BLIND SPARSE SOURCE SEPARATION FOR UNKNOWN NUMBER OF SOURCES
USING GAUSSIAN MIXTURE MODEL FITTING WITH DIRICHLET PRIOR
Shoko Araki, Tomohiro Nakatani, Hiroshi Sawada, Shoji Makino
NTT Communication Science Laboratories, NTT Corporation
2-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan
ABSTRACTIn this paper, we propose a novel sparse source separation methodthat can be applied even if the number of sources is unknown. Recently, many sparse source separation approaches with time-frequency
masks have been proposed. However, most of these approaches require information on the number of sources in advance. In our proposed method, we model the histogram of the estimated direction ofarrival (DOA) with a Gaussian mixture model (GMM) with a Dirichlet prior. Then we estimate the model parameters by using the maximum a posteriori estimation based on the EM algorithm. In order
to avoid one cluster being modeled by two or more Gaussians, we
utilize a sparse distribution modeled by the Dirichlet distributions as
the prior of the GMM mixture weight. By using this prior, without any specific model selection process, our proposed method canestimate the number of sources and time-frequency masks simultaneously. Experimental results show the performance of our proposed
method.
Index Terms— Blind source separation, Dirichlet distribution,prior, number of sources, sparse
1. INTRODUCTION
Blind source separation (BSS) is an approach for estimating source
signals that uses only the mixed signal information observed at eachsensor. The BSS technique for speech dealt with in this paper has
many applications, including hands-free teleconference systems andpreprocessing for an automatic speech recognizer.
Let us formulate the task. Suppose that Ns ≥ 2 speech sources
s1 , . . . , sNs are convolutively mixed and observed at Nm sensors,
P s P
(1)
xj (t) = N
i=1
l hji (l) si (t − l), j = 1, . . . , Nm ,In this paper, we propose a novel sparse source separation methodthat can estimate the number of sources and time-frequency masks
simultaneously. We model the histogram of the DOA with a Gaussian mixture model (GMM) with a Dirichlet prior [6], and we estimate the model parameters by using the maximum a posteriori estimation based on the EM algorithm. Here, it is expected that anindividual cluster in the histogram corresponds to one Gaussian. In
order to avoid one cluster being modeled by two or more Gaussians,
thus making it possible to estimate the number of sources correctly,
we propose utilizing a sparse distribution modeled by the Dirichlet
distribution as the prior of the GMM mixture weight. The authorsof [7, 8] also modeled the histogram with a GMM and derived the
EM algorithm. However, they still needed to know the number ofsources Ns in advance. On the other hand, our proposed algorithm
in this paper does not need information on the source number, thanks
to the weight prior. Moreover, our method can estimate the source
number without any specific model selection process [6], which isusually computationally expensive.
The experimental results show that our proposed method canestimate the number of sources from sensor observations and canseparate signals by time-frequency masks obtained by the posterior
probability for each cluster.

where hji (l) represents the impulse response from source i to sensor
j. Our goal is to obtain estimates yi of each source signal si fromthe sensor observations xj without information about the number ofsources Ns , the speech sources si or the mixing process hji .
Two approaches have been widely studied and employed to solve
the BSS problem: one is based on independent component analysis(ICA) (e.g., [1]) and the other relies on the sparseness of source signals (e.g., [2]). In this paper we focus on the latter approach, more
specifically, the time-frequency mask approach [2–4].
With the time-frequency mask approach, we assume that signals are sufficiently sparse and, therefore, that at most one source
is dominant at each time-frequency slot. If these assumptions hold,a histogram of the phase differences between sensor observations(or direction of arrival (DOA) estimated from the phase differences)
has Ns clusters. Because an individual cluster in the histogram corresponds to an individual source, we can separate each signal bycollecting the observation signal at time-frequency points in eachcluster. This is the time-frequency mask approach.

978-1-4244-2354-5/09/$25.00 ©2009 IEEE

In the previous work [2–4], to automatically estimate each cluster, the number of sources Ns is assumed to be known. For example,
in our previous work [4], we employed the k-means algorithm forthe clustering by assuming Ns is given. However, in real situations(e.g., a meeting situation [5]) we usually cannot obtain informationon the number of sources Ns in advance. Moreover, especially for
an underdetermined case (Ns > Nm ), the source number estimation
is difficult, and few papers have dealt with this problem.

2. MIXING AND SEPARATION PROCESSES
This paper employs a time-frequency domain approach. With an F point short-time Fourier transform (STFT), (1) is converted into:P s(2)
xj (f, τ ) = N
i=1 hji (f )si (f, τ ),where hji (f ) is the frequency response from source i to sensor j,si (f, τ ) is the STFT of a source si . f ∈ {0, F1 fs , . . . , FF−1 fs } isa frequency (fs is the sampling frequency) and τ (= 0, · · · , T − 1)
is a time-frame index.
In this paper, we assume the sparseness of the sources [2]:xj (f, τ ) ≈ hji (f )si (f, τ ),(3)

where si (f, τ ) is a dominant source at the time-frequency slot (f, τ ).
This is approximately true for speech signals in the time-frequency
domain (see [2, 4] and their references).

33

ICASSP 2009

2.1. Separation method

0.08

(a)

0.06

First, by assuming the source sparseness, we estimate the DOA value
at each time-frequency slot by using the time difference of arrival(TDOA) between sensors [9]:32cos ψ(f, τ ) cos ϕ(f, τ )4 sin ψ(f, τ ) cos ϕ(f, τ ) 5 = vD+ q(f, τ ).
(4)
sin ϕ(f, τ )

0.04
0.02
0
-150

50

100

150

0
-150

-100

-50

0

50

100

150

0.08

(c)

0.06
0.04
0.02
0
-150

-100

-50

0

50

100

150

DOA [deg.]

Fig. 1. An example GMM fitting result without prior (φ = 1.0)
when Ns = 4. (a) DOA histogram, (b) each Gaussian of GMM,(c) GMM plot. Two Gaussians are fit to the cluster around −115
degrees. Their mean values are -116.01 and -116.07 degrees, andtheir mixture weights are 0.13 and 0.15.

(5)

3.1. Problem
In this subsection, we explain a typical problem that occurs when weapply a GMM fitting method to a DOA histogram. Figure 1 shows
an example. Here we have four sources, and thus four clusters in the
DOA histogram (Fig. 1(a)). Figure 1(c) shows the fitting result of
GMM of eight Gaussians, and Fig. 1(b) plots each Gaussian. From
Fig. 1(b), we can see that two Gaussians are fit to the cluster around−115 degrees. However, we expect just one Gaussian for this peak.
In this paper, in order to avoid the case where one cluster is modeledby two or more Gaussians, we propose utilizing a sparse distribution
for the prior of the GMM mixture weight parameter.
3.2. Probabilistic model
If we observe one source from one direction, the DOA d(f, τ ) =ψ(f, τ ) at time-frequency slot (f, τ ) is an observation, and the powera(f, τ ) = |x(f, τ )|2 is considered as a weight. Moreover, we assume that the DOA observation follows a Gaussian distribution, whosemean value gives us the DOA of the source. Now, let n = τ F + f ,where F is the number of frequency bins, and let the observations be
the DOA values d = {d1 , d2 , · · · , dn , · · · , dN } and power values
a = {a1 , a2 , · · · , an , · · · , aN }. Here, N is the number of observations, i.e., when we have T time frames and F frequency bins,
N = T × F.
In our observed mixture, we assume that there are a sufficientnumber of source signals from different directions, where some aredominant and the others are much less dominant. We also assume
that observed data dn follows a Gaussian mixture model (GMM),
αm N (dn ; μm , σm ),0(b)

3. PROPOSED METHOD

M
X

-50

0.02

where ψ and ϕ are the source azimuth and elevation, respectively, v
is the sound velocity, D = [· · · , dj − dj  , · · · ]T is the microphone
coordinate where dj is the three-dimensional vector representing thelocation of the sensor j, q(f, τ ) is a TDOA vector consisting of the1
qjj  (f, τ ) = 2πfarg[xj (f, τ ) x∗j  (f, τ )] of all microphone pairs+j-j , and denotes the Moore-Penrose pseudo-inverse [9]. In thispaper, we utilize just the azimuth ψ(f, τ ) information as DOA. We
assume that the microphone spacing is sufficiently small to avoid thespatial aliasing problem.
Then we classify the estimated DOA values in some way. For
example, the k-means clustering is utilized in [4], and a GMM fittingwith the EM algorithm is employed in [7] and in this paper. Eachestimated cluster corresponds to an individual source.
Finally, we estimate the separated signals yi (f, τ ) with timefrequency masks Mi (f, τ ), which extract time-frequency points ofmembers in the i-th cluster:yi (f, τ ) = x1 (f, τ )Mi (f, τ ).

-100

0.04

(6)

m=1

where one Gaussian is assigned to one direction (= one source). We
prepare a sufficient number M of Gaussians for our GMM model34

2
and weight αm for eachand estimate the mean μm , variance σmGaussian m.
In this paper, in order to deal with the distribution of DOA values, we utilize a modified Gaussian distribution. When the DOA
value is observed, it is folded into a range between −π and π. However, if the mean of the given data is close to ±π, the distribution
wraps and becomes bimodal [10]. In order to handle such a distribution, we model the DOA data with a wrapped phase model
PM [10].
More
specifically,
instead
of
(6),
as
the
GMM
we
use
m=1 αmP∞k=−∞ N (dn + 2πk; μm , σm ), where −π ≤ dn < π and k is an
integer that is uniquely determined from the observed DOA.
In order to solve the problem mentioned in Section 3.1, that is,in order to model the observed DOA data by allocating one Gaussian
to each source, we assume the sparseness of the source directions.
For this purpose, as the prior of the mixture weight, we employ the
Dirichlet distribution:
M
1 Y φ−1p(α) =αm ,(7)B(φ) m=1
P
where α = {α1 , · · · , αm , · · · , αM }, M
m=1 αm = 1, 0 ≤ αm ≤1, and B(φ) is the beta distribution (regularization term). When we
make the hyper parameter φ small (φ < 1), the prior takes a larger
value as the number of mixture weights whose values are close tozero increases, which is desirable for representing the sparseness ofthe source direction [6]. In addition, the Dirichlet distribution is
known to be a conjugate prior of the mixture weight [6], and it canbe incorporated into the GMM fitting approach in a computationallyefficient manner.

3.3. Cost function based on GMM
Let θ = {αm , μm , σm , · · · } be a model parameter set. The observations are the DOA values d = {d1 , d2 , · · · , dn , · · · , dN } andpower values a = {a1 , a2 , · · · , an , · · · , aN } (see Sec. 3.2). In thefollowing, Gaussian indices m and k of the wrapped phase model
are assumed not to be observed, and therefore dealt with as hidden
variables.
The cost function of the maximum a posteriori estimation is defined based on a log of a joint probability density function as

L(θ)

=

Table 1. Example estimated parameters θ for (a) 40 and (b) 100 iterations for an Ns = 4 case (see Sec. 4 for experimental conditions).

log p(d, θ) = log p(d|θ) + log p(α) + const. (8)NX=

f (an ) log p(dn |θ) + log p(α) + const.

n=1
N
X

=

M
∞
X
X

f (an ) log

n=1(a) 40 iterations:
m
1
2
3
4
5
6
7
8
μm 64.2 63.8 79.8 161.2 252.9 241.1 352.5 326.4
σm 4.3 4.5 46.9 4.7 59.8 4.1
2.6
5.5
αm 0.00 0.33 0.13 0.15 0.12 0.15 0.00 0.17(b) 100 iterations:
m
1
2
3
4
5
6
7
8
μm 6.8 61.9 8.1 158.3 7.5 242.4 278.8 325.9
σm 2.5 14.6 2.0 16.1 2.2 12.7 27.4 10.8
αm 0.00 0.44 0.00 0.23 0.00 0.22 0.00 0.21

(9)!
p(m, k, dn |θ)

m=1 k=−∞+ log p(α) + const.,

(10)

where

«
„
−(dn + 2πk − μm )2
αm
√
(11)
,
exp
2
2
2σm
2πσm
and −π ≤ dn < π. We disregarded the priors of the model parameters except for α in (8), andp(m, k, dn |θ)=f (an ) = can /

N
X

an .

3.5. Source number estimation
Table 1 shows example estimated parameters θ after (a) 40 and (b)100 iterations when the number of sources Ns = 4. After a sufficient number of iterations (Table 1 (b)), four mixture weights αmbecome 0.00 and four αm indicate meaningful values. That is, thesource number Ns = 4 is estimated by thresholding the mixture
weight αm . However, such a large number of iterations requires
heavy computational time. On the other hand, when we use fewer
iterations (Table 1 (a)), six mixture weights αm still have significant values. However, we can see that two of them have very large
variance σm . Obviously, such Gaussians with large variance do not
represent one cluster.
Therefore, to save computational time, we determine the numberof sources Ns by counting the number of Gaussians whose parameters meet conditions αm ≥ and σm ≤ th, where is a sufficiently small threshod value and th is an appropriate threshold value( = 10−10 and th = 20 degrees are used in this paper).

(12)

n=1

gives a power weight and controls the importance of the observation
relative to the prior term (2nd term of (10)), where c is a control
parameter.
In (10), p(α) follows the Dirichlet distribution (7). As mentioned in Section 3.2, for the sparse representation of the GMM,
φ < 1 is preferred for the Dirichlet distribution (7). Note that φ = 1
is equivalent to the case without a prior for the mixture weight.
3.4. EM algorithm
Here we derive an algorithm for estimating parameter θ by the EM
algorithm.
The auxiliary function Q is given as˜ˆ(13)Q(θ|θt ) = E log p(d, θ)|dn ; θt
XXXˆ
˜
t
=
p(m, k|dn , θ )f (an ) log p(m, k, dn |θ)nm

3.6. Source Separation
Let xn and ynm be the observation vector at sensor 1 and the estimated m-th separated signal, respectively. The time-frequency mask
for the m-th separated source is obtained by marginalizing the estimated pdf (15) with respect to k,∞Xp(m, k|dn , θ).
(19)p(m|dn , θ) =k

+ log(α),(14)

where θt is the estimate of the parameters after the t-th iteration, andp(m, k, dn |θt ).
p(m, k|dn , θt ) = P Ptm
k p(m, k, dn |θ )
t

(15)

k=−∞

t

)
)
= 0 and ∂Q(θ|θ= 0, we obtain
By setting ∂Q(θ|θ2
For implementation, we summed up −1 ≤ k ≤ 1. Now, the sepa∂μm∂σm
P P
rated signal is obtained byt
k|dn , θ )f (an )(dn + 2πk)n Pk p(m,
P
μt+1
=
(16)
m
(20)
ynm = xn p(m|dn , θ),tnk p(m, k|dn , θ )f (an )P Por, with the Section 2.1 notation,k|dn , θt )f (an )(dn + 2πk)2
2 t+1
2
n
k p(m,PP(σm)=− (μt+1m ) .
ym (f, τ ) = x1 (f, τ )p(m|ψ(f, τ ), θ),(21)tnk p(m, k|dn , θ )f (an )
where
n
=
τ
F
+
f
.
(17)
P
Moreover, by using the Lagrange multiplier method, M
α
=
1
m
m
4. EXPERIMENTS
and (12), the mixture weight is obtained as follows:) 4.1. Experimental setup
(
XX
1
t+1
=
p(m, k|dn , θ t )f (an ) + (φ − 1) We performed experiments with measured impulse responses h in
αmji
c + M (φ − 1)nka room, whose reverberation time was 130 ms (see Fig. 4 of [4]).
(18) We utilized three microphones arranged at the apexes of an equilatSince αm ≥ 0, c > M (1 − φ) must hold from (18).
eral triangle, 4 cm on a side. Mixtures were made by convolving
the measured room impulse responses and 5-second English speech
In the E-step we calculate (15), then in the M-step the parameters
signals. The sampling rate was 8 kHz. The frame size F for STFT
θ are calculated by using (16), (17) and (18). Sometimes αm < 0
was 512 (64 ms), and the frame shift was 128 (16 ms).
occurs. In such a case, we can factor out the corresponding Gaussian
In the EM algorithm, we utilized M = 8 Gaussians, whose(by setting αm = , where is a very small value) and re-caluculate
initial values were μm =[25, 75, 115, 160, 205, 250, 295, 340] in dethe parameters.

35

0.08
0.06

Table 2. Experimental results. φ = 0.9: with prior; φ = 1.0:
without prior, K: with the k-means. InputSIR was 0.0 [dB] (Ns =2), −3.1 [dB] (Ns = 3), and −4.9 [dB] (Ns = 4).
Accuracy of N̂s estimation [%]Performance [dB]Ns φ
34
5 6 7 8 OutputSIR SDR
N̂s :1 2
2 0.9
100
19.6
11.3
1.0
0
10 25 35 20 10
11.6
4.2
K
given
15.9
14.5
3 0.9
5
95
16.0
8.9
1.0
35 15 30 20
14.5
7.5
K
given
12.2
11.4
4 0.9
100
13.7
7.8
1.0
20 75 5
12.8
6.7
K
given
10.7
9.7

(a)

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0.04

(b)
0.02

0
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0.08

(c)

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DOA [deg.]

From Table 2, we can see that with the prior (φ = 0.9) the
number of sources is almost perfectly estimated. On the other hand,
without the prior, the number of sources is overestimated, and the accuracy rate was quite low. This overestimation tendency is stronger
for a small number of sources Ns .
As for the separation performance, we obtained better performance by using the prior (φ = 0.9) than without the prior (φ = 1.0).

Fig. 2. GMM fitting result with prior (φ = 0.9) for the same example
as Fig. 1. (a) DOA histogram, (b) each Gaussian of GMM, (c) GMM
plot. We have just four (= Ns ) dominant Gaussians.
gree, σm = 40 in degree, and αm = 1/M = 0.125 for all m. For
the wrapped phase model, we summed up −1 ≤ k ≤ 1. The number of iterations was 40. As the hyper parameter for (7), we utilized
φ = 0.9 for our proposed method and φ = 1.0 for a conventional
EM algorithm that corresponds to the case without any prior for the
mixture weights. The control parameter c for (12) was 5.
We evaluated the signal-to-interference ratio (SIR) as a separation performance measure, and the signal-to-distortion ratio (SDR)
as a sound quality measure:P2
t |xJi (t)|InputSIRi = 10 log10 P P
[dB],
(22)
2
t|
k=i xJk (t)|P2
t |yii (t)|OutputSIRi = 10 log10 P P
[dB],
(23)
2
t|
k=i yik (t)|P2
t |xJi (t)|SDRi = 10 log10 P
[dB],
(24)
2
t |xJi (t) − βyii (t − Δ)|

5. CONCLUSION
In this paper, we proposed a sparse source separation method thatcan be applied even if the number of sources is unknown. We modeled the DOA histogram with a GMM. We proposed employing theDirichlet distribution as the prior of the GMM mixture weight toavoid the case where one cluster is modeled by two or more Gaussians. Our experimental results show that the proposed method canestimate the number of sources correctly without using any specific model selection process. We also confirmed that the proposed
method gives good separation performance.
6. REFERENCES[1] A. Hyvärinen, J. Karhunen, and E. Oja, Independent Component Analysis, John Wiley & Sons, 2001.
[2] O. Yilmaz and S. Rickard, “Blind separation of speech mixtures via
time-frequency masking,” IEEE Trans. Signal Processing, vol. 52, no.
7, pp. 1830–1847, July 2004.
[3] N. Roman and D. Wang, “Binaural sound segregation for multisource
reverberant environments,” in Proc. ICASSP 2004, May 2004, vol. II,
pp. 373–376.
[4] S. Araki, H. Sawada, R. Mukai, and S. Makino, “Underdeterminedblind sparse source separation for arbitrarily arranged multiple sensors,” Signal Processing, vol. 77, no. 8, pp. 1833–1847, Aug. 2007.
[5] S. Araki, M. Fujimoto, K. Ishizuka, H. Sawada, and S. Makino,“Speaker indexing and speech enhancement in real meeting / conversations,” in Proc. ICASSP’08, Apr. 2008, vol. I, pp. 93–96.
[6] C. M. Bishop, Pattern recognition and machine learning, Springer,
2008.
[7] M. Mandel, D. Ellis, and T. Jebara, “An EM algorithm for localizing
multiple sound sources in reverberant environments,” in Proc. Neural
Info. Proc. Sys. , 2006.
[8] P. O’Grady and B.Pearlmutter, “Soft-LOST: EM on a mixture of oriented lines,” in Proc. ICA 2004 (LNCS 3195), Sept. 2004, pp. 430–436.
[9] S. Araki, H. Sawada, R. Mukai, and S. Makino, “DOA estimation for
multiple sparse sources with normalized observation vector clustering,”
in Proc. ICASSP’06, May 2006, vol. 5, pp. 33–36.
[10] P. Smaragdis and P. Boufounos, “Learning source trajectories using
wrapped-phase hidden Markov models,” in Proc. of WASPAA’05, Oct.
2005, pp. 114–117.

where yik (t) is the sk component that appears at output yi (t):PNs
Pyi (t) =k=1 yik (t), xJi (t) =l hJi (l) si (t − l), and β and
Δ are parameters used to compensate for the amplitude and phasedifference, respectively, between xJi and yii .
We calculated SIR and SDR values for the separated sourcesthat are counted as the sources by the method in Section 3.5. We
tested 20 trials with different speech source combinations and location combinations, and then averaged the results.
4.2. Results
Figure 2 shows a GMM fitting result with prior (φ = 0.9) for thesame DOA distribution as that of Fig. 1. Although we had two Gaussians around -115 degrees in Fig. 1, we can see that just four (= Ns )Gaussians are dominant in this case. That is, using the prior ((7) with
φ = 0.9), more correct GMM fitting was performed.
Table 2 reports the experimental results. In the table, φ = 0.9
means the results with sparse prior (7) and φ = 1.0 indicates theresults without a prior. The percentage values are shown where themethod estimates the number of sources as N̂s within 20 trials. The
average separation performance results, SIR and SDR in dB, are also
reported. The results with the k-means method [4], where the number of sources was given, are also shown.

36