22. A system as claimed in claim 14 wherein the derivation of the data encoding vectors from the input signal involves the use of a second, low level GMM, whereby this second GMM provides the data encoding vectors to the original GMM that comprise elements derived from the second GMM's posterior probabilities.
22. A system as claimed in claim 14 wherein the derivation of the data encoding vectors from the input signal involves the use of a second, low level GMM, whereby this second GMM provides the data encoding vectors to the original GMM that comprise elements derived from the second GMM's posterior probabilities. 25. A system as claimed in claim 22 wherein the constant values for the modulus of each of the class mean vectors may be different at each level.