Voice activity detector

DETAILED DESCRIPTION What is claimed is: 1.
A voice activity detector for detecting the presence of speech in an input signal which comprises a noise component, said detector comprising: (a) means for storing an estimate of the noise component of an input signal; (b) means for recognizing a spectral similarity of the input signal and the stored estimate to produce an output decision signal; (c) means for updating the stored estimate; (d) an auxiliary detector arranged to indicate the absence of speech from the input signal and control the means for updating so that updating occurs only when speech is indicated by the auxiliary detector to be absent from the input signal; (e) means operable to calculate a prediction gain for the input signal; and (f) modifying means arranged to suppress said updating of the stored estimate in the event that the prediction gain exceeds a threshold value.
2.
A voice activity detector according to claim 1 in which the auxiliary detector includes a voiced speech detector responsive to signals derived from a Linear Predictive Coding residual signal.
3.
A voice activity detector according to claim 1 in which the prediction gain represents the prediction gain of a Linear Predictive Coding prediction of 6th order or less.
4.
A voice activity detector according to claim 3 in which the prediction gain represents the prediction gain of a Linear Predictive Coding prediction of 4th order or less.
5.
A voice activity detector according to claim 1 further including means for detecting one or more primary frequency components of the input signal, and means operable to compare the frequencies with a predetermined threshold and to permit suppression of said updating of the stored estimate only when said primary frequency component exceeds the threshold.
6.
A voice activity detector according to claim 1 in which the prediction gain calculation means is preceded by a filter to remove an upper portion of the frequency range of the input signal, the gain calculation being performed only for frequency components lying within the passband of the filter