| 摘要 |
The null transitions of an audio frequency signal are converted by Schmitt trigger circuits, one of which has a small hysteresis range centered on the null value and the other of which has a much larger hysteresis range likewise centered on the null value, into two binary pulse sequences of variable pulse lengths. The Schmitt trigger circuits are so constituted that a positive pulse length is produced by a negative null transition of the audio signal and vice versa and, moreover, the Schmitt trigger circuits return to their quiescent state 2 milliseconds after a positive null transition of the signal, also producing a positive pulse length, in this case beginning the indication of the pause. The pauses in the two binary pulse sequences thus produced, which exceed predetermined length (60 milliseconds in both cases and, additionally, 30 milliseconds in the case of the pulses formed by the Schmitt trigger with the narrower hysteresis range) and from the three different pause detection operations logic circuits derive either a speech recognition signal, a music recognition signal or an indication of an unidentifiable signal. The logic circuit uses as criteria the number of pauses and the time span of simultaneous or alternating appearance of signal pauses derived from the two different pulse sequences. |
