1992-09-28
1994-01-04
Knepper, David D.
G10L 500
Patent
active
052767655
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
A voice activity detector is a device which is supplied with a signal with the object of detecting periods of speech, or periods containing only noise. Although the present invention is not limited thereto, one application of particular interest for such detectors is in mobile radio telephone systems where the knowledge as to the presence or otherwise of speech can be used and exploited by a speech coder to improve the efficient utilisation of radio spectrum, and where also the noise level (from a vehicle-mounted unit) is likely to be high.
The essence of voice activity detection is to locate a measure which differs appreciably between speech and non-speech periods. In apparatus which includes a speech coder, a number of parameters are readily available from one or other stage of the coder, and it is therefore desirable to economise on processing needed by utilising some such parameter. In many environments, the main noise sources occur in known defined areas of the frequency spectrum. For example, in a moving car much of the noise (e.g., engine noise) is concentrated in the low frequency regions of the spectrum. Where such knowledge of the spectral position of noise is available, it is desirable to base the decision as to whether speech is present or absent upon measurements taken from that portion of the spectrum which contains relatively little noise. It would, of course, be possible in practice to pre-filter the signal before analysing to detect speech activity, but where the voice activity detector follows the output of a speech coder, prefiltering would distort the voice signal to be coded.
SUMMARY OF THE INVENTION
According to the invention there is provided a voice activity detection apparatus comprising means for receiving an input signal, means for periodically adaptively generating an estimate of the noise signal component of the input signal, means for periodically forming a measure M of the spectral similarity between a portion of the input signal and the noise signal component, means for comparing a parameter derived from the measure M with a threshold value T, and means for producing an output to indicate the presence or absence of speech in dependence upon whether or not that value is exceeded.
Preferably, the measure is the Itakura-Saito Distortion Measure.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects of the present invention are as defined in the claims.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a block diagram of a first embodiment of the invention;
FIG. 2 shows a second embodiment of the invention;
FIG. 3 shows a third, preferred embodiment of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
The general principle underlying a first Voice Activity Detector according to the a first embodiment of the invention is as follows.
A frame of n signal samples ##EQU1##
The zero order autocorrelation coefficient is the sum of each term squared, which may be normalized i.e. divided by the total number of terms (for constant frame lengths it is easier to omit the division); that of the filtered signal is thus ##EQU2## and this is therefore a measure of the power of the notional filtered signal s'--in other words, of that part of the signal s which falls within the passband of the notional filter.
Expanding, neglecting the first 4 terms, ##EQU3##
So R'.sub.0 can be obtained from a combination of the autocorrelation coefficients R.sub.i, weighted by the bracketed constants which determine the frequency band to which the value of R'.sub.0 is responsive. In fact, the bracketed terms are the autocorrelation coefficients of the impulse response of the notional filter, so that the expression above may be simplified to ##EQU4## where N is the filter order and H.sub.i are the (un-normalised) autocorrelation coefficients of the impulse response of the filter.
In other words, the effect on the signal autocorrelation coefficients of filtering a signal may be simulated by
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Rabiner et al., "Application of an LPC Distance Measure to the Voiced-Unvoiced-Silence Detection Problem", IEEE Trans. on ASSP, vol. ASSP-25, No. 4, Aug. 1977, pp. 338-343.
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Boyd Ivan
Freeman Daniel K.
British Telecommunications public limited company
Knepper David D.
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