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 measure of the prediction gain for the input signal; (f) modifying means arranged to suppress said updating of the stored estimate in the event that the prediction gain exceeds a threshold value.', '(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 measure of the prediction gain for the input signal;', '(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 measure of 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 measure of 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 measure calculation being performed only for frequency components lying within the passband of the filter.

7. A voice activity detector according to claim 6 having two filters having respective passbands and respective measure calculation means to calculate a measure of the prediction gain for the respective passbands, the modifying means being arranged to suppress the updating of the stored estimate in the event that the prediction gain within either passband exceeds a threshold value.

8. A voice activity detector according to claim 6 including means for subsampling a signal produced by the filter.

9. A voice activity detector according to claim 1 wherein said input signal includes signaling tones and the prediction gain exceeding a threshold value indicates the detection of signaling tones in the input signal and causes the modifying means to suppress the updating of the stored estimate

10. A method for detecting the presence of speech in an input signal which includes a noise component, said method comprising:(a) storing an estimate of the noise component of an input signal; (b) recognizing a spectral similarity of the input signal and the stored estimate to produce an output decision signal; (c) updating the stored estimate; (d) using an auxiliary detector arranged to indicate the absence of speech from the input signal and control the updating step so that updating occurs only when speech is indicated by the auxiliary detector to be absent from the input signal; (e) generating a measure of the prediction gain for the input signal; (f) suppressing said updating of the stored estimate in the event that the prediction gain exceeds a threshold value.', '(a) storing an estimate of the noise component of an input signal;', '(b) recognizing a spectral similarity of the input signal and the stored estimate to produce an output decision signal;', '(c) updating the stored estimate;', '(d) using an auxiliary detector arranged to indicate the absence of speech from the input signal and control the updating step so that updating occurs only when speech is indicated by the auxiliary detector to be absent from the input signal;', '(e) generating a measure of the prediction gain for the input signal;', '(f) suppressing said updating of the stored estimate in the event that the prediction gain exceeds a threshold value

11. A method as in claim 10 in which the auxiliary detector uses a voiced speech detector responsive to signals derived from a Linear Predictive Coding residual signal

12. A method as in claim 10 in which the measure of the prediction gain represents the prediction gain of a Linear Predictive coding prediction of 6th order or less

13. A method as in claim 12 in which the measure of the prediction gain represents the prediction gain of a Linear Predictive Coding prediction of 4th order or less

14. A method as in claim 10 further including:detecting one or more primary frequency components of the input signal, and comparing 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.', 'detecting one or more primary frequency components of the input signal, and', 'comparing 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

15. A method as in claim 10 in which the calculation of the measure of the prediction gain is preceded by filtered removal of an upper portion of the frequency range of the input signal, the measure calculation being performed only for frequency components lying within the passband of the filter

16. A method as in claim 15 wherein two filters are used having respective passbands and respective measure calculation means to calculate measures of the prediction gain for the respective passbands, the modifying step suppressing the updating of the stored estimate in the event that the prediction gain within either passband exceeds a threshold value

17. A method as in claim 15 including subsampling a signal produced by the filter

18. A method as in claim 10 wherein said input signal includes signaling tones and the prediction gain exceeding a threshold value indicates the detection of signaling tones in the input signal and causes the modifying step to suppress the updating of the stored estimate.']