| 摘要 |
A circuit serves to extract one of two or more pilot tones which are modulated on an audio-frequency signal to select an appropriate decoder, and which cause a changeover to the appropriate decoder or of a decoding signal processor. To this end, one of the stereo signals is digitized by means of a clocked analog-to-digital converter, and the audio signal is separated from the pilot-signal-containing component by means of a pilot-tone-range filter and then averaged over a number of clock periods of the analog-to-digital converter by means of a sampling averager. The output signal of the averager is applied to the inputs of pilot-tone filters, then rectified to measure the signal energy, and applied to the inputs of low-pass filters. The outputs of the low-pass filter control a logic circuit which causes the two audio-frequency signals to be applied to the input of the appropriate decoder.
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