System zur Unterscheiden zwischen Audio- und Nicht-Audio-Signalen

A threshold level for determining whether an input speech signal to a voice operated recorder is an audio or non-audio signal is changed in accordance with the ratio of the non-audio duration of the input speech signal to a predetermined period of time, as determined by a level changing control means (10 to 14), so as to decrease the influence of ambient noise on the operation of distinguishing between audio and non-audio signals. The threshold level may be changed directly at a variable voltage source (22 - Figure 2) connected to a comparator (18) which compares the input speech signal to the threshold level. Alternatively, the threshold level may be changed by changing the gain of a variable gain amplifier (16 - Figure 1) through which the input speech signal is passed before it arrives at the comparator (18).