14. A signal processing system for processing a plurality of multi-element data encoding vectors, the system: having means for deriving the data encoding vectors from input signals; being arranged to process the data encoding vectors using a Gaussian Mixture Model (GMM), the GMM having at least one class mean vector having multiple elements; being arranged to process the elements of the class mean vector(s) by an iterative optimisation procedure; characterised in that the system is also arranged to scale the elements of the class mean vector(s) during the optimisation procedure to provide for the class mean vector(s) to have constant modulus at each iteration, and to normalise the data encoding vectors input to the GMM.
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.