A speech processing system is provided which is operable to receive sets of signal
values representative of a speech signal generated by a speech source. The system
is operable to determine a measure of the quality of the speech signal by performing
a statistical analysis of the received sets of signal values. The system stores
data defining a predetermined function derived from a signal model which models
the speech source and which defines a probability density function which gives,
for a given set of model parameters, the probability that the signal model has
those model parameters given that the signal model is assumed to have generated
the received set of signal values. The system applies a current set of received
signal values to the stored probability density function and then draws samples
from it using a Gibbs sampler. The system then analyses the samples to determine
a measure of the variance of some of the samples and then outputs a signal indicative
of the quality of the received speech signal values in dependence upon the determined variance.