22. A speech coding method as claimed in claim 21, wherein: the step of measuring comprises measuring 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.
23. A speech coding method as claimed in claim 22, wherein the step of measuring comprises measuring the amplitudes of the utterance in two or more frequency bands during each of a series of successive time intervals.