GB1516100A - Audio signal processing apparatus - Google Patents

Abstract

1516100 Speech detector DEFENCE SECRETARY OF STATE FOR 17 Dec 1975 [17 Dec 1974 (2)] 54504/74 and 54506/74 Heading H4R In a speech detector an incoming signal is applied to first and second rectifying and smoothing circuits 6, 7 and 9, 10 respectively, the time constant of smoothing of the second circuit being larger than that of the first, and the output of the first circuit is applied directly to a comparator 8 whereas the output of the second circuit is coupled via an amplifier 11 of gain greater than unity to the comparator the output of which enables the speech circuit when the first circuit output exceeds the amplified second circuit output. The time constants are respectively 0�01 sec and 0�15 sec with FET 13 conducting and are such that with only slowly varying noise present both smoothing circuits give substantially the same output and hence comparator 8 gives a speech absent indication because the output from the second circuit as applied thereto exceeds that of the first circuit due to the presenting amplifier 11. However if a rapidly increasing input signal occurs the first circuit 6, 7 responds more quickly than the second circuit and output of amplifier 8 is switched to give a speech present indication via gate 15, integrated circuit monostable 17 and FET 18. The speech present signal also switches off FET 13 to increase the time constant of the second circuit 9, 10 to about 15 secs i.e. to a period greater than any continuous vocalization in normal speech. To guard against a sudden increase in noise level causing the muting to be removed a second comparator 25, integrated circuit monostable 26 and transistor 27 are provided. However the gain of the signal of the second circuit 9, 10 as applied to the comparator 25 is 1�2 as compared with a gain of 1�5 as applied to comparator 8. For a sudden increase of signal the circuit operates as before with both comparators giving a speech present indication. However in the case of noise which is not rapidly varying the output of comparator 25 reverts after about one second to cause FET 13 to be switched back on, thereby lowering the time constant of the second circuit to 0�15 seconds, and causing the output of comparator 8 to revert to a speech absent indication. Figure 1 (not shown) illustrates a simplified version of the above circuit in which components 22-27 are not provided, in which amplifier 11 has a gain of two, and in which the FET 18 is replaced by a photo resistor and photo emissive diode arrangement. The monostable device 17 uses transistor logic in this arrangement.