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Received: 18 November 2020

Accepted: 21 December 2020

CAAI Transactions on Intelligence Technology
DOI: 10.1049/cit2.12024
ORIGINAL RESEARCH
Enhancing direct‐path relative transfer function using deep
neural network for robust sound source localization
Bing Yang
1,2
Runwei Ding
1
Yutong Ban
3,4
Xiaofei Li
2
Hong Liu
1
1
Key Laboratory of Machine Perception, Shenzhen
Graduate School, Peking University, Beijing, China
2
Westlake University & Westlake Institute for
Advanced Study, Hangzhou, China
3
SAIIL, Massachusetts General Hospital, Boston,
Massachusetts, USA
4
CSAIL, Massachusetts Institute of Technology
(MIT), Cambridge, Massachusetts, USA
Correspondence
Runwei Ding, Shenzhen Graduate School, Peking
University, China.
Email: dingrunwei@pku.edu.cn
Funding information
National Natural Science Foundation of China,
Grant/Award Numbers: No.61673030, U1613209;
National Natural Science Foundation of Shenzhen,
Grant/Award Number: No.
JCYJ20200109140410340
Abstract
This article proposes a deep neural network (DNN)‐based direct‐path relative transfer
function (DP‐RTF) enhancement method for robust direction of arrival (DOA) estimation
in noisy and reverberant environments. The DP‐RTF refers to the ratio between the direct‐
path acoustic transfer functions of the two microphone channels. First, the complex‐value
DP‐RTF is decomposed into the inter‐channel intensity difference, and sinusoidal func
tions of the inter‐channel phase difference in the time‐frequency domain. Then, the
decomposed DP‐RTF features from a series of temporal context frames are utilized to train
a DNN model, which maps the DP‐RTF features contaminated by noise and reverberation
to the clean ones, and meanwhile provides a time‐frequency (TF) weight to indicate the
reliability of the mapping. The DP‐RTF enhancement network can help to enhance the
DP‐RTF against noise and reverberation. Finally, the DOA of a sound source can be
estimated by integrating the weighted matching between the enhanced DP‐RTF features
and the DP‐RTF templates. Experimental results on simulated data show the superiority of
the proposed DP‐RTF enhancement network for estimating the DOA of the sound source
in the environments with various levels of noise and reverberation.
1

INTRODUCTION
Sound source localization has a wide range of applications such
as teleconferencing, robot audition, hearing aids, and so forth.
With the development of deep learning techniques, lots of
data‐driven sound source localization works are built in a su
pervised manner [1]. According to the role of the deep learning
model plays, these methods are classied into four categories,
namely signal‐to‐location [2], feature‐to‐location [3, 4], spatial
spectrum‐to‐location [5], and feature‐to‐feature [6, 7]‐based
methods. Among these methods, the feature‐to‐feature‐based
method is simple and effective for improving the performance
of sound source localization in noisy and reverberant envi
ronments, as it is the data driven and the extracted features can
adapt to various acoustic conditions.
The spatial features utilized for localization include the time
and the intensity differences between dual‐microphone signals.
Inter‐channel time difference (ITD) is commonly estimated by
searching the maximum of the generalized cross‐correlation
(GCC) function [8]. Inter‐channel phase difference (IPD) is
another time difference feature and owns an approximate linear
property with respect to frequency [9]. Moreover, inter‐channel
intensity difference (IID) is computed as the energy ratio of the
signals captured by two microphones. Relative transfer function
(RTF) [10, 11] encodes time and intensity information in its
argument and magnitude respectively, which is the ratio between
the acoustic transfer functions of the two channels. Other
high‐level localization features include the cross‐correlation
function (CCF) [3], the eigen vectors of spatial correlation matrix
associated with signal subspace [12], and so forth. Overall, the
sound source can be easily localized with the aforementioned
localization features under a noise‐free and anechoic condition.
However, in practical acoustic scenes, noise and reverberation
often contaminate the direct‐path propagated source signal and
degrade the accuracy of localization feature estimation, which
furtherleadstoa signicantdroponthe localization performance.
Many methods aim to remove the effect of acoustic in
terferences on the direct‐path localization feature extraction.
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© 2021 The Authors. CAAI Transactions on Intelligence Technology published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology and Chongqing
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446

CAAI Trans. Intell. Technol. 2022;7:446–454. wileyonlinelibrary.com/journal/cit2
Some research works give high attention values to the direct
sound dominant time‐frequency (TF) regions using a TF
weighting scheme, which can be classied into unsupervised
methods [13–16] and supervised methods [17, 18]. However,
these methods do not rene the value of localization features.
Though the IPD enhancement method [6] has been used to
ne‐tune the localization features using deep neural network
(DNN), it only considers the time difference information of
current time frame, but intensity difference and temporal
context information are also important for localization. We aim
to investigate how to make full use of the time and intensity
difference information of both the historical and current time
frames, in order to recover the clean localization features from
the existing contaminated ones, so that the sound source can
be robustly localized.
This article designs a direct‐path relative transfer function
(DP‐RTF) enhancement network to preserve the time and
intensity difference information of direct‐path signal and
suppress the contamination of noise and reverberation. The
complex DP‐RTF is decomposed as the IID and the sinu
soidal functions of the IPD, in order to t the real‐value
DNN framework and to explicitly present the localization
cues. Then the DP‐RTF is enhanced using a DNN which non‐
linearly maps the contaminated DP‐RTF feature of multiple
temporal context frames to one single‐frame clean feature.
The DP‐RTF enhancement network can jointly predict the
clean DP‐RTF and the TF reliability weight, by adopting a
weighted mean square error (MSE) with a weight‐maximization
regularization term. The trained DP‐RTF enhancement
network can signicantly depress the effect of noise and
reverberation on the DP‐RTF estimation. For each TF bin,
the enhanced DP‐RTF are matched with the template of
candidate directions. The direction of arrival (DOA) of the
sound source is determined by integrating the matching
functions united with the predicted TF weight from multiple
TF bins. Experiments using simulated data demonstrate the
effectiveness of our method under noisy and reverberate
acoustic conditions. The main contributions of this paper are
summarized as follows:
(1)We design a DP‐RTFenhancement network to recover the
DP‐RTF features from the contaminated localization features for
robust sound source localization. Different from the method in
[6] which recovers the frame‐wise time difference information
using current time frame, the proposed model considers the
short‐term temporal context information, and jointly recovers
both time and intensity difference information.
(2) We use a weighted MSE with a weight‐maximization
regularization term to guarantee that the TF‐wise DP‐RTF
features are selectively recovered. Using the predicted TF
weight, the enhancement network is more concentrated on the
features with high weights, therefore high‐weights’ features will
dominate the DOA estimation.
The rest of this paper is organized as follows. Section 2
introduces the DP‐RTF based DOA estimation method.
Section 3details the proposed DNN based DP‐RTF
enhancement network. Experiments and discussions with
simulated data are presented in Section 4, and conclusions
are drawn in Section 5.
2

DP‐RTF BASED DOA ESTIMATION
In an enclosed environment with additive ambient noise, a
single source is observed by a pair of microphones. The signal
received by the mth microphone is formulated as
xmðtÞ ¼ hmðtÞsðtÞ þ vmðtÞ;ð1Þ
where m∈[1, 2] is the microphone index, s(t) denotes the
source signal, v
m
(t) denotes the received noise signal at the
mth microphone, and h
m
(t) is the acoustic impulse response
(AIR) from the source to the mth microphone. Here, ∗de
notes the convolution operation. Applying the short‐time
Fourier transform (STFT) to Equation (1), the signal at the
mth microphone can be rewritten in the TF domain as
Xmðn;fÞ ¼ Hmðf;θÞSðn;fÞ þ Vmðn;fÞ;ð2Þ
where n∈[1, N] denotes the index of time frame, f∈[1, F]
denotes the index of frequency, and θdenotes the horizontal
DOA of source. X
m
(n,f), S(n,f) and V
m
(n,f) represent the
STFT coefcients of x
m
(t), s(t), and v
m
(t), respectively. The
acoustic transfer function (ATF) H
m
(f,θ) is the Fourier
transform of h
m
(t). The ATF contains the direct and reected
propagation paths of sound source, that is,
Hmðf;θÞ ¼ Hd
mðf;θÞ þ Hr
mðf;θÞ;ð3Þ
where Hd
mðf;θÞand Hr
mðf;θÞdenote the ATFs of direct‐path
and reected propagations, respectively. The DP‐RTF [19] is
dened as the ratio between the two direct‐path ATFs, namely
Rdðf;θÞ ¼ Hd
2ðf;θÞ
Hd
1ðf;θÞ:ð4Þ
Under the anechoic and noise‐free condition, Hr
mðf;θÞ
and V
m
(n,f) are equal to zero. According to Equations (2) and
(3), the microphone signal is simplied as
Xmðn;fÞ ¼ Hd
mðf;θÞSðn;fÞ:ð5Þ
Using this simplication, the DP‐RTF can be estimated by
b
Rdðn;fÞ ¼ X2ðn;fÞ
X1ðn;fÞ:ð6Þ
The estimated DP‐RTF is decomposed and rewritten in a
vector form, namely
YANG ET AL.

447
brðn;fÞ
¼
20 log10 b
Rdðn;fÞ
ΔImax
;sin∠b
Rdðn;fÞ;cos∠b
Rdðn;fÞ
2
6
43
7
5
T
:
ð7Þ
The DOA of the sound source is estimated by matching
the estimated DP‐RTF vectors from reliable TF bins with the
template, namely
b
θ¼argmin
θ∈SX
N
n¼1X
F
f¼1b
wðn;fÞbrðn;fÞ−rðf;θÞð Þk k2;ð8Þ
where ‖⋅‖ denotes the Euclidean norm, Sis the set of
candidate directions, and b
wðn;fÞdenotes the TF weight that
indicates the reliability of brðn;fÞ. Here, the ground‐truth DP‐
RTF vector r(f,θ) is dened in Equation (13) (see Section 3.1),
and r(f,θ) of all candidate directions are used as the matching
template.
3

DP‐RTF ENHANCEMENT
NETWORK
The above‐mentioned framework provides a solution to the
single sound source localization on the anechoic and noise‐free
assumption (see Figure 1a). However, noise and reverberation
are inevitable in real‐world scenarios. The DP‐RTF estimation
using Equation (6) is a biased approximation under noisy and
reverberant conditions, which can introduce an obvious devi
ation of DOA estimation. Hence, we add a DP‐RTF
enhancement network to the above‐mentioned framework, in
order to recover the clean DP‐RTF feature from the
contaminated ones to suit the reverberant and noisy environ
ments (see Figure 1b).
3.1

Input and target
As the complex DP‐RTF estimates cannot be directly pro
cessed by the real‐value DNN, to t the real‐value DNN, the
input and target complex DP‐RTF values are transformed into
a real‐value form.
The theoretical direct‐path ATF, namely the head‐related
transfer function (HRTF) in the binaural localization case, is
formulated as
Hd
mðf;θÞ ¼ αmðf;θÞe−jωfτmðθÞ;ð9Þ
where ω
f
is the angular frequency of the fth frequency. α
m
(f)
and τ
m(θ)
denote the propagation attenuation factor and the
time of arrival from the source to the mth microphone,
respectively. Substituting Equation (9) into Equation (4), the
DP‐RTF is rewritten as
Rdðf;θÞ ¼ α2ðf;θÞ
α1ðf;θÞe−jωfτ2ðθÞ−τ1ðθÞð Þ:ð10Þ
It can be seen that the DP‐RTF encodes IID and IPD infor
mation in its magnitude and argument respectively. We extract
these two localization cues using a disjoint decomposition,
formally written as
ΔIðf;θÞ ¼ 20 log10 Rdðf;θÞ
;ð11Þ
ΔPðf;θÞ ¼ ∠Rdðf;θÞ;ð12Þ
where ∠is the phase operator of complex numbers. With these
two real‐value localization cues, the complex DP‐RTF value
can be recovered.
However, IPD is presented in the range of [−π,π], and
may be periodically wrapped with the increasing of frequency
or time difference. Hence, the mean squared IPD error cannot
be directly used to reect the DOA difference, and the DOA
estimation using such IPD will fail as well due to the phase
wrapping ambiguity. To avoid this ambiguity, the sinusoidal
FIGURE 1 Pipeline for the DP‐RTF based DOA estimation method. (a) Without DP‐RTF enhancement network. (b) With DP‐RTF enhancement network
448

YANG ET AL.
functions of IPD is used instead. Accordingly, the complex
DP‐RTF is decomposed into IID, the sine and cosine func
tions of IPD. The three decomposed parts are concatenated to
form the DP‐RTF vector ground truth associated with the
direction θ, that is,
rðf;θÞ ¼ ΔIðf;θÞ
ΔImax
;sin ΔPðf;θÞ;cos ΔPðf;θÞ
T
;ð13Þ
where (⋅)
T
denotes vector transpose, and ΔI
max
is an empiri
cally set maximum value of IID used for normalization. As the
original IID has a relatively wider range than the sinusoidal
functions, the IID is scaled to [−1,1] to balance the contri
bution of the IID and IPD information. The dimension of r
(f,θ) is 3 1, and each element is in the range from −1 to 1.
By using this transformation, we didn't lose any information of
the sound source location. Therefore, the DP‐RTF feature can
be recovered according to the IID and IPD information
contained in RTF vector.
Accordingly, for frame n, the input vector is a concatena
tion of the contaminated DP‐RTF vectors from a series of
(context) time frames and frequency bands, namely
CIðnÞ¼ ½brðn−Cþ1;1ÞT;…;brðn−Cþ1;FÞT;
…
brðn;1ÞT;…;brðn;FÞTiT;ð14Þ
and the learning target vector is a concatenation of the clean
DP‐RTF vectors from multiple frequency bands, namely
CRTF
TðnÞ ¼ ½ rð1;θÞT;…;rðF;θÞTT;ð15Þ
where Cdenotes the number of context time frames and F
refers to the number of utilized frequencies. The dimensions of
C
I
(n) and CRTF
TðnÞare 3CF 1 and 3F1, respectively. By
concatenating the DP‐RTF feature vectors of CF TF bins into
a long input feature vector, the acoustic context information
along the time and frequency axes can be captured. Full bands
are taken into account due to the mutual dependencies of
localization cues on different frequencies.
3.2

Network architecture
Considering the complex noise and reverberation generating and
mixing process, the mapping from contaminated DP‐RTFs to
clean DP‐RTFs is indeed a complicated non‐linear operation.
Since neural networks own many levels of non‐linearity, we
design a DNN model to approximate this highly non‐linear
relationship. The architecture of the designed DNN model for
DP‐RTF enhancement is illustrated in Figure 2. The DNN
model employs C‐frame contaminated DP‐RTF vector to pre
dict one‐frame clean DP‐RTF vector and TF reliability weight.
The input vector C
I
(n) is rst fed into three fully connected (FC)
layers to obtain the latent features. Each of the FC layers is with
2048 units and activated by a rectied linear unit (ReLU). Then
the latent features are pass to two FC layers, respectively.
Accordingly, the output contains two parts, one for DP‐RTF
vector and the other for TF weight, which are activated by a tan h
unit and a sigmoid unit, respectively. The output for DP‐RTF
vector is with a dimension of 3F1, namely
CRTF
OðnÞ ¼ ½ erðn;1ÞT;…;erðn;FÞTT:ð16Þ
The output for TF weight is with a dimension of F1, namely
CW
OðnÞ ¼ ½ e
wðn;1Þ;…;e
wðn;FÞ T;ð17Þ
where erðn;fÞand e
wðn;fÞare DP‐RTF vector and TF weight
predicted by DNN, respectively.
For network training, one commonly used loss function is
the MSE between the output and the target. This loss function
treats all frequencies equivalently, which is not suitable for the
present DP‐RTF enhancement problem. The TF bins where
source signal is silent or greatly contaminated by noise or
reverberation provide unreliable localization cues, and hence
should be disregarded in an indirect manner. To tackle this
problem, we add an adaptive weighting scheme to the loss
function, and thus the enhanced DP‐RTF vector and the TF
reliability weight can be jointly learned. The loss function is
then dened as
LðnÞ¼ 1
3FX
F
f¼1e
wðn;fÞerðn;fÞ−rðf;θÞð Þk k2þ
λ1
FX
F
f¼1
1−e
wðn;fÞj j;
ð18Þ
where r(f,θ) is the ground‐truth DP‐RTF vector which is free
of the affection of acoustic interferences, and hence it is uti
lized as the training target of erðf;θÞ. Here, λdenotes the
regularization factor. The regularization part is set to avoid
trivial solutions, that is zero weights, and to guarantee a
sufcient number of TF bins to be employed for training.
Using this weighted MSE, the DP‐RTF learning disregards
those TF bins that cannot provide accurate DP‐RTF estimates.
In the test stage, the trained DNN predicts a single‐frame
full‐band estimate of erðn;fÞand e
wðn;fÞ. The DOA of the
sound source is estimated using the enhanced DP‐RTFs and
TF weights of Ntime frames (and Ffrequency bands for each
frame), according to Equation (8).
4

EXPERIMENTS AND DISCUSSIONS
In this section, the performance of the proposed method is
measured. We rst give the details of the experimental setup,
and then show the experimental results and discussions.
YANG ET AL.

449
4.1

Experimental setup
4.1.1

Simulated data
We simulate different room congurations using the image
method [20] which is implemented by the Roomsim toolbox
[21]. Four acoustic congurations are generated for training and
three for test, as shown in Table 1. All experiments are carried
out with binaural microphones whose shadow effect is signi
cant. As illustrated in Figure 3, the speech sound source is
located in same horizontal plane as the binaural microphones,
and the candidate source directions ranges from −90° to 90°
with an interval of 5°. The binaural room impulse response
(BRIR) is generated using the Roomsim toolbox [21] and the
head‐related impulse response of the KEMAR dummy head
[22]. Speech recordings from TIMIT dataset
1
are truncated and
form speech segments with 0.5 s duration. These segments are
used as source signals, which are divided into training, validation
and test sets, respectively. White, Babble and Factory noise les
from the NOISEX‐92 database [24] are employed as noise
signals. Each type of noise signal segments is divided into
training, validation and test sets, respectively. Diffuse noise eld
[23] is generated using these noise segments. The sensor signals
are created by rst convolving the source signals with the
BRIRs, and then adding the scaled diffuse noise to the convo
luted signals according to a given signal‐to‐noise ratio (SNR).
4.1.2

Parameter settings and evaluation metrics
The sampling rate of the binaural signals used for localization is
16 kHz. The binaural signals are enframed by a window of 32 ms
with a frame shift of 16 ms. The frequency ranges from 0 to
4 kHz is used for localization (F=128). The maximum value of
IID ΔI
max
is set to 20. The DNN model is trained using the
Adam optimizer. The learning rate is set to 0.001. The accuracy
of DOA estimation is accessed by the mean absolute error
(MAE) and the localization accuracy. The MAE is dened as the
average error between the estimated and the ground‐truth
DOAs over different test instances. The localization accuracy
considers a prediction to be correct if the difference between the
DOA estimate and the true DOA is less than or equal to 5°.
4.2

Experimental result
4.2.1

Inuence of the IID information
To investigate the inuence of exploiting IID, we compare the
MAE of using only the enhanced IPD and using both
the enhanced IID and IPD under different sizes of rooms. The
experiments are carried in rooms with different levels of
reverberation and noise. The RT
60
is 0.2, 0.4, 0.6, 0.8 s with
SNR being set to 5 dB. The SNR is −5, 0, 10, 15 dB with RT
60
being set to 0.6 s. The experiment results present in Table 2are
an average of these acoustic conditions. It can be seen that the
IPD +IID method performs better than the IPD method in
Room 5 and Room 6 while slightly worse in Room 7. Overall,
with the IID information, the MAE of DOA estimation can be
reduced, which demonstrates the effectiveness of incorpo
rating the IID information in the present framework for
binaural sound source localization.
4.2.2

Inuence of the temporal context
information
We set the number of context time frames Cto different values
and give the MAE of DOA estimation under different sizes of
rooms in Table 3. The experiment data is the same as that used
in Section 4.2.1. It can be seen that the MAE of DOA esti
mation decreases when Cvaries from 1 to 7 and increases
when Cis larger than 7. Hence, Cis set to 7 in the following
experiments, as it is found to be the optimal value under
different acoustic conditions.
FIGURE 2 Architecture for the DP‐RTF enhancement network. The input is the DP‐RTF vector contaminated by noise and reverberation, and the output
is the enhanced DP‐RTF vector and the TF weight
1
https://catalog.ldc.upenn.edu/ldc93s1
450

YANG ET AL.
4.2.3

Inuence of the TF weighting scheme
The DOA estimation results without and with the TF
weighting scheme are present in Table 4. The experiment data
is the same as that used in Section 4.2.1. For the DOA esti
mation without TF weighting scheme, the TF weight b
wðn;fÞ
is set to one. For the loss with weighting scheme, the regula
rization factor λis set to 0.01 and 0.5, respectively. The result
show that compared with the DOA estimation without the TF
weighting scheme, using this weight achieves a smaller MAE,
which veries the effectiveness of the weighting scheme. Be
sides, with a larger regularization factor, the MAE of DOA
estimation become higher. This is because that a stronger
punishment of regularization term can result in that the TF
weight is closer to one, which will achieve a similar perfor
mance to that without TF weighting scheme. In the following
experiments, λis set to 0.01.
4.2.4

Robustness evaluation
To illustrate the effectiveness of the proposed DP‐RTF
enhancement network, we estimate the DP‐RTF without and
with the enhancement network, respectively, and plot the phase
and amplitude of the DP‐RTF estimate as a function of fre
quency bins in Figure 4. Each presented phase or amplitude
corresponds one DP‐RTF estimate in a certain TF bin. We use
31‐frame DP‐RTF estimates for evaluation. For the method
without the enhancement network, the TF weight is set to one.
It can be observed that the phase and amplitude of DP‐RTF
estimated without the enhancement network is scattered, while
that provided with the enhancement network is clustered
around the ground‐truth lines. The proposed enhancement
network provides more accurate DP‐RTF estimates, which
show the ability to preserve the direct‐path localization cues
and meanwhile reduce the effect of noise and reverberation.
The proposed method is compared with other three
methods, IPD‐NN [6], RTF and RTF‐CT [13]. The IPD‐NN
method uses a four‐layer DNN to map the contaminated IPD
features to the corresponding clean ones. The RTF method
TABLE 1Room conguration for training and test data
Dataset Training Test
Room Label 1 2 3 4 5 6 7
Room size (m
3
) 7.0 8.0 5.0 6.0 6.0 3.5 4.0 5.5 3.0 3.8 3.0 2.5 6.0 8.0 3.8 5.0 7.0 3.0 4.0 4.0 2.7
Array centre (m) (3.00, 3.50, 1.70) (3.50, 3.00, 1.65) (2.50, 2.50, 1.40) (1.20, 1.45, 1.55) (2.00, 4.00, 1.65) (2.50, 3.00, 1.50) (1.80, 1.70, 1.60)
Distance (m) 1.50: 0.50: 3.00, 3.40 1.75, 2.25 0.50, 1.00 0.75, 1.25 0.60: 0.90: 3.30 0.70, 1.40, 2.10 0.80, 1.30
RT
60
(s) 0: 0.17: 0.85 0: 0.22: 0.88 0: 0.25: 0.75 0: 0.3: 0.9 0.2: 0.2: 0.8 0.2: 0.2: 0.8 0.2: 0.2: 0.8
SNR (dB) −5: 5: 20 −5: 5: 20 −5: 5: 20 −5: 5: 20 −5: 5: 20 −5: 5: 20 −5: 5: 20
FIGURE 3 Illustration for the candidate directions of sound sources
TABLE 2Inuence of the IID information under different rooms
(C=1)
Method
MAE (degrees)
Room 5 Room 6 Room 7 AVG.
IPD [6] 14.28 18.47 20.23 17.66
DP‐RTF(IPD +IID) 13.93 17.97 20.64 17.51
TABLE 3Inuence of the temporal context information under
different rooms
Temporal Context
MAE (degrees)
Room 5 Room 6 Room 7 AVG.
C=1 13.93 17.97 20.64 17.51
C=3 9.18 11.99 13.45 11.54
C=5 8.43 10.78 11.87 10.36
C=7 7.35 9.40 10.38 9.04
C=9 7.66 9.95 11.16 9.59
C=11 7.64 9.88 11.20 9.57
C=13 7.86 10.02 10.64 9.51
C=15 8.42 10.64 11.99 10.35
TABLE 4Inuence of the TF weighting scheme under different
rooms
Method
MAE (degrees)
Room 5 Room 6 Room 7 AVG.
w/o weighting 7.35 9.40 10.38 9.04
w / weighting (λ=0.01) 7.05 8.98 9.99 8.67
w/ weighting (λ=0.5) 7.04 9.01 10.2 8.75
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451
means directly using the contaminated DP‐RTF features for
DOA estimation (namely the pipeline in Figure 1a). It uses a
TF weight equalling to one. The RTF‐CT method also follows
the pipeline in Figure 1a but sets the TF weight by coherence
test which is used to select direct path dominated TF bins. For
fair comparison, all the comparison methods estimate the
DOA by nding the optimal matching between the enhanced
feature and the template features of all candidate directions,
following the principle of the proposed method. The com
parison between these methods is carried out in the environ
ments with different levels of noise and reverberation. Tables 5
and 6respectively show the MAEs of the four methods under
Room 6 with various SNR and RT
60
conditions. Each test
signal segment used for DOA estimation is with a duration of
0.5 s. It can be seen that the proposed method and IPD‐NN
outperform RTF and RTF‐CT in all cases, which demonstrates
the superiority of DNN based methods for enhancing the
localization feature. The proposed method performs better
than the IPD‐NN method. This is due to that the proposed
method incorporates the IID and temporal context informa
tion to feature estimation, which is helpful for improving the
robustness of DOA estimation. Compared with the RTF
method, both RTF‐CT and our method add the DP‐RTF
enhancement process, but our method achieves a lower MAE
than the RTF‐CT. It can conclude that employing all data to
enhance the DP‐RTF is more benecial than only employing
the data selected by the coherence test. Besides, the RTF‐CT
method applies a hard selection on TF bins, while the pro
posed method applies a better TF weighting scheme, that is a
soft weight.
FIGURE 4 phase and amplitude of the DP‐
RTF as a function of frequency bins under a typical
acoustic condition that RT
60
=600 ms and
SNR =5 dB (babble noise) in Room 6. The DP‐
RTF is estimated without DP‐RTF enhancement
network in (a) (c), and with DP‐RTF enhancement
network in (b) (d). The sound source is located at 0°
in (a) (b), and 30° in (c) (d). The distance from the
sound source to the centre of the microphone array
is 2.1 m
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YANG ET AL.
5

CONCLUSION
This article proposes a DP‐RTF enhancement network for
sound source localization under adverse acoustic conditions.
Considering the complex non‐linear process of noise and
reverberation generating and mixing, we utilize a DNN to model
the non‐linear regression that discriminates the clean DP‐RTFs
from the contaminated ones. For training, a novel loss function
composed of a weighted MSE loss for DP‐RTF and a TF‐weight
regularization term are proposed to account for the fact that only
parts of TF bins contain reliable localization information due to
the TF sparsity of the (speech) source signal. Experiments with
binaural microphones verify the robustness of our method for
DOA estimation especially in scenarios with high level of noise
and reverberation. In this work, we focus on the concept of DP‐
RTF enhancement and TF weight estimation, and a generic
DNN model is adopted, which can further be revised with a
more advanced network structure, such as the recurrent neural
network, as a future work.
ACKNOWLEDGEMENTS
This work is supported by National Natural Science Founda
tion of China (No. 61673030, U1613209), Science and
Technology Plan Project of Shenzhen (No. JCYJ20200109
140410340).
CONFLICT OF INTEREST
The authors declared that they have no conicts of interest to
this work.
ORCID
Bing Yang
https://orcid.org/0000000289782322
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How to cite this article: Yang, B., et al.: Enhancing
direct‐path relative transfer function using deep neural
network for robust sound source localization. CAAI
Trans. Intell. Technol. 7(3), 446–454 (2022). https://
doi.org/10.1049/cit2.12024
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