9. Apparatus as described in claim 1 wherein said speech detection means includes: means for generating a start-of-speech indication for a portion of the signal in which audio signal amplitudes for a plurality of time periods exceed a speech threshold amplitude; and means for generating an end-of-speech indication for a second portion of the signal in which audio signal amplitudes for a plurality of time periods are below a no-speech threshold amplitude.
11. Apparatus as described in claim 9 wherein said means for deriving a background amplitude level includes selecting means for causing it to derive said level only from audio signal amplitudes for time periods which do not occur between a start-of-speech indication and its following end-of-speech indication generated by said means for generating.