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Speech enhancement using U-nets with wide-context units


In this article a new neural network for speech enhancement is proposed where single-channel noisy speech is processed in order to improve its intelligibility and quality. It is based on the U-net architecture, i.e. it is composed of two main blocks: encoder and decoder. Some of the corresponding layers in the encoder and decoder are connected with skip connections. In most of the encoder-decoder neural networks for speech enhancement known from the literature, the time-frequency resolution of the hidden feature maps is reduced. The main strategy in the presented approach is to maintain the time-frequency resolution of feature maps at all levels of the network while having large receptive field at the same time. In order to obtain features dependent on wide context we propose neural network units based on recurrent cells or dilated convolutions. The proposed neural network was evaluated using WSJ0 and TIMIT speech data mixed with noises from Noisex, DCASE and field recordings from Freesound online database. The results showed improvement over the baseline networks based on gated dilated convolutions or long-short term memory (LSTM) in terms of scale-independent speech-to-distortion ratio (SI-SDR), spectro-temporal objective intelligibility (STOI) and perceptual evaluation of speech quality (PESQ) measures.

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The work of Szymon Drgas was supported by grant 0211/SBAD/0222.

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Correspondence to Tomasz Grzywalski.

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The authors contributed equally to this work. The order of the authors is random.

Appendix: Gated recurrent unit

Appendix: Gated recurrent unit

There are many implementations and parameters of GRUs which can affect performance of the neural network. Therefore, in this appendix we provide information about variant of GRUs used in the experiments reported in this paper. The value of the reset gate is calculated as

$$ \textbf{r}_{t}=\sigma_{r}(\textbf{W}_{xr}\textbf{x}_{t}+\textbf{W}_{hr}\textbf{h}_{t-1}+\textbf{b}_{r}) , $$

where σr is the sigmoid, xt is input vector to the GRU for time index t, while ht− 1 is a vector representing previous state. Matrix Wxr, Whr, and br are parameters of the reset gate. Similarly, update gate

$$ \textbf{u}_{t}=\sigma_{u}(\textbf{W}_{xu}\textbf{x}_{t}+\textbf{W}_{hu}\textbf{h}_{t-1}+\textbf{b}_{u}) , $$

and candidate state is computed using the formula

$$ \textbf{c}_{t}=\sigma_{c}(\textbf{W}_{xc}\textbf{x}_{t}+\textbf{r}_{t}\odot (\textbf{W}_{hc}\textbf{h}_{t-1})+\textbf{b}_{c}) , $$

where σc is tanh function, Wxc, Whc and bc are weights and bias. Finally, state ht is computed using

$$ \textbf{h}_{t}=(\textbf{1}-\textbf{u}_{t})\odot \textbf{h}_{t-1} + \textbf{u}_{t} \odot \textbf{c}_{t} . $$

The processing performed by GRU can be described as a linear transformation

$$ \left[\begin{array}{cc} \textbf{W}_{xr} & \textbf{W}_{hr} \\ \textbf{W}_{xu} & \textbf{W}_{hu} \\ \textbf{W}_{xc} & \textbf{0} \\ \textbf{0} & \textbf{W}_{hc} \\ \textbf{0} & \textbf{I} \end{array}\right]\left[ \begin{array}{c} \textbf{x}_{t} \\ \textbf{h}_{t-1} \end{array}\right] + \left[ \begin{array}{c} \textbf{b}_{r} \\ \textbf{b}_{u} \\ \textbf{0} \\ \textbf{0} \\ \textbf{0} \end{array}\right] = \left[ \begin{array}{c} \textbf{z}_{r} \\ \textbf{z}_{u} \\ \textbf{z}_{xc} \\ \textbf{z}_{xh} \\ \textbf{h}_{t-1} \end{array}\right] , $$

which is transformed using the following nonlinear function

$$ f(\textbf{z}) = (1-\sigma(\textbf{z}_{u})\odot \textbf{h}_{t-1} + \sigma(\textbf{z}_{u})\odot \tanh(\textbf{z}_{xc}+\sigma(\textbf{z}_{r})\odot\textbf{z}_{hc}+\textbf{b}_{c}) . $$

Additionally, in all experiments featuring recurrent layers, the initial states of recurrences were also learned during training. We found this to have a small but consistent positive effect on the network’s performance.

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Grzywalski, T., Drgas, S. Speech enhancement using U-nets with wide-context units. Multimed Tools Appl 81, 18617–18639 (2022).

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  • Speech enhancement
  • U-nets
  • DNN