Data processing: speech signal processing – linguistics – language – Speech signal processing – Recognition
Reexamination Certificate
1998-09-26
2002-07-30
Chawan, Vijay B (Department: 2654)
Data processing: speech signal processing, linguistics, language
Speech signal processing
Recognition
C704S239000, C704S226000, C704S208000, C455S428000, C455S523000, C379S406010, C379S413020
Reexamination Certificate
active
06427134
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voice activity detector. It has particular utility in relation to an auxiliary voice activity detector comprised in a main voice activity detector and also when comprised in a noise reduction apparatus. A main voice activity detector incorporating such an auxiliary voice detector is especially suitable for use in mobile phones which may be required to operate in noisy environments.
2. Description of Related Art
Because of the limited regions of the electromagnetic spectrum which have been made available for use by cellular radio systems, the strong growth in the number of mobile phone users over the last decade has meant that cellular radio equipment suppliers have had to find ways to increase the efficiency with which the available electromagnetic spectrum is utilised.
One way in which this aim can be achieved is to reduce the size of the cells within the cellular radio system. However, it is found that cell size can only be reduced by so much before the level of interference from nearby cells (co-channel interference) becomes unacceptably high. In order to reduce co-channel interference, a technique called discontinuous transmission is used. This technique involves arranging the mobile phone to transmit speech-representing signals only when the mobile phone user is speaking and is based on the observation that, in a given conversation, it is usual for only one of the parties to speak at any one time. By implementing discontinuous transmission, the average level of co-channel interference can be reduced. This, in turn, means that the cell size in the system can be reduced and hence that the system can support more subscribers.
Another advantage of only transmitting sound-representing signals when the mobile phone user is speaking is that the lifetime of the electric battery within the mobile phone handset is increased.
A voice activity detector is used to enable discontinuous transmission. The purpose of such a detector is to indicate whether a given signal consists only of noise, or whether the signal comprises speech. If the voice activity detector indicates that the signal to be transmitted consists only of noise, then the signal is not transmitted.
Many mobile phones today use a voice activity detector similar to that described in European Patent No. 335521. In the voice activity detector described therein, the similarity between the spectrum of an input sound-representing signal and the spectrum of a noise signal is measured. The noise spectrum to be used in this comparison is obtained from earlier portions of the input signal which were determined to be noise. That judgement is made by an auxiliary voice activity detector which forms a component of the main voice activity detector. Since it is important that signals comprising speech are transmitted by the mobile phone and since the decision of the main voice activity detector is based on signals identified as noise by the auxiliary voice detector, it is desirable that the auxiliary voice detector tends, in borderline situations, towards a determination that the signal comprises speech. The proportion of a conversation which is identified as speech by a voice activity detector is called the voice activity factor (or simply “activity”) of the detector. The proportion of conversation which in fact comprises speech is typically in the range 35% to 40%. So, ideally, a main voice activity detector will have art activity lying within this range or slightly above it, whereas an auxiliary voice activity detector can have a significantly higher activity.
Although the known voice activity detectors exhibit good performance in a variety of environments, their performance has been found to be poor in noisy environments. A mobile phone may be required to operate in cars, in city streets, in busy offices, in train stations or in airports. There is therefore a requirement for a voice activity detector that can operate reliably in noisy environments.
BRIEF SUMMARY OF THE INVENTION
According to the first aspect of the present invention there is provided a voice activity detector comprising:
means arranged in operation to calculate at least one first spectral difference measure indicative of the degree of spectral similarity in a pair of time segments of a signal, one of the time segments of the pair lagging the other by a first time interval;
means arranged in operation to calculate at least one second spectral difference measure indicative of the degree of spectral similarity in a pair of time segments of a signal, one of the time segments of the pair lagging the other by a second time interval which differs from said first time interval;
means arranged in operation to calculate a spectral irregularity measure on the basis of at least said first and second spectral difference measures; and
means arranged in operation to compare said spectral irregularity measure with a threshold measure.
This voice activity detector has the advantage that it provides a reliable determination that an input signal consists of noise. As stated above, this is a desirable property for an auxiliary voice activity detector which is used to identify signals which are used as noise templates in other processes carried out in an apparatus. Also, by combining spectral difference measures derived in relation to different time intervals, a voice activity detector according to the present invention takes into account the degree of stationarity of the signal over different time intervals. For example, if a first spectral difference measure were to be calculated in relation to a first relatively long time interval and a second spectral difference measure were to be calculated in relation to a relatively short time interval, then both the short-term and long-term stationarity of the signal would influence a spectral irregularity measure which combines the first and second spectral difference measures. Since the spectrum of noise, unlike speech, is stationary at least over time intervals ranging from 80 ms to 1 s, the voice activity detector of the present invention provides a robust performance in noisy environments.
Preferably, the predetermined length of time is in the range 400 ms to 1 s. This has the advantage that the relatively rapidly time-varying nature of a speech spectrum can be best discriminated from the relatively slowly time-varying nature of a noise spectrum.
Preferably, said spectral irregularity measure calculating means are arranged in operation to calculate a weighted sum of said spectral difference measures. This has the advantage that, in making a speech
oise decision, more weight can be given to spectral difference measures derived from time intervals over which the difference in stationarity between speech spectra and noise spectra is most pronounced.
According to a second aspect of the present invention there is provided a voice activity detector including:
a voice activity detector according to the first aspect of the present invention operable as an auxiliary voice activity detector.
Since the auxiliary noise detector has a high activity, a determination that an input signal consists of noise can be relied on to be correct. Furthermore, because the correct functioning of the main voice activity detector relies on the auxiliary voice activity detector correctly identifying a noise signal, a voice activity detector according to the second aspect of the present invention makes a reliable determination of whether a signal comprises speech or consists only of noise.
According to a third aspect of the present invention there is provided a noise reduction apparatus comprising:
a voice activity detector according to the first aspect of the present invention;
means arranged in operation to provide an estimated noise spectrum on the basis of one or more spectra obtained from respective time segments determined to consist of noise by said voice activity detector; and
means arranged in operation to subtract said estimated noise spectrum from spectra obtained from su
Barrett Paul Alexander
Garner Neil Robert
British Telecommunications public limited company
Chawan Vijay B
Nixon & Vanderhye P.C.
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