| 摘要 |
<p>To decode control signals from a mixture of signals including audio signals, such as music, voice or speech in which the control signals are within the audio range; for example 2,350 Hz, on which a low-frequency signal, for example in the 100-200 Hz range is modulated, the control signal in the form of a control tone is filtered and demodulated by band pass filters (1,3), demodulators and rectifiers (2,4), and then applied to a timing circuit (5) which provides an output signal if and only if the demodulated control tone is continuously applied during the timing interval thereof. Suitable timing intervals are about 1 second. to provide an output signal which can switch a flipflop (7) on, for example to command radiation of a particular type of recognition signal by the transmitter (13), the setting of the flipflop (FF) (7) being fed back to the timing circuit to change the timing interval thereof to a lesser amount, for example 1/2 seconds to provide for a shorter signal to cause resetting of the FF (7). A second timing circuit (9) is provided to turn OFF the FF (7) in case it is overlooked to generate a 0.5 turn OFF signal and apply it to the first timing circuit (5).</p> |
