29. A method as claimed in claim 26 wherein the data encoding vectors are scaled in a pre-processing stage before being input to the GMM or GMM based HMM, such that the moduli of all data encoding vectors are equal.
29. A method as claimed in claim 26 wherein the data encoding vectors are scaled in a pre-processing stage before being input to the GMM or GMM based HMM, such that the moduli of all data encoding vectors are equal. 31. A method as claimed in claim 29 wherein elements forming spectral coefficients of a data encoding vector are arranged to be individually proportional to the square root of the power in their corresponding spectral band, divided by the square root of the overall power contained in spectral bands represented in the vector.