An Improved Real-Time Noise Suppression Method Based on RNN

and Long-Term Speech Information

Baoping Cheng

1,2

, Guisheng Zhang

, Xiaoming Tao

, Sheng Wang

, Nan Wu

and Min Chen

Department of Electronic Engineering, Tsinghua University, Beijing, 100084, China

ChinaMobile (Hangzhou) Information Technology Co., Ltd., Hangzhou, Zhejiang, 311121, China

taoxm@mail.tsinghua.edu.cn

Keywords: Noise Suppression, Recurrent Neural Network, Long-Term Spectral Divergence, Speech Enhancement.

Abstract: Speech enhancement based on deep learning can provide almost best performance when processing non-

stationary noise. Denoising methods that combine classic signal processing with Recurrent Neural Network

(RNN) can be implemented in real-time applications due to their low complexity. However, long term speech

information is omitted when selecting the features in these methods, which degrades the denoising perfor-

mance. In this paper, we extend a well-known RNN based denoising method called RNNoise with the long-

term spectral divergence (LTSD) feature. We also limited the amount of noisy speech attenuation to get a

better trade-off between noise removal level and speech distortion. Our proposed method outperforms the

RNNoise algorithm by 0.12 MOS points on average in the subjective listening test.

1 INTRODUCTION

Noise suppression (NS) is one of the most active

acoustic research area of speech enhancement (SE)

since 1970. It aims to improve the performance of hu-

man and computer speech interaction by enhancing

intelligibility and quality of speech signal which is in-

terfered by ambient noise (Loizou, 2017). Typical NS

application scenarios includes but not limited to voice

communication, online real-time audio-virtual com-

munications (Valin, 2018), automatic speech recogni-

tion, hearing aids, etc.

Conventional signal processing techniques used in

NS is to estimate the statistical characteristic of noisy

speech signal, and then filter out the noise part or ap-

ply a spectral suppression gain in the time-frequency

domain (Benesty et al., 2006). Although the conven-

tional NS methods can achieve a good performance in

processing stationary noise of speech signal, it cannot

cope with non-stationary noise, such as transient

noise, whose characteristic is hard to be estimated. In

the last decade, deep learning (DL) has been widely

used to present the data characteristic. Some research-

ers tried to use DL to estimate the characteristic of

non-stationary noise in NS task (DL Wang et al.,

Xiaoming Tao is the corresponding author of this paper.

2018).

In order to deal with both stationary and non-sta-

tionary noise, a recent trend of NS research is the de-

velopment of a hybrid system that combines DL tech-

niques and conventional methods (Xia et al., 2018).

A hybrid systems proposed in (Tu et al., 2018)

gives the estimated clean speech and the suppression

rule calculated by a long short-term memory (LSTM)-

based direct mapping regression model. The suppres-

sion rule is a geometric mean of the estimation of the

current frame using the conventional NS method and

the suppression rule of the previous frame. The first

step is used to decrease stationary noise interference.

The second step is to efficiently suppress non-station-

ary noise components. The algorithm proposed in

(Coto et al., 2018) follows a similar method with that

of Tu (2018). The noisy signal is first processed by

the conventional Wiener filter. Then the signal is fur-

ther processed by a multi-stream approach based on

LSTM networks.

In this paper, we investigate the NS performance

of the RNNoise system with additional long-term

spectral divergence (LTSD) feature. We also limit the

amount of noisy speech attenuation to get a trade-off

between noise suppress level and speech distortion.

476

Cheng, B., Zhang, G., Tao, X., Wang, S., Wu, N. and Chen, M.

An Improved Real-Time Noise Suppression Method Based on RNN and Long-Term Speech Information.

DOI: 10.5220/0011952700003612

In Proceedings of the 3rd International Symposium on Automation, Information and Computing (ISAIC 2022), pages 476-481

ISBN: 978-989-758-622-4; ISSN: 2975-9463

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

The RNNoise system (Valin, 2018) is briefly over-

viewed in Section 2. The definition of LTSD feature,

model architecture and the training and evaluation

procedure are present in Section 3. In Section 4, we

make the experiments in different scenarios and com-

pare the NS performance of the proposed method with

that of RNNoise system. The conclusion is given in

Section 5.

2 RNNoise SYSTEM

The work proposed in this paper is based on the

RNNoise that combines both conventional and DL

techniques, which is implemented by Jean Marc Val-

lin. The architecture of RNNoise system is shown in

Figure 1. A Vorbis window with 20 ms duration and

50% overlap is used in the NS process. 42 input fea-

tures are used in RNNoise to suppress noise. The first

22 features are Bark Frequency Cepstral Coefficients

(BFCCs), which are computed by applying the Dis-

crete Cosine Transformation (DCT) on the log spec-

trum of Opus scale (Valin et al., 2016). Next 12 fea-

tures are the first and second ordertemporal deriva-

tives of the first 6 BFCCs. The following 6 features

are the first 6 coefficients computed by the DCT on

the pitch correlation across frequency bands. The last

2 features are the pitch period and a spectral non-sta-

tionary metric respectively.

Figure 1: Architecture of RNNoise system

The input of RNNoise is 48 kHz full-band audio

signal. The consecutive audio signal is divided into

many frames by overlapped window, and then trans-

formed into frequency domain by FFT. Using the out-

puts of pitch analysis and FFT, 22 features can be ob-

tained, which are the BFCCs. RNN is used to calcu-

late an ideal ratio mask (IRM) for 22 triangular bands

derived from the Opus scale. The 22 band gains can

be applied to the DFT magnitudes of each window af-

ter an interpolation.

Before the IFFT operation, a comb filter defined

at the pitch period, also called a pitch filter, is applied

to each window. The aim of the pitch filter is to re-

move noise components among pitch harmonics in

voice segments.

The processed signal is the multiplication of the

outputs of pitch filter and band gain interpolation.

Then processed signal is converted by IFFT to obtain

the time domain signal. Finally, an overlap-add

(OLA) method is used to produce the denoised signal.

In practical application, the OLA method consecu-

tively process each frame when it arrives.

3 PROPOSED METHOD

In order to deal with the non-stationary noise, the

LTSD feature is attached to the 42 features used in

RNNoise, since the LTSD feature is useful for dis-

criminating speech and non-speech signal. We use a

long-term speech spectrum window instead of instan-

taneous one to track the spectral envelope and com-

pute LTSD by estimating of the long-term spectral en-

velope (LTSE).

3.1 Definitions of the LTSD Feature

Let 𝑥𝑛be a noisy speech signal which is divided

into overlapped frame segments. Its amplitude spec-

trum for the frame l at k band is defined as

𝑋𝑘, 𝑙.

The N-order LTSE is defined as

𝐿𝑇𝑆𝐸



𝑘, 𝑙 = 𝑚𝑎𝑥𝑋𝑘, 𝑙  𝑗





(1)

The average noise spectrum magnitude for the

frame l at k band is defined as S(k,l).The deviation of

the N-order LTSE respect to S(k,l) for the band, k = 0,

1..., NFFT-1, is defined as N-order LTSD, is de-

scribed as

NFFT 1

(,)

() 10log

NFFT

(,)

LTSE k l

LTSD l

−









(2)

The average noise spectrum magnitude value is

known in the training phase and its calculation for-

mula in the evaluation is given by

(, 1) (1 )(,),

if non-speech segment is detected

(,)

(, 1),

otherwise

Skl Skl

Skl

αα



−+−





−





(3)

(,) (,) (,)Skl Xkl mkl=⋅

(4)

An Improved Real-Time Noise Suppression Method Based on RNN and Long-Term Speech Information

477

(,)mkl

is the k frequency bin denoising gain com-

puted by the RNN at frame l. The speech and non-

speech segment detection is also performed by the

RNN which is introduced in Section 3.2. To get a

good trade-off between noise reduction and computa-

tion complexity, N is set to 6 (Ram et al., 2003).

3.2 Architecture and Implementation

We first extract the 42 features presented in (Valin,

2018) and extend them with the additional LTSD fea-

ture to evaluate our proposed system. We pass this

data along with the input audio files to the evaluation

tool which computes, interpolates and applies the

opus scale gains along with a pitch filter to the audio

file.

Figure 2: Deep recurrent neural network topology

The architecture of the neural network used foll-

ows the original RNNoise architecture with the differ-

ence that the input layer created a tensor whose size

is modified to fit that of the increased number of fea-

tures. The detailed topology is shown in Figure 2 and

the network contains 215 units and 4 hidden layers.

As described in (Valin, 2018), the entire network

architecture is designed so that it follows the structure

of most conventional noise suppression methods. The

denoising system can be divided into three modules:

a voice activity (VAD), a noise spectral estimation

module and a spectral subtraction module. Each mod-

ule includes a recurrent layer and applying GRU.

VAD module contributes significantly to the train-

ing phase by assisting the system differentiate noise

from speech. It also outputs a voice activity flag used

to compute the average noise spectrum in the evalua-

tion phase.

The full training and evaluation phase is visual-

ized in Figure 3. We first use Sound eXchange (SoX)

to concatenate and convert the input clean speech and

noise to RAW format. Then we apply the appropriate

tool to produce the samples used for training, by mix-

ing clean speech and noise samples in different SNRs,

and extract the original 42 features. After, we process

the training samples to extract the additional LTSD

feature using formula (1) and (2).

Figure 3: Training and Evaluation overview

We train the proposed system using the Keras

toolchain with Tensorflow backend. Both reference

and proposed network are trained through the course

of 160 epochs with 8 steps. We set the learning rate to

0.001 by applying the Adam optimizer. We use the

loss function (3) (as proposed by Vallin), where 𝑚 is

the ground truth NS gain,

is the mask calculated

by the RNN, 𝛾=1/2 is a parameter that tunes the NS

aggressiveness and K is the number of bands, set to

22 in our training. During training, both systems are

fed with 6000000 audio frames, each with a non-over-

lapping 10 ms duration.

(

()

)

()

(,)

10 min( 1,1) 10 ( )

ˆˆ

( ) 0.01 log

20.5log

iii

ii i i

iii

Lmm

mmm

mm m m

mmm



⋅+⋅⋅−





+− −⋅⋅



−⋅ ⋅ − ⋅ ⋅







(5)

ISAIC 2022 - International Symposium on Automation, Information and Computing

478

Figure 4: PESQ MOS-LQO quality evaluation for living, office, cafe, and bus noise environment

Figure 5: STOI in different acoustical environments under various SNR levels

An Improved Real-Time Noise Suppression Method Based on RNN and Long-Term Speech Information

479

We apply the clean speech datasets included in the

Edinburgh Datasets (Botinhao, 2017) which is com-

prised of audio recordings, including 28 English

speakers (14 men and 14 women), sampled at 48 kHz,

to train our proposed system. For noise audios, we

used a subset of the noise environments available in

the DEMAND datasets (Thiemann et al., 2013).

These noise environments were then not included

inthe test set. The DEMAND datasets include noise

recordings corresponding to six distinct acoustic

scenes (Domestic, Nature, Office, Public, Street and

Transportation), which are further subdivided in mul-

tiple more specific noise sources (Thiemann et al.,

2013). Note that while we used clean speech and noise

included in the Edinburgh Datasets, the samples used

for training the systems are not the noisy samples

found in the noisy speech subset of the Edinburgh Da-

tasets, but rather samples mixed using the method de-

scribed in (Valin, 2018).

The model generalization is achieved by data aug-

mentation. Since cestrum mean normalization is not

applied, the speech and noise signal are filtered inde-

pendently for each training example through a second

order filter as

()

rz rz

−−

(6)

where each of

rr

are following the uniform dis-

tribution in the [-3/8, 3/8] range. We vary the final

mixed signal level to achieve robustness to the input

noisy speech signal. The amount of noisy speech at-

tenuation is also limited to get a better trade-off be-

tween noise removal level and speech distortion.

The test set used is the one provided in the Edin-

burgh Datasets, which has been specifically created

for SE applications and consists of wide-band (48

kHz) clean and noisy speech audio tracks. The noisy

speech in the set contains four different SNR levels

(2.5dB, 7.5dB, 12.5dB, 17.5dB). The clean speech

tracks included in the set are recordings of two Eng-

lish language speakers, a male and a female. As for

the noise recordings that were used in the mixing of

the noisy speech tracks, those were selected from the

DEMAND database. More specifically, the noise pro-

files found in the testing set are:

• Office: noise from an office with keyboard typ-

ing and mouse clicking

• Living: noise inside a living room

• Cafe: noise from a cafe at a public square

• Bus: noise from a public bus

The selected evaluation metrics is of great im-

portance in the effort of regular evaluation of one sys-

tem. In order to evaluate our system, we used a metric

that focuses on the intelligibility of the voice signal

(STOI) ranges from 0 to 1 and a metric that focuses

on the sound quality (PESQ) ranges from -0.5 to 4.5,

with higher values corresponding to better quality.

4 EXPERIMENTAL RESULTS

AND DISCUSSIONS

It was appropriate to present our results in a compari-

son between the reference RNNoise system and the

proposed method that makes use of the LTSD feature.

As seen in Figure 4, it becomes apparent that the pro-

posed method has better performance in most acoustic

scenarios and in all SNR levels, especially in lower

SNRs, comparing the two methods with the PESQ

quality metric. Our proposed method outperforms the

RNNoise algorithm by 0.12 MOS points on average.

Similarly, examining the STOI intelligibility meas-

ure, as depicted in Figure 5, it is indicated that the pro-

posed method also has better intelligibility perfor-

mance.

We observe a noticeable improvement in perfor-

mance than RNNoise method. Having also compared

several spectrograms of both methods, it was ob-

served that in general the proposed method does in-

deed subtract more noise components.

Having taken these results into consideration, it is

demonstrated that more detailed research is required

in future work to reduce speech distortion and pro-

mote noise removal level for different application sce-

narios.

Firstly, we find that adding more hidden layers in-

deed be beneficial for our proposed system. Given

that we provide the system with more and diverse in-

put information, the RNN might be able to better use

the proposed features with additional hidden layers.

Secondly, studying samples processed by our ex-

tended system, we speculate that the system could

benefit from changing how aggressively the noise

suppression occurs. This can be achieved by fine-tun-

ing the value of the 𝛾 parameter in the loss function

(3), keeping in mind that smaller 𝛾 values lead to

more aggressive suppression. According to our exper-

iment, setting 𝛾=1/2 is an optimal balance.

Finally, we believe that further research can be

done regarding the performance of our proposed sys-

tems as the training datasets increases in size and di-

versity.

ISAIC 2022 - International Symposium on Automation, Information and Computing

480

5 CONCLUSIONS

In this paper we extend and improve a hybrid noise

suppression method. We proposed LTSD feature

which we believed would be beneficial to the de-

noising process and regard it as one input to the net-

work. We describe our implementation for training

the system with extended input features and make a

comparison against the reference RNNoise trained by

the same training parameters and datasets. We make

a discussion about our findings from this process,

concluding that the extra LTSD feature have obvious

positive effect on NS performance. In the future, we

will explore the further improvement of the base sys-

tem by tuning hidden layers, loss function and da-

tasets.

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