1. A speech coding apparatus comprising: means for measuring the value of at least one feature of an utterance during each of a series of successive time intervals to produce a series of feature vector signals representing the feature values; means for storing a plurality of prototype vector signals, each prototype vector signal having at least one parameter value and having an identification value, at least two prototype vector signals having different identification values; classification rules means for storing classification rules mapping each feature vector signal from a set of all possible feature vector signals to exactly one of at least two different classes of prototype vector signals, each class containing a plurality of prototype vector signals and each class of prototype vector signals is at least partially different from other classes of prototype vector signals, wherein each class of prototype vector signals contains less than 1/N times the total number of prototype vector signals in all classes, where 5.ltoreq.N.ltoreq.150; classifier means for mapping, by the classification rules, a first feature vector signal to a first class of prototype vector signals; means for comparing the closeness of the feature value of the first feature vector signal to the parameter values of only the prototype vector signals in the first class of prototype vector signals to obtain prototype match scores for the first feature vector signal and each prototype vector signal in the first class; and means for outputting at least the identification value of at least the prototype vector signal having the best prototype match score as a coded utterance representation signal of the first feature vector signal.