Deep learning-based noise reduction method using bone conduction sensors and microphone signals

Description

BACKGROUND OF THE INVENTION1. Field of the Invention

[0001] The invention relates to a method of noise reduction for an electronic voice capturing device and more particular to a deep learning based noise reduction method using both one bone-conduction sensor and one microphone signals.2. Description of Related Art

[0002] Noise reduction is a technology for separating speech from background noise. The technology is widely employed in many electronic voice capturing devices. Conventionally, either a single microphone or microphone array for reducing noise is involved in the technology. However, the conventional technology has a number of drawbacks. In detail, the traditional single microphone technology assumes noise is stationary, so it is not highly adaptable, and has many limitations. The microphone array technology requires two or more microphones, which increases cost, requires a very complicated product design, and has limited applications in terms of product structure. Furthermore, the microphone array technology such as beamforming relies on spatial difference of target speech and interference noise. When target speech and noise source originate from the same direction, this method fails to separate them.

[0003] The Chinese Patent Publication Number CN109346075A, entitled "system for identifying voice of a user to control an electronic device through human vibration", comprises a vibration sensor for sensing body vibration of a user, a processor circuit coupled to the vibration sensor for activating a voice pickup device to begin voice pickup when the output signal of vibration sensor detects voice of the user; and a communication module coupled to both the processor circuit and the pickup device for communicating between the processor circuit and the pickup device.

[0004] The Chinese Patent Publication Number CN107452389A, entitled "A general real time noise reduction method for monaural sound", the method comprises the steps of receiving noisy speech in an electronic form, which includes target speaker voice and interfered non-speech noise; extracting the magnitude spectrum of Short-time Fourier transform (STFT) as acoustic features in a frame by frame manner; using a deep recurrent neural network (RNN) having a long short-term memory (LSTM) to generate ideal ratio masks in a frame by frame manner; multiplying the estimated ratio mask and the magnitude spectrum of the noisy speech; combining the magnitude spectrum and the original phases of the noisy speech to form a clean voice waveform. This patent disclosed a supervised learning method for noise reduction. It further disclosed using a deep RNN with LSTM to generate ideal ratio mask. The RNN uses a large amount of noisy speeches for training, including various noises and microphone impulse responses. As a result, a general noise reduction method is realized which is independent from speakers, background noises and transmission channels. The monaural noise reduction method involves only processing signals recorded by a single microphone. Compared with microphone array noise reduction method which requires multiple microphones, the monaural noise reduction method has wider applications and low cost.

[0005] The supervised learning based noise reduction method disclosed by the Chinese Patent Publication Number CN107452389A disclosed using deep RNN with LSTM to generate ideal ratio mask. It does not rely on information of future time frames and achieves a highly efficient computation of RNN model in the noise reduction process without adversely affecting the noise reduction performance. It achieves real time noise reduction by further simplifying computational demand and building a relative small scale RNN model.

[0006] However, the microphone array based technology for noise reduction has the following drawbacks: cost is proportional to the number of microphones. That is, the more of the microphones the higher of the cost. More microphones mean stringent specifications requirements for products and more limitations for the products. The microphone array based technology relying on directional information for reducing noise is incapable of suppressing noise from the target speech direction. The single microphone based technology for reducing noise has the following drawbacks: it relies on noise estimation. When the noise is non-stationary, the performance becomes worse, particular in low SNR conditions. Thus, the need for improvement still exists.

[0007] The non-patent literature "A wearable bone-conducted speech enhancement system for strong background noises", 2017 18TH INTERNATIONAL CONFERENCE ON ELECTRONIC PACKAGING TECHNOLOGY (ICEPT), IEEE, 16 August 2017 (2017-08-16), pages 1682-1684, XP033154500, relates to a high-performance wearable bone-conducted speech enhancement system which is developed to reduce the distortion of the environment noises. Both bone-conducted and air-conducted voices are used to train the equalization function of bone-conducted speech to air-conducted speech based on the deep neural network.

[0008] Further non-patent literature "A Spectra-Based Equalization-Generation Combined Framework for Throat Microphone Speech Enhancement", IEEE ACCESS, vol. 6, pages 71455-71463, XP011699372, investigates the low-band and high-band spectral structure between TM (throat microphone) and AM (acoustic microphone) speech, respectively, and proposes a spectra-based band-division mapping framework for TM speech enhancement based on the investigation.

[0009] Further relevant prior art is discussed in US 2018 / 033449 A1. Non-patent literature "Multisensory speech enhancement in noisy environments using bone-conducted and air-conducted microphones", 2014 IEEE China Summit & International Conference on Signal and Information Processing (ChinaSIP), 9 July 2014, pages 1-5, XP093258698, proposes a speech enhancement algorithm for estimating the clean speech using samples of air-conducted and bone-conducted speech signals.SUMMARY OF THE INVENTION

[0010] The invention proposes a deep learning based noise reduction method by using both bone-conduction sensor signal and a microphone signal. It helps to solve current technical problems relating to stringent product structure design of multiple microphones, high cost, and many limitations related to single microphone noise reduction technique.

[0011] To solve above technical difficulties, this invention proposes a deep learning based noise reduction method by using both bone-conduction sensor signal and a microphone signal. The invention is set out in the appended set of claims. It comprises the following steps; collecting speech signal from a microphone; collecting bone-conducted signal by bone-conduction sensor; passing the bone-conducted signal to a high pass filter; transmitting both the high pass filtered bone-conducted signal and microphone signal to a deep neural network (DNN) module; the DNN module is to fuse both the filtered bone-conducted signal and the microphone based signal and make predictions, in order to obtain clean speech.

[0012] The above and other objects, features and advantages of the invention will become apparent from the following detailed description taken with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a flowchart of a deep learning based noise reduction method by fusing signals collected by both a bone-conduction sensor and a microphone according to a first preferred embodiment of the invention; FIG. 2 is a flowchart of a deep learning based noise reduction method by fusing signals collected by both a bone-conducted sensor and a microphone according to a second preferred embodiment of the invention; FIG. 3 is a block diagram of details of the high frequency restructuring step; FIG. 4 is a block diagram of deep neural network (DNN) incorporated into the invention; FIG. 5 is a spectrogram of signal collected by the bone-conduction sensor of the invention; FIG. 6 is a spectrogram of signal collected by the microphone of the invention; FIG. 7 is a spectrogram of speech signal processed by the invention; and FIG. 8 is a table of comparing the noise reduction method of the invention with the deep learning based noise reduction method without incorporating a bone sensor for monaural sound in terms of eight different noisy environments and showing advantageous noise reduction results of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Referring to FIG. 1, a flowchart of a deep learning based noise reduction method of fusing signals from both a bone-conduction sensor and a microphone according to a first preferred embodiment of the invention is illustrated. The method comprises the steps of (S1) collecting speech signals from a microphone; (S2) collecting bone-conducted signals from a bone-conduction sensor; (S3) transmitting the bone-conducted signals to a high-pass filter to filter out low frequency noise; (S4) transmitting both the filtered bone-conducted signals and the speech signals to a deep neural network (DNN) module; and (S5) activating the DNN module to fuse both the filtered bone-conducted signals and the speech signals and making predictions, thereby obtaining a clean speech.

[0015] The invention has the following advantageous effects in comparison with the prior art: A bone-conduction sensor is utilized. The bone-conduction sensor is not interfered by acoustic noise propagated through air. Through combining both the filtered bone-conducted signal and the microphone signal to the DNN module, and activating the DNN module to analyze and process the combination signals, satisfactory performance of noise reduction can be achieved even in super low signal-to-noise ratio (SNR) conditions.

[0016] The most advanced noise reduction method so far is based on deep learning which uses a large amount of data for training. While the method is capable of separating the speech of a specific person from background noise without being trained, this model is speaker independent.

[0017] To improve the performance of noise reduction for an unspecific person, the most effective method is to add the voices of many persons to training set. However, in this case, the DNN cannot suppress the interfering voice effectively. Even worse, the DNN may erroneously take interfering voice as target speaker voice and suppress the true target speaker voice.

[0018] Preferably, the bone-conduction sensor is capable of collecting low frequency bone vibration and is not interfered by air conduced acoustic noise. It is possible to effectively reduce noise in a very low SNR in full frequency band by combining both the filtered bone-conducted sensor signal and the microphone signal with the DNN module, and activating the DNN module to analyze and fuse the combination signals.

[0019] The bone sensor of the embodiment is a known technique.

[0020] Speech signals have a strong correlation in time which is critical to voice separation. For improving the performance of voice separation in terms of context, the DNN is used to concatenate the previous frames, the current frame and the subsequent frames as a vector having an increased dimension and the vector is taken as a characteristic of input. The method of the invention is performed by running a program on a computer. Acoustic features are extracted from noisy speech. An ideal time frequency ratio mask is estimated. Together they are combined again to form a voice waveform. The method involves at least one module which can be executed by any system or hardware having computer executable instructions.

[0021] Preferably, the high-pass filter modifies the direct current offset of the bone sensor signal and filters out low frequency noise signal.

[0022] More preferably, the high-pass filter is a digital filter.

[0023] Referring to FIG. 2, a flowchart of a deep learning based noise reduction method of fusing combined signals collected by both a bone-conduction sensor and a microphone according to a second preferred embodiment of the invention is illustrated. The method comprises the steps of (T1) collecting speech signals from a microphone; (T2) collecting bone-conducted signals from a bone-conduction sensor; (T3) transmitting the bone-conducted signals to a high-pass filter to filter out low frequency noise; (T4) designing a high frequency reconstruction module to extend the frequency of the filtered bone-conducted signals to more than 2kHz (i.e., high frequency restructuring for increasing a bandwidth of the filtered bone-conducted signals); (T5) transmitting both the filtered bone-conducted signals having an extended frequency range and the speech signals to a deep neural network (DNN) module; and (T6) activating the DNN module to fuse both the filtered bone-conducted signals having an extended frequency range and the speech signals and making predictions, thereby obtaining a clean speech.

[0024] Preferably, many methods are capable of restructuring high frequency. The DNN is the most effective method so far. In the embodiment, only one kind of DNN is described as an exemplary example.

[0025] Referring to FIG. 3, it is a block diagram of details of the high frequency reconstructing step. The purpose of the high frequency reconstruction is to increase the frequency range of the filtered bone-conduction sensor signal. The DNN is used to reconstruct high frequencies. There are many implementations of the DNN and FIG. 3 shows, as one of them, a method of DNN based restructuring high frequency based on a long short-term memory (LSTM).

[0026] Preferably, the DNN module comprises a signal fusing unit for fusing the filtered bone-conduction signal and the microphone signal and making predictions to obtain a clean speech.

[0027] Preferably, one of the implementations of the DNN module is a convolutional neural network (CNN) which can obtain a clean speech magnitude spectrum (SMS) by making predictions.

[0028] More preferably, the CNN is used in the DNN based combination model as an example, and the CNN can be replaced by LSTM or deep full CNN.

[0029] Referring to FIG. 4, it is a block diagram of DNN incorporated into the invention. The CNN is implemented, i.e., a training target of the DNN is clean speech magnitude spectrum (SMS). First, the clean speech is subjected to Short-time Fourier transform (STFT) to obtain a clean speech magnitude spectrum (SMS) as a training target (e.g., a target magnitude spectrum (TMS)). For example, the DNN module includes three CNNs, three LSTMs and three deconvolutional neural networks.

[0030] Input signals of the DNN module are generated by stacking both the magnitude spectrum of the bone-conduction sensor signal and the magnitude spectrum of the microphone signal. First, both the bone-conduction sensor signal and the microphone signal are subjected to STFT to obtain two magnitude spectrums. The magnitude spectrums are configured to stack.

[0031] Preferably, the stacked magnitude spectrums are processed by the DNN module to generate an estimated magnitude spectrum (EMS) which is in turn outputted.

[0032] Preferably, each of the TMS and the EMS are subjected to mean squared error (MSE) which is used to measures the average of the squares of the errors, i.e., the average squared difference between the estimated values and the true values.

[0033] More preferably, back propagation gradient descent is used to update network parameters in the training. In detail, training data is continuously sent to the network to update the network parameters until the network converges.

[0034] Preferably, inference is used to subject the microphone data to STFT to generate phases which are combined with the EMS to recover a clean speech.

[0035] In comparison to the conventional noise reduction methods, a single microphone is employed by the invention as input and thus the invention has advantages of being robust, having economical cost and simple specifications requirements. In the invention, the robustness means the performance of the noise reduction system is not influenced by the perturbation of microphone consistence and strong robustness means there are no requirements for microphone consistence and location of the microphone. In brief, the invention is applicable to various types of microphones.

[0036] Referring to FIG. 5, it is a chart of spectrogram of bone-conducted signal collected by the bone-conduction sensor of the invention.

[0037] Referring to FIG. 6, it is a chart of spectrogram of voice signal collected by the microphone of the invention.

[0038] Referring to FIG. 7, it is a chart of spectrogram of speech signals processed by the invention.

[0039] Referring to FIG. 8, it is a table of comparing the noise reduction method of the invention with the conventional noise reduction method without incorporating a bone-conduction sensor for monaural sound corresponding to deep learning in terms of eight different noisy environments and showing advantageous noise reduction results of the invention. Specifically, the table tabulates the comparison of the noise reduction method of the invention (i.e., sensor-microphone) with the conventional noise reduction method (i.e., Chinese Patent Publication Number CN107452389A) without incorporating a bone sensor for monaural sound corresponding to deep learning (i.e., microphone only) in terms of eight different noisy environments and showing advantageous noise reduction results of the invention. The eight different noise sources are bar noise, road noise, intersection noise, railroad noise, noise made by a car running at 130km per hour, cafeteria noise, eating noise, and office noise. Both the invention and the conventional method are subjected to perceptual evaluation of speech quality (PESQ) having a range of [-0.5, 4.5]. As shown, the PESQ score of each noise source is greatly increased after being subjected to the method of the invention. Average of the increases scores is 0.26. In brief, the method of invention can reproduce high-quality sound and has a strong capability of cancelling noise.

[0040] The invention has the following advantageous effects in comparison with the prior art: The bone sensor is capable of collecting low frequency voice and is not interfered by air conducted acoustic noise. It is possible of effectively reducing noise in a very low SNR by transmitting both the bone-conduction sensor signal and the microphone signal to the DNN module, and activating the DNN module to analyze and fuse the combined signals.

[0041] In comparison with the conventional method of using a single microphone for noise reduction, the invention can reproduce high-quality sound, has a strong capability of cancelling noise, and effectively extracts target speech from noisy background by employing the strong modeling capability of the DNN.

[0042] The method of the invention is applicable to conversation earphone or a cellular phone contacted an ear (or any of other body parts). In contrast to the conventional noise reduction method of employing only bone sensor signals while having installed bone sensor and microphone, the method of the invention takes the bone sensor signals as input by taking advantage of the bone sensor signals not being affected by acoustic noise interference. Further, the method of the invention transmits both the bone-conduction sensor signal and the microphone signal to the DNN module, and activates the DNN module to fuse both signals and make predictions, thereby obtaining a clean speech as implemented in the first embodiment; or transmits both the filtered bone sensor signals having an increased frequency and the microphone signals from the microphone to the DNN module, and activates the DNN module to fuse both signals and make predictions, thereby obtaining a clean speech in the second embodiment.

[0043] The method of the invention can generate low frequency signals of high quality by taking advantage of the bone sensor. Further, the method of the invention can greatly increase estimation accuracy of the DNN, thereby obtaining a clean speech. Alternatively, the filtered bone sensor signals having an increased frequency can be outputted.

[0044] The method of the invention is different from Chinese Patent Publication Number CN109346075A which takes the bone sensor voice signals as a target to be processed. The method of the invention transmits both the bone sensor signals and the microphone signals to the DNN module, and activates the DNN module to fuse both signals and make predictions, thereby obtaining a clean speech.

[0045] The invention provides a deep learning based noise reduction method of fusing signals from both a bone sensor and a microphone by taking advantage of the bone sensor signals and the microphone signals. Further, the invention can reproduce high-quality sound, has a strong capability of suppressing noise, and effectively collect speech from noisy background by employing the strong modeling capability of the DNN. Thus, a clean speech with noise being substantially suppressed is reproduced. Finally, both complexity and cost are greatly decreased by taking advantage of a single microphone.

[0046] While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modifications. The scope of the invention is defined by the scope of the appended claims.

Claims

1. A deep learning based noise reduction method, comprising the steps of: collecting speech signals from a microphone (S1, T1); collecting bone-conducted signals from a bone-conduction sensor (S2, T2); transmitting the bone-conducted signals to a high-pass filter to filter out low frequency noise (S3, T3); using a high frequency reconstruction module to extend the frequency of the filtered bone-conducted signals to more than 2kHz (T4); transmitting both the filtered bone-conducted signal having an extended frequency range and the speech signals to a deep neural network, DNN, module (S4, T5); and activating the DNN module to fuse both the filtered bone-conducted signal having an extended frequency range and the speech signals and making predictions, thereby obtaining a clean speech (S5, T6), wherein input signals of the DNN module are generated by stacking a magnitude spectrum of the bone sensor based signal and a magnitude spectrum of the microphone based voice signal; wherein both the bone sensor based signal and the microphone based voice signal are subjected to Short-time Fourier transform, STFT, to obtain the two magnitude spectrums; and wherein the magnitude spectrums are configured to stack.

2. The deep learning based noise reduction method of claim 1, wherein the high-pass filter modifies a direct current offset of the bone sensor signal and filters out low frequency noise signals.

3. The deep learning based noise reduction method of claim 1, wherein the DNN module comprises a signal fusing unit for fusing the filtered bone-conduction signal and the microphone signal and making predictions to obtain a clean speech.

4. The deep learning based noise reduction method of claim 1, wherein one of a plurality of implementations of the DNN module is a convolutional neural network, CNN, which is capable of obtaining a clean speech magnitude spectrum, SMS, by making predictions.

5. The deep learning based noise reduction method of claim 1, wherein the DNN module comprises a plurality of the CNNs, a plurality of long short-term memories, LSTMs, and a plurality of deconvolutional neural networks.

6. The deep learning based noise reduction method of claim 1, wherein the clean speech is subjected to Short-time Fourier transform, STFT, to obtain a clean speech magnitude spectrum, SMS, as a target magnitude spectrum, TMS.

7. The deep learning based noise reduction method of claim 6, wherein the TMS is subjected to mean squared error, MSE.

8. The deep learning based noise reduction method of claim 1, wherein the stacked magnitude spectrums are processed by the DNN module to generate an estimated magnitude spectrum, EMS, to be outputted.

9. The deep learning based noise reduction method of claim 8, wherein the EMS is subjected to mean squared error, MSE.