver derives a reference signal from the received carrier wave, having a frequency which is a predetermined fraction of the frequency of the carrier wave, detects the command signal, and compares a time parameter of the command signal (either its frequency or duration) with the frequency of the reference signal. If a predetermined relationship exists, a control signal is provided for controlling a muting/demuting circuit in the receiver. Selective addressing of a particular receiver may be effected by providing the transmitter with means for deriving a plurality of command signals, each having a different time parameter corresponding to a particular receiver, and selective keying means for selecting one of the command signals for transmission. The carrier wave may be a pilot carrier which, in turn, modulates a main carrier wave of the transmitter.
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