This application claims the priority of Korean Patent Application Nos. 10-2003-0028340 and 10-2004-0013029 filed on May 2, 2003 and Feb. 26, 2004, respectively, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a microphone array method and system, and more particularly, to a microphone array method and system for effectively receiving a target signal among signals input into a microphone array, a method of decreasing the amount of computation required for a multiple signal classification (MUSIC) algorithm used in the microphone array method and system, and a speech recognition method and system using the microphone array method and system.
2. Description of the Related Art
With the development of multimedia technology and the pursuit of a more comfortable life, controlling household appliances such as televisions (TVs) and digital video disc (DVD) players with speech recognition has been increasingly researched and developed. To realize a human-machine interface (HMI), a speech input module receiving a user's speech and a speech recognition module recognizing the user's speech are needed. In an actual environment of a speech interface, a user's speech, as well as interference signals, such as music, TV sound, and ambient noise, are present. To implement a speech interface for a HMI in the actual environment, a speech input module capable of acquiring a high-quality speech signal regardless of ambient noise and interference is needed.
A microphone array method uses spatial filtering in which a high gain is given to signals from a particular direction and a low gain is given to signals from other directions, thereby acquiring a high-quality speech signal. A lot of research and development for increasing the performance of speech recognition by acquiring a high-quality speech signal using such a microphone array method has been conducted. However, because a speech signal has a wider bandwidth than a narrow bandwidth which is a primary condition in array signal processing technology, and due to problems caused by, for example, various echoes in an indoor environment, it is difficult to actually use the microphone array method for a speech interface.
To overcome these problems, an adaptive microphone array method based on a generalized sidelobe canceller (GSC) may be used. Such an adaptive microphone array method has advantages of a simple structure and a high signal to interface and noise ration (SINR). However, performance deteriorates due to an incidence angle estimation error and indoor echoes. Accordingly, an adaptive algorithm robust to the estimation error and echoes is desired.
In addition, there are wideband minimum variance (MV) methods in which a minimum variance distortionless response (MVDR) may be applied to wideband signals. Wideband MV methods are divided into MV methods and maximum likelihood (ML) methods according to a scheme of configuring an autocorrelation matrix of a signal. In each method, a variety of schemes of configuring the autocorrelation matrix have been proposed for example, a microphone array based on a wideband MV method may be used by, etc.
The following description concerns a conventional microphone array method. When D signal sources are incident on a microphone array having M microphones in directions θ=, assuming that θ1 is a direction of a target signal and the remaining directions are those of interference signals. Discrete Fourier transforming data input to the microphone array and signal modeling are performed by expressing a vector of frequency components obtained by the discrete Fourier transformation, shown in Equation (1). Hereinafter, the vector of frequency components is referred to as a frequency bin.
xk=Aksk+nk (1)
Here, xk=[X1,k . . . Xm,k . . . XM,k]T, Ak=[ak(θ1) . . . ak(θd) . . . ak(θD)], sk=[S1,k . . . Sd,k . . . SD,k]T, nk=[N1,k . . . Nm,k . . . NM,k]T, and “k” is a frequency index. Xm,k and Nm,k are discrete Fourier transform (DFT) values of a signal and background noise, respectively, observed at an m-th microphone, and Sd,k is a DFT value of a d-th signal source. ak(θd) is a directional vector of a k-th frequency component of the d-th signal source and can be expressed as Equation (2).
ak(θd)=[e−jw
Here, τk,m(θd) is the delay time taken by the k-th frequency component of the d-th signal source to reach the m-th microphone.
An incidence angle of a wideband signal is estimated by discrete Fourier transforming an array input signal, applying a MUSIC algorithm to each frequency component, and finding the average of MUSIC algorithm application results with respect to a frequency band of interest. A pseudo space spectrum of the k-th frequency component is defined as Equation (3).
Here, Un,k indicates a matrix consisting of noise eigenvectors with respect to the k-th frequency component, and ak(θ) indicates a narrowband directional vector with respect to the k-th frequency component. When the incidence angle of the wideband signal ak(θ) is identical to an incidence angle of a temporary signal source, the denominator of the pseudo space spectrum becomes “0” because a directional vector is orthogonal to a noise subspace. As a result, the pseudo space spectrum has an infinite peak. An angle corresponding to the infinite peak indicates an incidence direction. Here, an average pseudo space spectrum can be expressed as Equation (4).
Here, kL and kH respectively indicate indexes of a lowest frequency and a highest frequency of the frequency band of interest.
In a wideband MV algorithm, a wideband speech signal is discrete Fourier transformed, and then a narrowband MV algorithm is applied to each frequency component. An optimization problem for obtaining a weight vector is derived from a beam-forming method using different linear constraints for different frequencies.
Here, a spatial covariance matrix Rk is expressed as Equation (6).
Rk=E[xkxkH] (6)
When Equation (6) is solved using a Lagrange multiplier, a weight vector wk is expressed as Equation (7).
Wideband MV methods are divided into two types of methods according to a scheme of estimating the spatial covariance matrix Rk in Equation (7): (1) MV beamforming methods in which a weight is obtained in a section where a target signal and noise are present together; and (2) SINR beamforming methods or Maximum Likelihood (ML) methods in which a weight is obtained in a section where only noise without a target signal is present.
The above discussed conventional system reliably operates when estimating a spatial covariance matrix in a section having only an interference signal without a speech signal. However, when obtaining a spatial covariance matrix in a section having a target signal, the conventional system removes the target signal as well as the interference signal. This result occurs because the target signal is transmitted along multiple paths as well as a direct path due to echoing. In other words, echoed target signals transmitted in directions other than a direction of a direct target signal are considered as interference signals, and the direct target signal having a correlation with the echoed target signals is also removed.
To overcome the above-discussed problem, a method or a system for effectively acquiring a target signal with less effect of an echo is desired.
In addition, a method of decreasing the amount of computation required for the MUSIC algorithm is also desired because the wideband MUSIC module 5 performs a MUSIC algorithm with respect to each frequency bin, which puts a heavy load on the system.
The invention provides a microphone array method and system robust to an echoing environment.
The invention also provides a speech recognition method and system robust to an echoing environment using the microphone array method and system.
The invention also provides a method of decreasing the amount of computation required for a multiple signal classification (MUSIC) algorithm, which is used to recognize a direction of speech, by reducing the number of frequency bins.
According to an aspect of the invention, there is provided a microphone array system comprising an input unit which receives sound signals using a plurality of microphones; a frequency splitter which splits each sound signal received through the input unit into a plurality of narrowband signals; an average spatial covariance matrix estimator which uses spatial smoothing, by which spatial covariance matrices for a plurality of virtual sub-arrays, which are configured in the plurality of microphones comprised in the input unit, are obtained with respect to each frequency component of the sound signal processed by the frequency splitter and then an average spatial covariance matrix is calculated, to obtain a spatial covariance matrix for each frequency component of the sound signal; a signal source location detector which detects an incidence angle of the sound signal based on the average spatial covariance matrix calculated using the spatial smoothing; a signal distortion compensator which calculates a weight for each frequency component of the sound signal based on the incidence angle of the sound signal and multiplies the weight by each frequency component, thereby compensating for distortion of each frequency component; and a signal restoring unit which restores a sound signal using distortion compensated frequency components.
The frequency splitter uses discrete Fourier transform to split each sound signal into the plurality of narrowband signals, and the signal restoring unit uses inverse discrete Fourier transform to restore the sound signal.
According to another aspect of the invention, there is provided a speech recognition system comprising the microphone array system, a feature extractor which extracts a feature of a sound signal received from the microphone array system, a reference pattern storage unit which stores reference patterns to be compared with the extracted feature, a comparator which compares the extracted feature with the reference patterns stored in the reference pattern storage unit, and a determiner which determines based on a comparison result whether a speech is recognized.
According to another aspect of the invention, there is provided a microphone array method comprising receiving wideband sound signals from an array comprising a plurality of microphones, splitting each wideband sound signal into a plurality of narrowbands, obtaining spatial covariance matrices for a plurality of virtual sub-arrays, which are configured to comprise a plurality of microphones constituting the array of the plurality of microphones, with respect to each narrowband using a predetermined scheme and averaging the obtained spatial covariance matrices, thereby obtaining an average spatial covariance matrix for each narrowband, calculating an incidence angle of each wideband sound signal using the average spatial covariance matrix for each narrowband and a predetermined algorithm, calculating weights to be respectively multiplied by the narrowbands based on the incidence angle of the wideband sound signal and multiplying the weights by the respective narrowbands, and restoring a wideband sound signal using the narrowbands after being multiplied by the weights respectively.
In the microphone array method, discrete Fourier transform is used to split each sound signal into the plurality of narrowband signals, and inverse discrete Fourier transform is used to restore the sound signal.
According to another aspect of the invention, there is provided a speech recognition method comprising extracting a feature of a sound signal received from the microphone array system, storing reference patterns to be compared with the extracted feature, comparing the extracted feature with the reference patterns stored in the reference pattern storage unit, and determining based on a comparison result whether a speech is recognized.
Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the U.S. Patent and Trademark Office upon request and payment of the necessary fee. The above and other features and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:
FIGS. 10(A)(1)-(3) shows a waveform of an output signal with respect to a reference signal in a conventional method;
FIGS. 15(A)-(C) illustrates a distribution of averaged speech presence probabilities (SPPs) with respect to individual channels according to an embodiment of the present invention;
Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.
As shown in
M sound signals input through the M microphones are input to a discrete Fourier transformer 102 to be decomposed into narrowband frequency signals. In an aspect of the invention, a wideband sound signal such as a speech signal is decomposed into N narrowband frequency components using a discrete Fourier transform (DFT). However, the speech signal may be decomposed into N narrowband frequency components by methods other than a discrete Fourier transform (DFT).
The discrete Fourier transformer 102 splits each sound signal into N frequency components. An average spatial covariance matrix estimator 104 obtains spatial covariance matrices with respect to the M sound signals referring to the sub-arrays of L microphones and averages the spatial covariance matrices, thereby obtaining N average spatial covariance matrices for the respective N frequency components. Obtaining average spatial covariance matrices will be described later with reference to
A wideband multiple signal classification (MUSIC) unit 105 calculates a location of a signal source using the average spatial covariance matrices. A wideband minimum variance (MV) unit 106 calculates a weight matrix to be multiplied by each frequency component using the result of calculating the location of the signal source and compensates for distortion due to noise and an echo of a target signal using the calculated weight matrices. An inverse discrete Fourier transformer 107 restores the compensated N frequency components to the sound signal.
In the speech recognition module, a feature extractor 201 extracts a feature vector of a signal source from a digital sound signal received through the inverse discrete Fourier transformer 107. The extracted feature vector is input to a pattern comparator 202. The pattern comparator 202 compares the extracted feature vector with patterns stored in a reference pattern storage unit to search for a sound similar to the input sound signal. The pattern comparator 202 searches for a pattern with a highest match score, i.e., a highest correlation, and transmits the correlation, i.e., the match score, to a determiner 204. The determiner 204 determines sound information corresponding to the searched pattern as being recognized when the match score exceeds a predetermined value.
The concept of spatial smoothing (SS) will be described with reference to
Here, an i-th sub-array input vector is given as Equation (9).
x(i)(t)=BD(i−1)s(t)+n(i)(t) (9)
Here, D(i−1) is given as Equation (10).
D(i−1)=diag(e−jω
Here, τ(θd) indicates a time delay between microphones with respect to a d-th signal source.
In addition, B is a directional matrix comprising L-dimensional sub-array directional vectors reduced from M-dimensional directional vectors of the entire equal-interval linear array and is given as Equation (11).
B=[ã(θ1)ã(θ2) . . . ã(θD)] (11)
Here, ã(θ1) is given as Equation (12).
A calculation of obtaining spatial covariance matrices for the respective “p” sub-arrays and averaging the spatial covariance matrices is expressed as Equation (13), where “H” designates a conjugate transpose.
Here,
When p≧D, a rank of
Wideband SS according to the invention will be described with reference to
A calculation of obtaining spatial covariance matrices for the respective “p” sub-arrays of microphones and averaging the spatial covariance matrices is expressed as Equation (16).
Estimation of an incidence angle of a target signal source and beamforming can be performed using
Based on the results shown in Table 1, 6 was chosen as the optimal number of microphones in each sub-array.
Table 2 shows average speech recognition ratios obtained when the experiments were performed in various noises environments to compare the invention with conventional technology.
While the performance of an entire system depends on the performance of a speech signal detector in conventional technology, stable performance is guaranteed regardless of existence or non-existence of a target signal by using SS in the invention. Meanwhile, the wideband MUSIC unit 105 shown in
As described above, a MUSIC algorithm performed by the wideband MUSIC unit 105 is typically applied to all frequency bins, thereby causing a speech recognition system using the MUSIC algorithm to be overloaded in calculation. To overcome this problem, a frequency bin selector 1110 is added to a signal distortion compensation module, as shown in
As shown in
In the embodiment of the present invention, since 16 channels are defined, the VAD 1320 outputs 16 SPPs for the respective 16 channels. Thereafter, a channel selector 1330 lines up the 16 SPPs and selects K channels having highest SPPs and transmits the K channels to a channel-bin converter 1340. The channel-bin converter 1340 converts the K channels into frequency bins. The covariance selector 1210, included in the wideband MUSIC unit 105 shown in
For example, let's assume that 5th and 10th channels shown in
Since channels include different numbers of frequency bins as shown in
Referring to
Meanwhile, it is necessary to select (L-M) frequency bins from the K-th channel including the L-th frequency bin. The (L-M) frequency bins may be selected in descending order of power. More specifically, a second channel-bin converter 1640 converts the K-th channel into frequency bins. Then, a remaining bin selector 1650 selects (L-M) frequency bins in descending order of power from among the converted frequency bins so that the covariance selector 1210 included in the wideband MUSIC unit 105 additionally selects the converted (L-M) frequency bins and performs the MUSIC algorithm thereon. Here, a power measurer 1660 measures power of signals input to the VAD 1320 with respect to each frequency bin and transmits measurement results to the remaining bin selector 1650 so that the remaining bin selector 1650 can select the (L-M) frequency bins in descending order of power.
In the experimental environment shown in
When the results of experiments (1) through (3) are analyzed, an entire amount of computation decreases by approximately 66% in the invention. This average decreasing ratio is almost the same as a ratio at which the number of frequency bins subjected to the MUSIC algorithm decreases. As the amount of computation decreases, a success ratio in detecting a direction of the speech speaker 1710 may also decrease. This is shown in Table 9. However, it can be seen from Table 9 that a decrease in the success ratio is minimal.
According to the present invention, since removal of a wideband target signal is reduced in a location, for example, in an indoor environment, where an echo occurs, the target signal can be optimally acquired. A speech recognition system of the present invention uses a microphone array system that reduces the removal of the target signal, thereby achieving a high speech recognition ratio. In addition, since the amount of computation required for a wideband MUSIC algorithm is decreased, performance of the microphone array system can be increased.
Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
Number | Date | Country | Kind |
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10-2003-0028340 | May 2003 | KR | national |
10-2004-0013029 | Feb 2004 | KR | national |
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20040220800 A1 | Nov 2004 | US |