Method and apparatus for speech coding

Technical Field

This invention relates to methods for speech decoding and apparatuses for speech decoding. Particularly, this invention relates to a method for speech decoding and apparatus for speech decoding for reproducing a high quality speech at low bit rates.

In the related art, code-excited linear prediction (Code-Excited Linear Prediction: CELP) coding is well-known as an efficient speech coding method, and its technique is described in "<nplcit id="ncit0001" npl-type="s"><text>Code-excited linear prediction (CELP): High-quality speech at very low bit rates," ICASSP '85, pp. 937 - 940, by M. R. Shroeder and B. S. Atal in 1985</text></nplcit>.

<figref idref="f0006">Fig. 6</figref> illustrates an example of a whole configuration of a CELP speech coding and decoding method. In <figref idref="f0006">Fig. 6</figref>, an encoder 101, decoder 102, multiplexing means 103, and dividing means 104 are illustrated.

The encoder 101 includes a linear prediction parameter analyzing means 105, linear prediction parameter coding means 106, synthesis filter 107, adaptive codebook 108, excitation codebook 109, gain coding means 110, distance calculating means 111, and weighting-adding means 138. The decoder 102 includes a linear prediction parameter decoding means 112, synthesis filter 113, adaptive codebook 114, excitation codebook 115, gain decoding means 116, and weighting-adding means 139.

In CELP speech coding, a speech in a frame of about 5 - 50 ms is divided into spectrum information and excitation information, and coded.

Explanations are made on operations in the CELP speech coding method. In the encoder 101, the linear prediction parameter analyzing means 105 analyzes an input speech S101, and extracts a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameter coding means 106 codes the linear prediction parameter, and sets a coded linear prediction parameter as a coefficient for the synthesis filter 107.

Explanations are made on coding of excitation information.

An old excitation signal is stored in the adaptive codebook 108. The adaptive codebook 108 outputs a time series vector, corresponding to an adaptive code inputted by the distance calculator 111, which is generated by repeating the old excitation signal periodically.

A plurality of time series vectors trained by reducing a distortion between a speech for training and its coded speech for example is stored in the excitation codebook 109. The excitation codebook 109 outputs a time series vector corresponding to an excitation code inputted by the distance calculator 111.

Each of the time series vectors outputted from the adaptive codebook 108 and excitation codebook 109 is weighted by using a respective gain provided by the gain coding means 110 and added by the weighting-adding means 138. Then, an addition result is provided to the synthesis filter 107 as excitation signals, and a coded speech is produced. The distance calculating means 111 calculates a distance between the coded speech and the input speech S101, and searches an adaptive code, excitation code, and gains for minimizing the distance. When the above-stated coding is over, a linear prediction parameter code and the adaptive code, excitation code, and gain codes for minimizing a distortion between the input speech and the coded speech are outputted as a coding result.

Explanations are made on operations in the CELP speech decoding method.

In the decoder 102, the linear prediction parameter decoding means 112 decodes the linear prediction parameter code to the linear prediction parameter, and sets the linear prediction parameter as a coefficient for the synthesis filter 113. The adaptive codebook 114 outputs a time series vector corresponding to an adaptive code, which is generated by repeating an old excitation signal periodically. The excitation codebook 115 outputs a time series vector corresponding to an excitation code. The time series vectors are weighted by using respective gains, which are decoded from the gain codes by the gain decoding means 116, and added by the weighting-adding means 139. An addition result is provided to the synthesis filter 113 as an excitation signal, and an output speech S103 is produced.

Among the CELP speech coding and decoding method, an improved speech coding and decoding method for reproducing a high quality speech according to the related art is described in "<nplcit id="ncit0002" npl-type="s"><text>Phonetically - based vector excitation coding of speech at 3.6 kbps," ICASSP '89, pp. 49 - 52, by S. Wang and A. Gersho in 1989</text></nplcit>.

<figref idref="f0007">Fig. 7</figref> shows an example of a whole configuration of the speech coding and decoding method according to the related art, and same signs are used for means corresponding to the means in <figref idref="f0006">Fig. 6</figref>.

In <figref idref="f0007">Fig. 7</figref>, the encoder 101 includes a speech state deciding means 117, excitation codebook switching means 118, first excitation codebook 119, and second excitation codebook 120. The decoder 102 includes an excitation codebook switching means 121, first excitation codebook 122, and second excitation codebook 123.

Explanations are made on operations in the coding and decoding method in this configuration. In the encoder 101, the speech state deciding means 117 analyzes the input speech S101, and decides a state of the speech is which one of two states, e.g., voiced or unvoiced. The excitation codebook switching means 118 switches the excitation codebooks to be used in coding based on a speech state deciding result. For example, if the speech is voiced, the first excitation codebook 119 is used, and if the speech is unvoiced, the second excitation codebook 120 is used. Then, the excitation codebook switching means 118 codes which excitation codebook is used in coding.

In the decoder 102, the excitation codebook switching means 121 switches the first excitation codebook 122 and the second excitation codebook 123 based on a code showing which excitation codebook was used in the encoder 101, so that the excitation codebook, which was used in the encoder 101, is used in the decoder 102. According to this configuration, excitation codebooks suitable for coding in various speech states are provided, and the excitation codebooks are switched based on a state of an input speech. Hence, a high quality speech can be reproduced.

A speech coding and decoding method of switching a plurality of excitation codebooks without increasing a transmission bit number according to the related art is disclosed in Japanese Unexamined Published Patent Application <patcit id="pcit0001" dnum="JP8185198A"><text>8 - 185198</text></patcit>. The plurality of excitation codebooks is switched based on a pitch frequency selected in an adaptive codebook, and an excitation codebook suitable for characteristics of an input speech can be used without increasing transmission data.

As stated, in the speech coding and decoding method illustrated in <figref idref="f0006">Fig. 6</figref> according to the related art, a single excitation codebook is used to produce a synthetic speech. Non-noise time series vectors with many pulses should be stored in the excitation codebook to produce a high quality coded speech even at low bit rates. Therefore, when a noise speech, e.g., background noise, fricative consonant, etc., is coded and synthesized, there is a problem that a coded speech produces an unnatural sound, e.g., "Jiri-Jiri" and "Chiri-Chiri." This problem can be solved, if the excitation codebook includes only noise time series vectors. However, in that case, a quality of the coded speech degrades as a whole.

In the improved speech coding and decoding method illustrated in <figref idref="f0007">Fig. 7</figref> according to the related art, the plurality of excitation codebooks is switched based on the state of the input speech for producing a coded speech. Therefore, it is possible to use an excitation codebook including noise time series vectors in an unvoiced noise period of the input speech and an excitation codebook including non-noise time series vectors in a voiced period other than the unvoiced noise period, for example. Hence, even if a noise speech is coded and synthesized, an unnatural sound, e.g., "Jiri-Jiri," is not produced. However, since the excitation codebook used in coding is also used in decoding, it becomes necessary to code and transmit data which excitation codebook was used. It becomes an obstacle for lowing bit rates.

According to the speech coding and decoding method of switching the plurality of excitation codebooks without increasing a transmission bit number according to the related art, the excitation codebooks are switched based on a pitch period selected in the adaptive codebook. However, the pitch period selected in the adaptive codebook differs from an actual pitch period of a speech, and it is impossible to decide if a state of an input speech is noise or non-noise only from a value of the pitch period. Therefore, the problem that the coded speech in the noise period of the speech is unnatural cannot be solved.

This invention was intended to solve the above-stated problems. Particularly, this invention aims at providing speech coding and decoding methods and apparatuses for reproducing a high quality speech even at low bit rates.

Disclosure of the Invention

According to the first aspect of the present invention a speech decoding apparatus according to code-excited linear prediction which receives a coded speech including a gain code an synthesizes a speech, comprises: a gain decoder for inputting the gain code and for decoding a gain of a speech in a concerning decoding period based on the gain code input; a noise level evaluator for evaluating a noise level of the speech in the concerning decoding period by using the gain decoded by the gain decoders; an excitation codebook storing time series vectors; and a noise level controller for changing a noise level of the time series vectors output from the excitation codebook based on an evaluation result of the noise level evaluator.

According to the second aspect of the present invention a speech decoding apparatus according to code-excited linear prediction, which receives a coded speech including a linear prediction parameter code, an adaptive code, an excitation code, and a gain code and synthesizes a speech, comprises: an adaptive codebook, which stores an old excitation signal, for inputting the adaptive code and for outputting a time series vector corresponding to the old excitation signal based on the adaptive code input; an excitation codebook, which stores a plurality of time series vectors corresponding to a plurality of predetermined excitation signals, for inputting the excitation code and for outputting a time series vector corresponding to an excitation signal based on the excitation code input; a gain decoder for inputting the gain code and decoding a gain of a speech in a concerning decoding period from the gain code input; a noise level evaluator for inputting the gain decoded by the gain decoder and for evaluating a noise level of the speech in the concerning decoding period by using the gain input; a noise level controller for inputting an evaluation result of the noise level evaluator and the time series vector output from the excitation codebook and for changing a noise level of the time series vector output from the excitation codebook based on the evaluation result of the noise level evaluator; a weighting-adder for inputting the time series vector output from the adaptive codebook and the time series vector output from the excitation codebook and the gain decoded by the gain decoder, for weighting the time series vector output from the adaptive codebook and the time series vector output from the adaptive codebook and the time series vector output from the excitation codebook by using the gain, for adding a time series vector weighted by using the gain and a time series vector weighted by using the gain, and for outputting an addition result, a linear prediction parameter decoder for inputting the linear prediction parameter code and for decoding and outputting a linear prediction parameter from the linear prediction parameter code input; and a synthesis filter for inputting the linear prediction parameter output from the linear prediction parameter decoder and the addition result output from the weighting-adder and for synthesizing the speech using the linear prediction parameter and the addition result.

According to the third aspect of the present invention a speech decoding method according to code-excited linear prediction, which receives a coded speech including a gain code and synthesizes a speech, comprises: inputting the gain code and decoding a gain of a speech in a concerning decoding period from the gain code; evaluating a noise level of the speech in the concerning decoding period by using the gain decoded; providing an excitation codebook storing time series vectors; and changing a noise level of the time series vectors output from the excitation codebook based on an evaluation result.

According to the fourth aspect of the present invention a speech decoding method according to code-excited linear prediction for decoding a coded speech including a linear prediction parameter code, an adaptive code, an excitation code, and a gain code and synthesizing a speech, comprises: inputting the adaptive code to an adaptive codebook, which stores an old excitation signal, and outputting, from the adaptive codebook, a time series vector corresponding to the old excitation signal based on the adaptive code input; providing an excitation codebook, which stores a plurality of time series vectors corresponding to a plurality of predetermined excitation signals; inputting the excitation code to the excitation codebook and outputting, from the excitation codebook, a time series vector corresponding to an excitation signal based on the excitation code input; inputting the gain code and decoding a gain of a speech in a concerning decoding period from the gain code input; inputting the gain decoded an evaluating a noise level of the speech in the concerning decoding period by using the gain input; inputting an evaluation result and time series vector output from the excitation codebook, and changing a noise level of the time series vector output from the excitation codebook based on the evaluation result; inputting the time series vector output from the adaptive codebook and the time series vector output from the excitation codebook and the gain decoded, weighting the time series vector output from the adaptive codebook and the time series vector output from the excitation codebook by using the gain, adding a time series vector weighting by using the gain and a time series vector weighted by using the gain, and outputting an addition result, inputting the linear prediction parameter code, and decoding and outputting a linear prediction parameter from the linear prediction parameter code input; and inputting the linear prediction parameter output and the addition result output and synthesizing the speech using the linear prediction parameter and the addition result.

Brief Description of the Drawings

<dl id="dl0001"><dt>Fig. 1</dt><dd>shows a block diagram of a whole configuration of a speech coding and speech decoding apparatus in embodiment 1 of this invention.</dd><dt>Fig. 2</dt><dd>shows a table for explaining an evaluation of a noise level in embodiment 1 of this invention illustrated in <figref idref="f0001">Fig. 1</figref>.</dd><dt>Fig. 3</dt><dd><figref idref="f0003">Fig. 3</figref> shows a block diagram of a whole configuration of a speech coding and speech decoding apparatus in embodiment 3 of this invention.</dd><dt>Fig. 4</dt><dd><figref idref="f0004">Fig. 4</figref> shows a block diagram of a whole configuration of a speech coding and speech decoding apparatus in embodiment 5 of this invention.</dd><dt>Fig. 5</dt><dd><figref idref="f0005">Fig. 5</figref> shows a schematic line chart for explaining a decision process of weighting in embodiment 5 illustrated in <figref idref="f0004">Fig. 4</figref>.</dd><dt>Fig. 6</dt><dd><figref idref="f0006">Fig. 6</figref> shows a block diagram of a whole configuration of a CELP speech coding and decoding apparatus according to the related art.</dd><dt>Fig. 7</dt><dd>shows a block diagram of a whole configuration of an improved CELP speech coding and decoding apparatus according to the related art.</dd></dl>

Best Mode for Carrying Out the Invention Explanations are made on embodiments of this invention with reference to drawings.

Embodiment 1.

<figref idref="f0001">Fig. 1</figref> illustrates a whole configuration of a speech coding method and speech decoding method in embodiment 1 according to this invention. In <figref idref="f0001">Fig. 1</figref>, an encoder 1, a decoder 2, a multiplexer 3, and a divider 4 are illustrated. The encoder 1 includes a linear prediction parameter analyzer 5, linear prediction parameter encoder 6, synthesis filter 7, adaptive codebook 8, gain encoder 10, distance calculator 11, first excitation codebook 19, second excitation codebook 20, noise level evaluator 24, excitation codebook switch 25, and weighting-adder 38. The decoder 2 includes a linear prediction parameter decoder 12, synthesis filter 13, adaptive codebook 14, first excitation codebook 22, second excitation codebook 23, noise level evaluator 26, excitation codebook switch 27, gain decoder 16, and weighting-adder 39. In <figref idref="f0001">Fig. 1</figref>, the linear prediction parameter analyzer 5 is a spectrum information analyzer for analyzing an input speech S1 and extracting a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameter encoder 6 is a spectrum information encoder for coding the linear prediction parameter, which is the spectrum information and setting a coded linear prediction parameter as a coefficient for the synthesis filter 7. The first excitation codebooks 19 and 22 store pluralities of non-noise time series vectors, and the second excitation codebooks 20 and 23 store pluralities of noise time series vectors. The noise level evaluators 24 and 26 evaluate a noise level, and the excitation codebook switches 25 and 27 switch the excitation codebooks based on the noise level.

Operations are explained.

In the encoder 1, the linear prediction parameter analyzer 5 analyzes the input speech S1, and extracts a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameter encoder 6 codes the linear prediction parameter. Then, the linear prediction parameter encoder 6 sets a coded linear prediction parameter as a coefficient for the synthesis filter 7, and also outputs the coded linear prediction parameter to the noise level evaluator 24.

Explanations are made on coding of excitation information.

An old excitation signal is stored in the adaptive codebook 8, and a time series vector corresponding to an adaptive code inputted by the distance calculator 11, which is generated by repeating an old excitation signal periodically, is outputted. The noise level evaluator 24 evaluates a noise level in a concerning coding period based on the coded linear prediction parameter inputted by the linear prediction parameter encoder 6 and the adaptive code, e.g., a spectrum gradient, short-term prediction gain, and pitch fluctuation as shown in <figref idref="f0002">Fig. 2</figref>, and outputs an evaluation result to the excitation codebook switch 25. The excitation codebook switch 25 switches excitation codebooks for coding based on the evaluation result of the noise level. For example, if the noise level is low, the first excitation codebook 19 is used, and if the noise level is high, the second excitation codebook 20 is used.

The first excitation codebook 19 stores a plurality of non-noise time series vectors, e.g., a plurality of time series vectors trained by reducing a distortion between a speech for training and its coded speech. The second excitation codebook 20 stores a plurality of noise time series vectors, e.g., a plurality of time series vectors generated from random noises. Each of the first excitation codebook 19 and the second excitation codebook 20 outputs a time series vector respectively corresponding to an excitation code inputted by the distance calculator 11. Each of the time series vectors from the adaptive codebook 8 and one of first excitation codebook 19 or second excitation codebook 20 are weighted by using a respective gain provided by the gain encoder 10, and added by the weighting-adder 38. An addition result is provided to the synthesis filter 7 as excitation signals, and a coded speech is produced. The distance calculator 11 calculates a distance between the coded speech and the input speech S1, and searches an adaptive code, excitation code, and gain for minimizing the distance. When this coding is over, the linear prediction parameter code and an adaptive code, excitation code, and gain code for minimizing the distortion between the input speech and the coded speech are outputted as a coding result S2. These are characteristic operations in the speech coding method in embodiment 1.

Explanations are made on the decoder 2. In the decoder 2, the linear prediction parameter decoder 12 decodes the linear prediction parameter code to the linear prediction parameter, and sets the decoded linear prediction parameter as a coefficient for the synthesis filter 13, and outputs the decoded linear prediction parameter to the noise level evaluator 26.

Explanations are made on decoding of excitation information. The adaptive codebook 14 outputs a time series vector corresponding to an adaptive code, which is generated by repeating an old excitation signal periodically. The noise level evaluator 26 evaluates a noise level by using the decoded linear prediction parameter inputted by the linear prediction parameter decoder 12 and the adaptive code in a same method with the noise level evaluator 24 in the encoder 1, and outputs an evaluation result to the excitation codebook switch 27. The excitation codebook switch 27 switches the first excitation codebook 22 and the second excitation codebook 23 based on the evaluation result of the noise level in a same method with the excitation codebook switch 25 in the encoder 1.

A plurality of non-noise time series vectors, e.g., a plurality of time series vectors generated by training for reducing a distortion between a speech for training and its coded speech, is stored in the first excitation codebook 22. A plurality of noise time series vectors, e.g., a plurality of vectors generated from random noises, is stored in the second excitation codebook 23. Each of the first and second excitation codebooks outputs a time series vector respectively corresponding to an excitation code. The time series vectors from the adaptive codebook 14 and one of first excitation codebook 22 or second excitation codebook 23 are weighted by using respective gains, decoded from gain codes by the gain decoder 16, and added by the weighting-adder 39. An addition result is provided to the synthesis filter 13 as an excitation signal, and an output speech S3 is produced. These are operations are characteristic operations in the speech decoding method in embodiment 1.

In embodiment 1, the noise level of the input speech is evaluated by using the code and coding result, and various excitation codebooks are used based on the evaluation result. Therefore, a high quality speech can be reproduced with a small data amount.

In embodiment 1, the plurality of time series vectors is stored in each of the excitation codebooks 19, 20, 22, and 23. However, this embodiment can be realized as far as at least a time series vector is stored in each of the excitation codebooks.

Embodiment 2.

In embodiment 1, two excitation codebooks are switched. However, it is also possible that three or more excitation codebooks are provided and switched based on a noise level.

In embodiment 2, a suitable excitation codebook can be used even for a medium speech, e.g., slightly noisy, in addition to two kinds of speech, i.e., noise and non-noise. Therefore, a high quality speech can be reproduced.

Embodiment 3.

<figref idref="f0003">Fig. 3</figref> shows a whole configuration of a speech coding method and speech decoding method in embodiment 3 of this invention. In <figref idref="f0003">Fig. 3</figref>, same signs are used for units corresponding to the units in <figref idref="f0001">Fig. 1</figref>. In <figref idref="f0003">Fig. 3</figref>, excitation codebooks 28 and 30 store noise time series vectors, and samplers 29 and 31 set an amplitude value of a sample with a low amplitude in the time series vectors to zero.

Operations are explained. In the encoder 1, the linear prediction parameter analyzer 5 analyzes the input speech S1, and extracts a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameter encoder 6 codes the linear prediction parameter. Then, the linear prediction parameter encoder 6 sets a coded linear prediction parameter as a coefficient for the synthesis filter 7, and also outputs the coded linear prediction parameter to the noise level evaluator 24.

Explanations are made on coding of excitation information. An old excitation signal is stored in the adaptive codebook 8, and a time series vector corresponding to an adaptive code inputted by the distance calculator 11, which is generated by repeating an old excitation signal periodically, is outputted. The noise level evaluator 24 evaluates a noise level in a concerning coding period by using the coded linear prediction parameter, which is inputted from the linear prediction parameter encoder 6, and an adaptive code, e.g., a spectrum gradient, short-term prediction gain, and pitch fluctuation, and outputs an evaluation result to the sampler 29.

The excitation codebook 28 stores a plurality of time series vectors generated from random noises, for example, and outputs a time series vector corresponding to an excitation code inputted by the distance calculator 11. If the noise level is low in the evaluation result of the noise, the sampler 29 outputs a time series vector, in which an amplitude of a sample with an amplitude below a determined value in the time series vectors, inputted from the excitation codebook 28, is set to zero, for example. If the noise level is high, the sampler 29 outputs the time series vector inputted from the excitation codebook 28 without modification. Each of the times series vectors from the adaptive codebook 8 and the sampler 29 is weighted by using a respective gain provided by the gain encoder 10 and added by the weighting-adder 38. An addition result is provided to the synthesis filter 7 as excitation signals, and a coded speech is produced. The distance calculator 11 calculates a distance between the coded speech and the input speech S1, and searches an adaptive code, excitation code, and gain for minimizing the distance. When coding is over, the linear prediction parameter code and the adaptive code, excitation code, and gain code for minimizing a distortion between the input speech and the coded speech are outputted as a coding result S2. These are characteristic operations in the speech coding method in embodiment 3.

Explanations are made on the decoder 2. In the decoder 2, the linear prediction parameter decoder 12 decodes the linear prediction parameter code to the linear prediction parameter. The linear prediction parameter decoder 12 sets the linear prediction parameter as a coefficient for the synthesis filter 13, and also outputs the linear prediction parameter to the noise level evaluator 26.

Explanations are made on decoding of excitation information. The adaptive codebook 14 outputs a time series vector corresponding to an adaptive code, generated by repeating an old excitation signal periodically. The noise level evaluator 26 evaluates a noise level by using the decoded linear prediction parameter inputted from the linear prediction parameter decoder 12 and the adaptive code in a same method with the noise level evaluator 24 in the encoder 1, and outputs an evaluation result to the sampler 31.

The excitation codebook 30 outputs a time series vector corresponding to an excitation code. The sampler 31 outputs a time series vector based on the evaluation result of the noise level in same processing with the sampler 29 in the encoder 1. Each of the time series vectors outputted from the adaptive codebook 14 and sampler 31 are weighted by using a respective gain provided by the gain decoder 16, and added by the weighting-adder 39. An addition result is provided to the synthesis filter 13 as an excitation signal, and an output speech S3 is produced.

In embodiment 3, the excitation codebook storing noise time series vectors is provided, and an excitation with a low noise level can be generated by sampling excitation signal samples based on an evaluation result of the noise level the speech. Hence, a high quality speech can be reproduced with a small data amount. Further, since it is not necessary to provide a plurality of excitation codebooks, a memory amount for storing the excitation codebook can be reduced.

Embodiment 4.

In embodiment 3, the samples in the time series vectors are either sampled or not. However, it is also possible to change a threshold value of an amplitude for sampling the samples based on the noise level. In embodiment 4, a suitable time series vector can be generated and used also for a medium speech, e.g., slightly noisy, in addition to the two types of speech, i.e., noise and non-noise. Therefore, a high quality speech can be reproduced.

Embodiment 5.

<figref idref="f0004">Fig. 4</figref> shows a whole configuration of a speech coding method and a speech decoding method in embodiment 5 of this invention, and same signs are used for units corresponding to the units in <figref idref="f0001">Fig. 1</figref>.

In <figref idref="f0004">Fig. 4</figref>, first excitation codebooks 32 and 35 store noise time series vectors, and second excitation codebooks 33 and 36 store non-noise time series vectors. The weight determiners 34 and 37 are also illustrated.

Operations are explained. In the encoder 1, the linear prediction parameter analyzer 5 analyzes the input speech S1, and extracts a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameter encoder 6 codes the linear prediction parameter. Then, the linear prediction parameter encoder 6 sets a coded linear prediction parameter as a coefficient for the synthesis filter 7, and also outputs the coded prediction parameter to the noise level evaluator 24.

Explanations are made on coding of excitation information. The adaptive codebook 8 stores an old excitation signal, and outputs a time series vector corresponding to an adaptive code inputted by the distance calculator 11, which is generated by repeating an old excitation signal periodically. The noise level evaluator 24 evaluates a noise level in a concerning coding period by using the coded linear prediction parameter, which is inputted from the linear prediction parameter encoder 6 and the adaptive code, e.g., a spectrum gradient, short-term prediction gain, and pitch fluctuation, and outputs an evaluation result to the weight determiner 34.

The first excitation codebook 32 stores a plurality of noise time series vectors generated from random noises, for example, and outputs a time series vector corresponding to an excitation code. The second excitation codebook 33 stores a plurality of time series vectors generated by training for reducing a distortion between a speech for training and its coded speech, and outputs a time series vector corresponding to an excitation code inputted by the distance calculator 11. The weight determiner 34 determines a weight provided to the time series vector from the first excitation codebook 32 and the time series vector from the second excitation codebook 33 based on the evaluation result of the noise level inputted from the noise level evaluator 24, as illustrated in <figref idref="f0005">Fig. 5</figref>, for example. Each of the time series vectors from the first excitation codebook 32 and the second excitation codebook 33 is weighted by using the weight provided by the weight determiner 34, and added. The time series vector outputted from the adaptive codebook 8 and the time series vector, which is generated by being weighted and added, are weighted by using respective gains provided by the gain encoder 10, and added by the weighting-adder 38. Then, an addition result is provided to the synthesis filter 7 as excitation signals, and a coded speech is produced. The distance calculator 11 calculates a distance between the coded speech and the input speech S1, and searches an adaptive code, excitation code, and gain for minimizing the distance. When coding is over, the linear prediction parameter code, adaptive code, excitation code, and gain code for minimizing a distortion between the input speech and the coded speech, are outputted as a coding result.

Explanations are made on the decoder 2. In the decoder 2, the linear prediction parameter decoder 12 decodes the linear prediction parameter code to the linear prediction parameter. Then, the linear prediction parameter decoder 12 sets the linear prediction parameter as a coefficient for the synthesis filter 13, and also outputs the linear prediction parameter to the noise evaluator 26.

Explanations are made on decoding of excitation information. The adaptive codebook 14 outputs a time series vector corresponding to an adaptive code by repeating an old excitation signal periodically. The noise level evaluator 26 evaluates a noise level by using the decoded linear prediction parameter, which is inputted from the linear prediction parameter decoder 12, and the adaptive code in a same method with the noise level evaluator 24 in the encoder 1, and outputs an evaluation result to the weight determiner 37.

The first excitation codebook 35 and the second excitation codebook 36 output time series vectors corresponding to excitation codes. The weight determiner 37 weights based on the noise level evaluation result inputted from the noise level evaluator 26 in a same method with the weight determiner 34 in the encoder 1. Each of the time series vectors from the first excitation codebook 35 and the second excitation codebook 36 is weighted by using a respective weight provided by the weight determiner 37, and added. The time series vector outputted from the adaptive codebook 14 and the time series vector, which is generated by being weighted and added, are weighted by using respective gains decoded from the gain codes by the gain decoder 16, and added by the weighting-adder 39. Then, an addition result is provided to the synthesis filter 13 as an excitation signal, and an output speech S3 is produced.

In embodiment 5, the noise level of the speech is evaluated by using a code and coding result, and the noise time series vector or non-noise time series vector are weighted based on the evaluation result, and added. Therefore, a high quality speech can be reproduced with a small data amount.

Embodiment 6.

In embodiments 1 - 5, it is also possible to change gain codebooks based on the evaluation result of the noise level. In embodiment 6, a most suitable gain codebook can be used based on the excitation codebook. Therefore, a high quality speech can be reproduced.

Embodiment 7.

In embodiments 1 - 6, the noise level of the speech is evaluated, and the excitation codebooks are switched based on the evaluation result. However, it is also possible to decide and evaluate each of a voiced onset, plosive consonant, etc., and switch the excitation codebooks based on an evaluation result. In embodiment 7, in addition to the noise state of the speech, the speech is classified in more details, e.g., voiced onset, plosive consonant, etc., and a suitable excitation codebook can be used for each state. Therefore, a high quality speech can be reproduced.

Embodiment 8.

In embodiments 1 - 6, the noise level in the coding period is evaluated by using a spectrum gradient, short-term prediction gain, pitch fluctuation. However, it is also possible to evaluate the noise level by using a ratio of a gain value against an output from the adaptive codebook.

Industrial Applicability

In the speech coding method, speech decoding method, speech coding apparatus, and speech decoding apparatus according to this invention, a noise level of a speech in a concerning coding period is evaluated by using a code or coding result of at least one of the spectrum information, power information, and pitch information, and various excitation codebooks are used based on the evaluation result. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, a plurality of excitation codebooks storing excitations with various noise levels is provided, and the plurality of excitation codebooks is switched based on the evaluation result of the noise level of the speech. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, the noise levels of the time series vectors stored in the excitation codebooks are changed based on the evaluation result of the noise level of the speech. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, an excitation codebook storing noise time series vectors is provided, and a time series vector with a low noise level is generated by sampling signal samples in the time series vectors based on the evaluation result of the noise level of the speech. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, the first excitation codebook storing noise time series vectors and the second excitation codebook storing non-noise time series vectors are provided, and the time series vector in the first excitation codebook or the time series vector in the second excitation codebook is weighted based on the evaluation result of the noise level of the speech, and added to generate a time series vector. Therefore, a high quality speech can be reproduced with a small data amount.