KR20060085118A - Method and apparatus for bandwidth extension of speech - Google Patents

Abstract

Disclosed are a method and apparatus for widening a voice signal for generating a wideband voice signal from a narrowband voice signal.

The apparatus for extending a speech signal according to the present invention comprises means for estimating a highband speech signal from the narrowband speech signal using spectral folding and nonlinearization techniques, and adjusting a spectral envelope of the estimated highband speech signal. Means for doing so.

According to the present invention, the spectral folding technique and the nonlinearization technique can be used to estimate the highband speech signal from the narrowband speech signal, thereby effectively extending the bandwidth of the speech signal, and can be used at the receiving side of the communication system to improve A voice signal can be provided.

Description

Method and apparatus for band extension of speech signal

1 is a block diagram showing a schematic configuration of a band extension device according to the present invention.

2 is a block diagram showing a specific configuration of a high band signal generation unit according to the present invention;

3 is a block diagram showing a specific configuration of a high band estimation unit according to the present invention;

4 illustrates a spectral window used in the present invention.

5 is a flowchart illustrating a method for extending a band of a voice signal according to the present invention;

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the processing of speech signals, and more particularly, to a method and apparatus for widening a speech signal for generating a wideband speech signal from a narrowband speech signal.

In general, since the bandwidth of the telephone network is narrow to 300 to 3200 ㎐, the frequency band of the voice signal transmitted through the telephone network is limited. That is, signals in the 0 to 300 (Hz) and 3.2 to 8 (KHz) bands transmitted through the telephone network are lost, resulting in deterioration of the voice signal.

As one method for solving this problem, a bandwidth extension method has been studied for a long time. The band extension method receives an audio signal at a transmitting side, cuts out data of a high frequency band over a predetermined frequency, and generates additional information necessary to restore the cut out data of the high frequency band. When the signal and the additional information are transmitted, the receiver operates by recovering data of a high frequency band using the additional information. A representative example of such a bandwidth extension technology is Coding Technology's SBR (Spectral Band Replication) technology. A detailed description of the SBR is disclosed in Convention Paper 5560, presented at the Audio Engineering Society 112th Convention, May 10-13, 2002.

The band extension technology using the SBR has been a great success in audio signals such as music. Recently, aaaPLUS using Advanced Audio Coding (AAC) and the SBR has been selected as a standard for third generation communication. However, other band extension techniques, including the SBR, focus on audio signals, and there are relatively few band extension techniques suitable for speech signals such as speech.

Accordingly, an object of the present invention is to provide a method and apparatus for extending a band of a voice signal. In particular, it is an object of the present invention to provide a method and apparatus for extending a band of a voice signal which can effectively expand a band of a voice signal such as speech.

In addition, the present invention can be used in the decoder on the receiving side to generate a frequency component of 4 ~ 8 (KHz) to enable an increase in the sampling rate up to 16 (KHz), thereby providing a voice signal that can provide a voice signal of improved quality An object of the present invention is to provide a method and a device for expanding a band of a channel.

In order to solve the above technical problem, the present invention provides an apparatus for extending a speech signal, comprising: means for estimating a highband speech signal from the narrowband speech signal using spectral folding and nonlinearization techniques; And means for adjusting the spectral envelope of the estimated high band speech signal.

The means for estimating a highband speech signal from the narrowband speech signal comprises performing mirror sampling with the narrowband speech signal through upsampling, filtering with a high pass filter, and downsampling. A spectral folding unit for outputting a signal having a component; And a nonlinearity unit for non-linearizing the narrowband speech signal, and linearly combining the spectral folding unit and an output signal of the nonlinearization unit to estimate a highband speech signal from the narrowband speech signal. .

The means for adjusting the spectral envelope of the estimated high band speech signal may include: a mapping unit configured to output a high band speech signal corresponding to the narrowband speech signal using a predetermined table that is trained in advance; And an envelope adjusting unit for adjusting the spectrum of the estimated high band speech signal to match the high band speech signal output from the mapping unit.

Here, it is preferable that the mapping unit and the envelope adjusting unit use characteristics of the voice signal provided by the linear frequency hysteresis coefficient (LFCC).

In addition, the band extension method of the speech signal of the present invention removes the pitch component of the narrowband speech signal, and estimates the highband speech signal from the narrowband speech signal from which the pitch component is removed by using spectral folding and nonlinearization techniques. Doing; And extracting a linear frequency hysteresis coefficient (LFCC) of the narrowband speech signal, searching for a highband speech signal corresponding to the narrowband speech signal, and matching the estimated highband speech signal with the estimated highband speech signal. Adjusting the spectral envelope of the speech signal.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a block diagram showing a schematic configuration of a band extension device according to the present invention.

In the method of band extension of a narrowband speech signal, in consideration of the generation process of the speech signal, the envelope of the narrowband speech signal or the highband speech signal is mutually dependent, so if the narrowband speech signal is known, the narrowband speech signal is known. It is based on the fact that it can generate high-band speech signals from signals.

Referring to FIG. 1, the band extension device 100 according to the present invention includes first and second upsampling units 110 and 150, a low pass filter 120, a high band signal generator 130, and a high pass filter 160. , And a coupling unit 170.

 The first upsampling unit 110 upsamples the narrowband speech signal 101 that is input to the band extension device 100 twice. This upsampled signal is sampled at 16 (KHz). As a result, the signal output from the upsampling unit 110 is the same as the input signal in the band of 0 to 4 (KHz), and the folded version of the input signal in the high band of 4 to 8 (KHz) and It will have the same spectrum.

The low pass filter 120 filters the upsampled signal to remove the folded version and outputs a low band signal 121 having the same spectral characteristics as the input signal.

The high band signal generator 130 estimates a high band speech signal of 4 to 8 (KHz) from the narrow band speech signal 101, and a high band signal estimator and a high band spectral envelope as described below. A high frequency signal is estimated from the narrowband speech signal, a highband speech signal corresponding to the narrowband speech signal is searched using a predetermined table, and the corrected highband signal is corrected and output. It works in such a way.

The high band speech signal output from the high band signal generator 130 is upsampled twice by the second upsampling unit 150, and the high pass filter 160 is 4 to 8 from the upsampled signal. The audio signal 161 of the (KHz) band is extracted.

The coupling unit 170 is a low pass voice signal of 0 to 4 (KHz) output from the low pass filter 120 and a high pass voice of 4 to 8 (KHz) output from the high pass filter 160. The signal 161 is combined to output a wideband voice signal 171 band extended to 0 to 8 (KHz) as a whole.

2 is a block diagram showing a specific configuration of the high band signal generation unit 130 according to the present invention.

The high band signal generator 130 estimates a high band speech signal from the narrowband speech signal inputted therein, and adjusts a spectral envelope of the estimated high band speech signal. Separated by.

The portion A for estimating the high band speech signal includes a pitch filter 134, a pitch inverse filter 135, and a high band estimator 136.

Specifically, the pitch filter 134 obtains information about the third-order pitch filter coefficients and the pitch from the input narrowband speech signal. The pitch filter 134 uses a modified covariance method.

The pitch inverse filter 135 removes the pitch component included in the output signal of the pitch filter 134. This is because high pitch voice signals generally have fewer pitch components.

The high band estimator 136 estimates a high band speech signal from a narrow band speech signal from which the pitch component output from the pitch inverse filter 135 is removed.

Specifically, the high band estimating unit 136 includes a spectral folding unit 137 and a nonlinearizing unit 138.

The spectral folding unit 137 upsamples the narrowband speech signal from which the pitch component has been removed twice, filters through a high pass filter, and then downsamples the result twice to compare with the original narrowband speech signal. Output a signal with mirror symmetrical spectral components.

The nonlinearizer 138 upsamples the narrowband speech signal from which the pitch component has been removed twice, and passes the filtered signal through a low pass filter through a nonlinearization means such as a full wave rectifier. In addition, the nonlinearizer 138 again filters the nonlinearized signal through a high pass filter and downsamples it twice.

The output signals from the spectral folding unit 137 and the non-linearization unit 138 are linearly combined to output a high band voice signal, that is, an estimated voice signal of 4 to 8 (KHz) band.

A detailed configuration of the high band estimator 136 will now be described with reference to FIG. 3.

Referring to FIG. 3, the high band estimator 136 mainly includes a spectral folding unit 137 that is effective for restoring a high frequency band of an unvoiced sound component, and a nonlinearizer 138 for generating a harmonic structure of the voiced sound components. ).

The spectral folding unit 137 upsamples a band limited signal (narrowband speech signal) by the upsampling unit 137a, thereby restoring a high frequency component to a symmetrical form of a low frequency component. Through this process, components are also generated in the high frequency band. In general, since the unvoiced sound component of the voice has a considerable energy even in the high frequency band, the spectral folding unit 137 is effective to restore the high frequency band of the unvoiced sound component.

The nonlinearizer 138 is used to create a harmonic structure of voiced sound components, unlike the spectral folding unit 137. In general, since the voiced sound component is composed of a large number of harmonic structures, nonlinearization can be used to estimate the high frequency band of the voiced sound component. In detail, after the original signal is filtered through the low pass filter 138a to obtain only low frequency components, the low frequency band is included in the high frequency band by extending it to the high frequency band through the nonlinearization function (x (n) ² ) 138b. Extends the strong harmonics. Here, the non-linearization function may be applied in various forms in addition to the above.

The signals obtained by the spectral folding unit 137 and the nonlinearizing unit 138 are weighted and summed by being multiplied by α and 1-α in the weighting units 137b and 138c, respectively. Α denotes which of the spectral folding and non-linearization techniques are to be given more weight. The α may be estimated from the measured periodicity after measuring the pitch of the speech in consideration that the spectral folding technique and the nonlinearization technique are suitable for the unvoiced and voiced components, respectively. There are various methods for measuring the periodicity of speech, but in general, the gain value of the pitch filter used to process the periodicity in the speech codec may be used, and in addition, a normalized autocorrelation coefficient may be used.

The weighted summation signal as described above is filtered through the high pass filter 137d, down-sampled 137e twice, and the signal about the high band is estimated and output.

As described above, in the present invention, the spectral folding technique and the nonlinearization technique are applied together to improve the bandwidth extension.

Referring back to FIG. 2, portions B for adjusting the spectral envelope of the estimated high-band speech signal may include first and second discrete cosine transform units 131 and 139 and first and second cepstral coefficient extractors. 132 and 140, a mapping unit 133, and an envelope adjusting unit 141.

The first and second discrete cosine transform (DCT) units 131 and 139 convert the input voice signal into a frequency domain.

The spectral coefficient extracting units 132 and 140 extract the spectral coefficients representing the characteristics of the speech signal from the speech signal converted into the frequency domain.

As is well known, Mel-Frequency Cepstral Coefficients (MFCC) are coefficients used to characterize speech in the field of speech recognition. In the present invention, the characteristic of the speech signal is extracted using the linear frequency cepstral coefficient (hereinafter referred to as "LFCC") modified from the MFCC. In the present invention, since the linear scale frequency band is used, the characteristic of the speech signal is represented by using a linear frequency Histral coefficient named LFCC, which is distinguished from the conventional MFCC using Mel-scale units.

In the present invention, in extracting the LFCC, DCT can be used instead of Discrete Fourier Transform / Fast Fourier Transform (DFT / FFT). This is because the DCT can be implemented at a lower cost than the DFT / FFT.

In addition, referring to FIG. 4 illustrating a spectral window used in the present invention, the spectral window used in the process of extracting the LFCC has first and last flat edges. This spectral window form allows for improved spectral envelope adjustment compared to the window used for conventional MFCC / LFCC.

The discrete cosine transforming units 131 and 139 and the cepstral coefficient extracting units 132 and 140 respectively identify characteristics of an input speech signal and search for a high-band speech signal mapped from a narrowband speech signal and the high-band estimator. It is used to obtain the characteristics of the high band speech signal estimated from 136.

Specifically, the first discrete cosine transform unit 131 connected to the input stage converts the input narrowband speech signal into the frequency domain, and the first Histral coefficient extractor 132 converts the speech signal into the frequency domain. An LFCC indicating characteristics of the voice signal is output.

 Using the LFCC output from the first Histral coefficient extractor 132, the mapping unit 133 searches for a high-band speech signal corresponding to a narrowband speech signal input from a predetermined table not shown. . The high frequency speech signal corresponding to the narrowband speech signal is trained and stored in the table embedded in the mapping unit 133 in advance using a vector quantizer.

On the other hand, the high-band speech signal estimated by the high-band estimator 136 is converted into a frequency domain by the second discrete cosine transform unit 139, and the second Histral coefficient extracting unit 140 estimates the high-band speech signal. Outputs an LFCC indicating the characteristics of the high band speech signal.

The envelope adjusting unit 141 adjusts the spectrum of the estimated high-band speech signal so that the spectrum of the estimated high-band speech signal matches the high-band speech signal output from the mapping unit 133.

The high-band speech signal of the frequency domain adjusted by the envelope adjusting unit 141 is converted into a time-domain signal by an inverse discrete cosine transform (IDCT) 142 and finally, 4-8 (KHz) band. Output a high-band speech signal.

5 is a flowchart illustrating a method for extending a band of a voice signal according to the present invention.

Referring to FIG. 5, the method for extending a speech signal according to the present invention includes estimating a high band speech signal from a narrowly input narrowband speech signal (200) and corresponding to the pre-trained and stored narrowband speech signal. In step 250, the envelope of the estimated high-band speech signal is adjusted using information of the high-band speech signal.

First, in order to estimate the high-band speech signal, pitch information is obtained using the pitch filter 134 and the pitch inverse filter 134, and the pitch is removed by removing the pitch components included in the narrowband speech signal. (Step 202).

Next, by using the narrowband speech signal from which the pitch component has been removed, the spectral folding unit 137 sequentially performs upsampling, filtering using a high pass filter, and downsampling to sequentially spectrum symmetric to the original signal. Outputs a signal having a component, and the nonlinearizer 138 outputs a nonlinearized signal by performing an upsampling, nonlinearization, filtering using a high pass filter, and a downsampling process, and the spectral folding unit 137 ) And the high-band speech signal is estimated by the narrow-band speech signal input by linearly combining the signals output from the non-linearization unit 138 (step 204).

Next, a process for correcting the estimated high-band speech signal more accurately will be described. First, an input narrowband speech signal is converted into a frequency domain, and the LFCC indicating the characteristics of the narrowband speech signal is extracted as described above. . The mapping unit 133 outputs, to the envelope adjusting unit 141, the LFCC information related to the high-band speech signal corresponding to the input narrow-band speech signal pre-trained with the LFCC information and stored in a predetermined table (step 254). .

The envelope adjusting unit 141 uses the LFCC information output from the mapping unit 133 to convert the spectrum of the high band speech signal estimated by the high band estimating unit 136 into the high band speech signal of the mapping unit 133. The spectral envelope of the high band speech signal is adjusted to match (step 256).

The high-band speech signal, which has been estimated and adjusted by the above-described steps, is converted into the time domain by the inverse discrete transform unit and outputted. As described with reference to FIG. 1, the high-band speech signal is finally combined with the low-band speech signal. A wideband signal having a band of 0 to 8 (KHz) is output.

As such, it will be understood by those skilled in the art that the present invention may be implemented in a modified form without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments should be considered in descriptive sense only and not for purposes of limitation. The scope of the present invention is shown in the claims rather than the foregoing description, and all differences within the scope will be construed as being included in the present invention.

According to the present invention as described above, it is possible to effectively expand the band of the speech signal by estimating the highband speech signal from the narrowband speech signal using the spectral folding technique and the nonlinearization technique.

In addition, the present invention can be used at the receiving side of a communication system to provide an improved quality speech signal.

Claims (5)

In the band expansion device of a narrowband speech signal, Means for estimating a highband speech signal from the narrowband speech signal using spectral folding and nonlinearization techniques; And Means for adjusting the spectral envelope of the estimated high band speech signal. The method of claim 1, Means for estimating a highband speech signal from the narrowband speech signal, A spectral folding unit configured to output a signal having a mirror symmetrical spectral component with respect to the narrowband speech signal by performing upsampling, filtering using a high pass filter, and downsampling the narrowband speech signal; And And a nonlinearizer for nonlinearizing the narrowband speech signal, and linearly combining the spectral folding unit and an output signal of the nonlinearizer to estimate a highband speech signal from the narrowband speech signal. Bandwidth extension device for voice signals. The method of claim 1, Means for adjusting the spectral envelope of the estimated high band speech signal, A mapping unit for outputting a high band speech signal corresponding to the narrowband speech signal using a predetermined table; And And an envelope adjusting unit for adjusting a spectrum of the estimated high band speech signal to match the high band speech signal output from the mapping unit. The method of claim 3, wherein And the mapping unit and the envelope adjusting unit use characteristics of the speech signal provided by the linear frequency hysteresis coefficient (LFCC). In the method of extending the band of a narrowband speech signal, Removing a pitch component of the narrowband speech signal and estimating a highband speech signal from the narrowband speech signal from which the pitch component has been removed using spectral folding and nonlinearization techniques; And Extracts a linear frequency cepstral coefficient (LFCC) of the narrowband speech signal, retrieves a highband speech signal corresponding to the narrowband speech signal, and estimates the estimated highband speech to match the found highband speech signal. Adjusting the spectral envelope of the signal.

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