JP3230790B2 - Wideband audio signal restoration method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for generating a wideband audio signal from a narrowband audio signal. More specifically, the present invention relates to a method for generating a narrowband audio signal which is currently output as telephone voice or AM radio. The present invention relates to a method for improving the quality of a wideband audio signal output from an audio set, FM radio, or the like.

[0002]

2. Description of the Related Art A case of telephone voice will be described as an example of a narrowband voice signal. The signal band that can be transmitted by the existing telephone system is about 300 Hz to 3.4 kHz. However, in recent years, voices who are not satisfied with the sound quality and desire higher quality are increasing. As a method of solving this demand, it is conceivable to destroy the existing telephone system and reconstruct a telephone system capable of transmitting a wideband signal. Doing so would take a considerable amount of time. From this point, as can be seen in JP-A-6-118995, by restoring a signal that exists in a wideband signal but does not exist in a narrowband signal from a narrowband signal,
It has been proposed to generate a wideband signal. This method is useful because broadband speech can be obtained regardless of the configuration of the telephone network.

The method disclosed in the above publication will be briefly described with reference to FIG. Input narrowband audio signal is LPC
The analysis unit 101 performs an LPC analysis, and the analysis result is subjected to fuzzy vector quantization in a quantization unit 102 using a codebook 103 of a narrowband audio signal. Fuzzy vector quantization is described in, for example, H. Tseng,
M. Sabin, E .; Lee, "Fuzzy Vect
or Quantization Applied t
o Hidden Markov Modelin
g, "ICASSP '87 15.5, Apr. 198
7. It is described in. Quantization unit 3 to reduce the amount of calculation
The processing of 02 may be ordinary vector quantization. The decoding unit 104 decodes the code subjected to the fuzzy vector quantization using the codebook 105 of the wideband audio signal.

[0004] The decoded output X 'is the spectrum envelope of the voice, and is used by the voice synthesizer 106 to generate an LPC.
Combine to obtain a wideband audio signal. That is, the synthesis filter is driven by the excitation signal corresponding to the pitch obtained from the analysis result of the LPC analysis unit 101, and the filter coefficient of the synthesis filter is controlled according to the decoded output X '.
To obtain a synthesized speech signal having a level corresponding to the power obtained from the analysis result. The synthesized audio signal may be output as a restored wideband audio signal.

[0005] The wideband audio signal obtained by the above processing is
Although the input narrowband audio signal includes a signal that does not exist outside the band and is a wideband signal, the input narrowband audio signal also includes a signal different from the input narrowband audio signal within the band. If the audio signal is used as it is, there is a side effect that the signal existing in the input narrowband audio signal is distorted. Therefore, the following processing is performed, and the signal existing in the original input narrowband audio signal is used as it is. That is, the wideband audio signal synthesized by LPC synthesis section 106 is supplied to bandpass filter 107, and the out-of-band component of the input narrowband audio signal, that is, a frequency component of 300 Hz or less and a frequency component of 3.4 kHz or more are extracted. It is. On the other hand, the input narrowband audio signal is upsampled by the upsampling unit 108 to an 8 kHz band. The output of the up-sampling unit 108 and the output of the bandpass filter 107 are combined by the adding unit 109,
A restored wideband audio signal is obtained. In the upsampling, for example, a zero sample is inserted between each sample point, passes through an all-pass filter, and its output is sampled at twice the speed to double the frequency band.

A code book 10 for a wideband audio signal
Reference numeral 5 is created using a large number of wideband speech signals for learning, for example, in the same manner as a codebook used for vector quantization of speech. The codebook 103 of the narrowband audio signal is the codebook 1 of the wideband audio signal.
05, and its preparation method is described in the above-mentioned publication.

[0007]

According to the method disclosed in the above-mentioned publication, LPC synthesis is performed.
A pitch, that is, a fundamental frequency of voice is extracted from the analysis, and an impulse train of the fundamental frequency is used as a sound source signal. However, there are various methods for extracting the fundamental frequency of voice, but it is difficult to accurately extract the fundamental frequency. For this reason, the method of the above-mentioned publication cannot restore a high-quality wideband audio signal.

[0008]

According to the present invention, an input narrow band signal is encoded by an encoding method based on an analysis method by synthesis, and various signals in the encoding obtained as a final result of the encoding are obtained. Use That is, claim 1
According to the invention, the sound source signal for speech synthesis finally used in the encoding is used as a sound source signal for restoring a wideband speech signal. In the invention of claim 2, the synthesized speech signal finally used in the encoding is used as a sound source signal for restoring a wideband speech signal. In the invention of claim 5,
A low-band component is added to the input narrow-band audio signal from the synthesized audio signal finally used in the encoding to obtain a wide-band audio signal.

[0009]

According to the present invention, a narrowband audio signal is encoded by an encoding method based on an analysis method based on synthesis, and a wideband audio signal is restored by using a signal finally obtained by the encoding. In addition, it is not necessary to explicitly extract the fundamental frequency from the input narrowband audio signal, and the influence of the fundamental frequency extraction error can be avoided.

[0010]

FIG. 1 shows an embodiment of the first to third aspects of the present invention, in which parts corresponding to those in FIG. 4 are denoted by the same reference numerals. According to the present invention, the input narrowband audio signal is
It is encoded by an encoding method based on an analysis method by synthesis.
Coding methods based on analysis by synthesis are CELP, VSE
LP, etc., and basically the input narrowband audio signal is LP
C analysis is performed, and the spectral parameter as the analysis result is vector-quantized. This vector quantization can use the quantized output of the vector quantization unit 102 using the codebook 103 of the narrowband audio signal. The filter coefficient of the LPC synthesis section 202 in the coding section 201 is set by the quantized spectrum. LPC synthesis section 202 is driven by the sound source signal searched from sound source signal search section 203. The difference between the audio signal synthesized by the LPC synthesis unit 202 and the input narrowband audio signal is obtained by a difference circuit 204, and the difference calculation unit 205 calculates a distortion of a scale based on the difference, and minimizes this distortion. Then, the sound source signal in the sound source signal search unit 203 is searched. The parameters indicating the excitation signal, the gain, and the like when the distortion is minimized in this way, and the quantized spectrum become the encoded output.

In the present invention, the signal finally used in the encoding is used for wideband speech signal restoration. That is, according to the first aspect of the present invention, the sound source signal searched by the sound source search unit 203 is zero-padded by the zero padding processing unit 206 to generate a broadband sound source signal. For example, in order to change from a 4 kHz band sound source signal to an 8 kHz band sound source signal, zero is filled for each point (one sample). This zero-padded sound source signal is supplied as a sound source signal to LPC combining section 106 in the configuration shown in FIG. Other processes are the same as those in FIG.

According to the second aspect of the present invention, as shown by the broken line in FIG.
The synthesized speech signal from the synthesizing section 202 is zero-padded to
6 to obtain a wideband signal, which is
06 as a sound source signal. In the invention of claim 4, as shown in FIG. 2, the invention of claim 1 or 2 is premised. That is, the excitation signal or the synthesized speech signal in the final encoding of the encoding section 201 is converted by the LPC synthesis section 106. However, only the high frequency component of the wideband audio signal obtained by the LPC synthesis unit 106 is used for restoring the wideband audio signal. That is, the LPC synthesis unit 106 is
, A component higher than the input narrow-band audio signal, for example, a component of 3.4 kHz or higher, is extracted and supplied to the adding unit 109. On the other hand, the synthesized audio signal from the LPC synthesizing unit 202 in the final encoding of the encoding unit 201 is supplied to a low-pass filter 208, and a low-frequency component, for example, a component of 306 Hz or lower than the input narrow-band audio signal is extracted. The extracted low-frequency component is amplified by the amplification unit 209, and the amplified low-frequency signal is up-sampled by the up-sampling unit 210. This upsampling is performed in the same manner as the upsampling unit 108. The up-sampled low-frequency component is supplied to the adder 109, and the high-pass filter 2
07 and the narrowband audio signal from the up-sampling unit 108 are added to obtain a restored wideband audio signal. In this example, the analysis result of the LPC analysis unit 101 is quantized by the quantization unit 211 of the encoding unit 201 and supplied to the LPC synthesis unit 202 as a filter coefficient, and the quantization unit 102 performs fuzzy vector quantization. .

According to the fifth aspect of the present invention, in the case of restoring only a low-frequency component of the input narrow-band audio signal and restoring a wide-band audio signal, the same reference numerals in FIG. 3 denote parts corresponding to those in FIG. 2 is omitted, and the restored output of the low-frequency component from the encoding unit 201, that is, the output of the up-sampling unit 210 and the input narrow-band audio signal from the up-sampling unit 108 are omitted. A wideband audio signal restored by the addition in the addition unit 109 is obtained.

In the embodiment shown in FIG. 1, the output of the LPC synthesizing section 106 may be a restored wideband audio signal. The input narrow-band audio signal is not limited to a telephone audio signal, but may be a decoded audio signal, such as that used in a mobile phone. In this case, the excitation signal selected by the decoder or the synthesized audio signal as the excitation signal or the synthesized audio signal finally used in the encoding may be supplied to the zero padding processing unit 206, and encoding is not particularly required. . In the above description, the codebook 10 of the narrowband audio signal is used.
3 and the codebook 105 of the wideband audio signal, but the present invention can be applied to a case where the audio spectrum is restored by another method.

[0015]

As described above, according to the present invention, there is no need to extract the fundamental frequency of the input narrowband audio signal.
There is no quality deterioration due to erroneous extraction of the fundamental frequency, and a high-quality wideband audio signal can be restored accordingly.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a processing procedure according to an embodiment of each of the first to third aspects of the present invention.

FIG. 2 is a flowchart showing a processing procedure according to an embodiment of the present invention.

FIG. 3 is a flowchart showing a processing procedure according to an embodiment of the invention of claim 5;

FIG. 4 is a flowchart showing a processing procedure of a conventional wideband audio signal restoring method.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G10L 19/08

Claims (6)

(57) [Claims] 1. A wide-band audio signal restoration method for generating and outputting a wide-band audio signal from an input narrow-band audio signal, wherein the input narrow-band audio signal is encoded by an encoding method based on an analysis method by synthesis. A wideband audio signal restoring method characterized in that a sound source signal for speech synthesis finally used in (1) is used as a sound source signal for restoring the wideband audio signal. 2. A wide-band audio signal restoration method for generating and outputting a wide-band audio signal from an input narrow-band audio signal, wherein said input narrow-band audio signal is encoded by an encoding method based on an analysis method by synthesis. A wideband audio signal restoring method characterized in that the synthesized audio signal finally used in (1) is used as a sound source signal for restoring the wideband audio signal. 3. An out-of-band component of the input narrow-band audio signal is extracted from a wide-band audio signal synthesized using the sound source signal, and the extracted out-of-band component is added to the input narrow-band audio signal. 3. The wideband audio signal restoring method according to claim 1, wherein the restored wideband audio signal is obtained. 4. A low-frequency component is extracted from the input narrow-band audio signal from the synthesized audio signal finally used in the encoding, and the input narrow-band is extracted from a wide-band audio signal synthesized using the sound source signal. Extracting a high-frequency component from the audio signal, adding the extracted low-frequency component, the extracted high-frequency component, and the input narrowband audio signal to obtain the restored wideband audio signal. 3. The method for restoring a wideband audio signal according to claim 1, wherein 5. A wide-band audio signal restoration method for generating and outputting a wide-band audio signal from an input narrow-band audio signal, wherein the input narrow-band audio signal is encoded by an encoding method based on an analysis method by synthesis. A wideband audio signal restoring method characterized by adding a lower frequency component of the synthesized narrowband signal than the input narrowband signal to the input narrowband audio signal to obtain the wideband audio signal. 6. The input narrow-band audio signal is obtained by decoding an encoded audio signal encoded by an encoding method based on an analysis method based on synthesis, and includes a sound source signal finally used in the encoding. 2. The method according to claim 1, wherein the audio signal is a sound source signal used for decoding the encoded audio signal.