13. The speech coding apparatus as claimed in claim 12, wherein: the measuring means measures the values of at least two features of an utterance during each of a series of successive time intervals to produce a series of feature vector signals representing the feature values; and a scalar function of a feature vector signal comprises the value of only a single feature of the feature vector signal.
13. The speech coding apparatus as claimed in claim 12, wherein: the measuring means measures the values of at least two features of an utterance during each of a series of successive time intervals to produce a series of feature vector signals representing the feature values; and a scalar function of a feature vector signal comprises the value of only a single feature of the feature vector signal. 14. The speech coding apparatus as claimed in claim 13, characterized in that the measuring means comprises a microphone.