Speech coding apparatus, speech decoding apparatus, speech coding and decoding method and a phase amplitude characteristic extracting apparatus for carrying out the method

A speech coding and decoding apparatus for improving the quality of synthesized speech. A coding portion includes a filter for adding a short-term phase amplitude characteristic to an excitation signal, and a coding circuit for quantizing and coding a phase amplitude characteristic. A decoding portion includes a decoding circuit for decoding the coded phase amplitude characteristic, and a filter for adding the same phase amplitude characteristic as that in the coding portion. Thus, it is possible to synthesize high-quality speech with good reproducibility of the phase amplitude characteristic of an excitation signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a code-excited linear prediction speech 
coding apparatus for compressing and coding a speech signal into a digital 
signal, a code driving linear prediction speech decoding apparatus for 
decoding the compressed signal, a speech coding and decoding method and a 
phase amplitude characteristic extracting apparatus which is available for 
this method. 
2. Description of the Prior Art 
FIG. 7 shows the overall structure of an example of a conventional 
code-excited linear prediction speech coding and decoding apparatus which 
is shown in "Improved Speech Quality and Efficient Vector Quantization in 
SELP" by W. B. Kleijn, D. J. Krasinski, R. H. Ketchrum (ICASSP 88, pp. 155 
to 158, 1988). 
This apparatus includes a coding portion 1, a decoding portion 2, a 
multiplexing means 3 and a separating means 4. Input speech 5 is input to 
these elements and output therefrom as output speech 6. This apparatus 
further includes a linear prediction parameter analysis means 7, a linear 
prediction parameter coding means 8, and synthesis filters 9, 18. Adaptive 
codebooks 10, 14, random codebooks 11, 15, and an optimum code searching 
means 12 constitute an excitation signal generating means. The gains of 
codevectors are coded by an excitation gain coding means 13. The decoding 
portion 2 includes an excitation gain decoding means 16 and a linear 
prediction parameter decoding means 17. 
The operation of the conventional code-excited linear prediction speech 
coding and decoding apparatus will now be explained. 
In the coding portion 1, the linear prediction parameter analysis means 7 
first extracts a linear prediction parameter by analyzing the input speech 
5. The linear prediction parameter coding means 8 then quantizes the 
linear prediction parameter, and outputs the code corresponding to the 
parameter to the multiplexing means 3 and the quantized linear prediction 
parameter to the synthesis filter 9. 
The adaptive codebook 10 stores excitation signals which have been obtained 
and outputs an adaptive vector which corresponds to an adaptive code L 
input from the optimum code searching means 12. The random codebook 11 
stores N random vectors which are produced from random noise, for example, 
and outputs a random vector which corresponds to a random code I input 
from the optimum code searching means 12. The synthesis filter 9 generates 
synthesized speech by using the quantized linear prediction parameter and 
an excitation signal which is obtained by adding the adaptive vector and 
the random vector which are multiplied by excitation gains .beta. and 
.gamma., respectively. 
The optimum code searching means 12 evaluates the perceptual weighted 
distortion constituting a residual signal between the synthesized speech 
and the input speech 5, obtains the adaptive code L, the random code I and 
the excitation gains .beta. and .gamma. which minimize the distortion, and 
outputs the adaptive code L and the random code I to the multiplexing 
means 3 and the excitation gains .beta. and .gamma. to the excitation gain 
coding means 13. The excitation gain coding means 13 quantizes the 
excitation gains .beta. and .gamma. and outputs those codes to the 
multiplexing means 3. 
The adaptive codebook 10 updates the contents of the codebook 10 by using 
the excitation signal generated by using the adaptive vector corresponding 
to the adaptive code L, the random vector corresponding to the random code 
I and the quantized excitation gains .beta. and .gamma. which minimize the 
distortion. 
As a result of the above-described operation, the multiplexing means 3 
supplies the code which corresponds to the quantized linear prediction 
parameter, and the codes which correspond to the adaptive code L, the 
random code I and the excitation gains .gamma. and .beta. to a 
transmission path. 
The operation of the decoding portion 2 will now be explained. 
The separating means 4 which receives the outputs from the multiplexing 
means 3 separates the outputs and transmits the supplied adaptive code L 
to the adaptive codebook 14, the random code I to the random codebook 15, 
the codes of the excitation gains .gamma. and .beta. to the excitation 
gain decoding means 16, and the code of the linear prediction parameter to 
the linear prediction parameter decoding means 17. 
The adaptive codebook 14 outputs the adaptive vector which corresponds to 
the adaptive code L, and the random codebook 15 outputs the random vector 
which corresponds to the random code I. The excitation gain decoding means 
16 decodes the excitation gains .beta. and .gamma. and as to multiply the 
adaptive vector by the gain .beta. and the random vector by the gain 
.gamma.. 
The linear prediction parameter decoding means 17 decodes the linear 
prediction parameter which corresponds to the code of the linear 
prediction parameter and outputs the decoded linear prediction parameter 
to the synthesis filter 18. The synthesis filter 18 synthesizes an 
excitation signal which is obtained by adding the adaptive vector and the 
random vector by using the linear prediction parameter, and outputs the 
output speech 6. 
The adaptive codebook 14 updates the contents of the codebook by using the 
excitation signal in the same way as the adaptive codebook 10 of the 
coding portion 1. 
Another coding and decoding apparatus is shown in FIG. 8. 
FIG. 8 shows an apparatus having coding and decoding means for coding and 
decoding the phase characteristic of an excitation signal which is shown 
in "Speech Coding Using All-pass Filter Response" by Ikeda, Nakamura and 
Asada (Technical Reports of the Institute of Electronics, Information and 
Communication Engineers SP 91-72, pp. 45 to 52, 1991). The structure of 
this apparatus is different from that of the apparatus shown in FIG. 7 in 
that the former further includes pulse train generating means 19, 25, 
phase characteristic codebooks 20, 26, phase characteristic adding filters 
21, 27, an optimum excitation.phase characteristic searching means 22, a 
pulse position coding means 23 and a pulse position decoding means 24. 
In the coding portion 1, the pulse train generating means 19 outputs a 
pulse train which corresponds to the position of the head pulse and the 
pulse interval which are input from the optimum excitation.phase 
characteristic searching means 22. The phase characteristic adding filter 
21 is, for example, an N-order all-pass filter whose transfer function 
H(z) is represented by the following formula (1): 
##EQU1## 
The phase characteristic codebook 20 stores a plurality of filter 
coefficients which are created on the assumption that the impulse response 
of the phase characteristic adding filter 21, for example, is given as a 
random sequence of numbers, and outputs the filter coefficient which 
corresponds to the code input from the optimum excitation.phase 
characteristic searching means 22 to the phase characteristic adding 
filter 21. The phase characteristic adding filter 21 adds a phase 
characteristic by using the filter coefficient to the excitation signal 
which is obtained by multiplying the pulse train output from the pulse 
train generating means 19 by an excitation gain g mission, by using the 
filter coefficient, and outputs the phase characteristic added excitation 
signal to the synthesis filter 9. The synthesis filter 9 generates 
synthesized speech by using the quantized linear prediction parameter 
which is input from the linear prediction parameter coding means 8 and the 
excitation signal to which the phase characteristic is added. 
The optimum excitation.phase characteristic searching means 22 obtains the 
position of the head pulse and the pulse interval of the pulse train, the 
excitation gain g and the code of the phase characteristic which minimize 
the perceptual weighted distortion of a residual signal between the 
synthesis speech and the input speech 5, and outputs the position of the 
head pulse and the pulse interval of the pulse train to the pulse position 
coding means 23, the excitation gain g to the excitation gain coding means 
13, and the code of the phase characteristic to the multiplexing means 3. 
The pulse position coding means 23 quantizes the position of the head pulse 
and the pulse interval of the pulse train and outputs the codes to the 
multiplexing means 3. 
The multiplexing means 3 which has received these codes transfers the code 
which corresponds to the linear prediction parameter, the code of the 
phase characteristic, the codes which correspond to the quantized position 
of the head pulse and the pulse interval of the pulse train, and the code 
corresponding to the quantized excitation gain g to the separating means 
4. 
The operation of the decoding portion 2 will now be explained. 
The separating means 4 which has received the outputs of the multiplexing 
means 3 outputs the codes which correspond to the quantized position of 
the head pulse and the pulse interval of the pulse train to the pulse 
position decoding means 24, the code of the excitation gain g to the phase 
characteristic codebook 28, and the code of the linear prediction 
parameter to the linear prediction parameter decoding means 17. 
The pulse position decoding means 24 decodes the position of the head pulse 
and the pulse interval which correspond to the codes of the position of 
the head pulse and the pulse interval of the pulse train and outputs the 
decoded position and pulse interval to the pulse train generating means 
25. The pulse train generating means 25 outputs the pulse train which 
corresponds to the position of the head pulse and the pulse interval to 
the phase characteristic adding filter 27. 
The excitation gain decoding means 18 decodes the excitation gain g which 
corresponds to the code of the excitation gain. The phase characteristic 
codebook outputs the filter coefficient which corresponds to the code of 
the phase characteristic to the phase characteristic adding filter 27. 
The phase characteristic adding filter 27 adds the phase characteristic to 
the excitation signal which is obtained by multiplying the pulse train by 
the excitation gain g, by using the filter coefficient, and outputs the 
excitation signal obtained to the synthesis filter 1B. The synthesis 
filter 18 outputs the output speech 6 by using the linear prediction 
parameter which is input from the linear prediction decoding means 17 and 
the excitation signal with the phase characteristic added thereto. 
A conventional apparatus for obtaining the short-term phase amplitude 
characteristic of the linear prediction residual signal of speech is shown 
in FIG. 9. This is an apparatus described in "Speech Encoding Based on 
Phase Equalization" by Honda and Morlya (Transactions of the Committee on 
Speech Research The Acoustical Society of Japan S84-05, pp. 33 to 40, 
1984). 
In FIG. 9, speech is input as input speech 101, and a phase amplitude 
characteristic 102 is obtained. This apparatus includes a linear 
prediction parameter analysis means 103, a linear predictive inverse 
filter 104, a pitch extracting means 105, a pitch position extracting 
means 106, and a phase amplitude characteristic adding filter coefficient 
calculator 107. 
The process for obtaining the short-term phase amplitude characteristic of 
the linear prediction residual signal of speech will be explained. 
When the input speech 101 is input, the linear prediction parameter 
analysis means 103 analyzes the input speech 101 so as to extract the 
linear prediction parameter and outputs the extracted linear prediction 
parameter to the linear predictive inverse filter 104. The linear 
predictive inverse filter 104 generates a linear prediction residual 
signal from the input speech 101 by using the linear prediction parameter, 
and outputs the linear prediction residual signal to the pitch position 
extracting means 106 and the phase amplitude characteristic adding filter 
coefficient calculator 107. 
The pitch extracting means 105 extracts the pitch period of the input 
speech 101 by a known method and outputs the extracted pitch period to the 
pitch position extracting means 106. The pitch position extracting means 
106 extracts the pitch position at every pitch period as the position at 
which the linear prediction residual signal has the maximum amplitude in 
one pitch period, and outputs the pitch position to the phase amplitude 
characteristic adding filter coefficient calculator 107. 
The phase amplitude characteristic adding filter coefficient calculator 107 
obtains the function of a phase amplitude characteristic adding filter 
(FIG. 10) having an impulse response which outputs the linear prediction 
residual signal when a pulse train, in which pulses exist only at pitch 
positions, is input, and outputs the function as the phase amplitude 
characteristic 102. The phase amplitude characteristic adding filter is, 
for example, an N-order filter whose transfer function H(z) is represented 
by the following formula (2). 
##EQU2## 
Alternatively, the phase amplitude characteristic adding filter may be, 
for example, an N-order all-pass filter whose transfer function H(z) is 
represented by the formula (1). 
The above-described prior art has the following problems. 
Speech is composed of voiced speech and unvoiced speech. The 
reproducibility of voiced speech exerts a great influence on the quality 
of synthesized speech. It is possible to model the excitation of a voiced 
sound in the form of a signal having a pitch periodicity and a short-term 
phase characteristic in the pitch periodicity. 
In the conventional code-excited linear prediction speech coding apparatus, 
the excitation signal is represented by the sum of an adaptive vector and 
a random vector. This method does not directly represent the phase 
characteristic of the excitation signal. Therefore, there is a case in 
which the phase characteristic of the excitation signal is not reproduced, 
which leads to a deterioration of the quality of synthesized speech. 
This problem is serious, for example, at a transitional portion from 
unvoiced speech to voiced speech or at a voiced speech where the pitch 
period changes greatly. At such a portion, an adaptive vector does not 
adequately work so that it is necessary to reproduce the pitch period and 
the phase characteristic using only the random vector. 
In the conventional coding and decoding apparatus for coding the phase 
characteristic of an excitation signal, although the phase characteristic 
of an excitation signal is coded, since an excitation signal is assumed to 
have a simple pulse train, when an appropriate phase characteristic is not 
found in the phase characteristic codebook, it is impossible to complete 
the phase characteristic using an excitation signal, which leads to a 
deterioration of the quality of synthesized speech. 
In the case of adopting the conventional method of obtaining the short-term 
phase amplitude characteristic of the linear prediction residual signal of 
speech, although it is necessary to obtain the pitch period and the pitch 
position, since it is not always possible to obtain the exact pitch period 
and pitch position, the difference between the phase amplitude 
characteristic obtained from the inexact pitch period and pitch position 
and that obtained from the exact ones will increase according to the 
degree of the error. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to eliminate the 
above-described problems in the prior art and to provide a code-excited 
linear prediction speech coding and decoding apparatus and a speech coding 
and decoding method which can avoid a deterioration in the quality of 
synthesized speech and generate synthesized speech having a good quality. 
To achieve this end, in a first aspect of the present invention there is 
provided a speech coding apparatus comprising: a linear prediction 
parameter analysis means; a linear prediction parameter coding means; an 
excitation signal generating means; a synthesis filter for synthesizing 
the output signal of the linear prediction parameter coding means and the 
excitation signal output from the excitation signal generating means; a 
phase amplitude characteristic coding means for quantizing and coding the 
phase amplitude characteristic which is obtained by analyzing the linear 
prediction residual signal of an input speech signal; and a phase 
amplitude characteristic adding filter for adding a short-term phase 
amplitude characteristic to the excitation signal. 
According to this structure, the short-term phase amplitude characteristic 
of an excitation signal is quantized and coded, so that the phase 
amplitude characteristic is positively added to the excitation signal. As 
a result, it is possible to synthesize speech of a high quality with a 
good reproducibility of the phase characteristic of the excitation signal. 
In a second aspect of the present invention, there is provided a speech 
decoding apparatus comprising: a linear prediction parameter decoding 
means; an excitation signal generating means; a synthesis filter for 
synthesizing the output signal of the linear prediction parameter decoding 
means and the excitation signal output from the excitation signal 
generating means; a phase amplitude characteristic decoding means for 
decoding a coded short-term phase amplitude characteristic; and a phase 
amplitude characteristic adding filter for adding the decoded phase 
amplitude characteristic to the excitation signal. 
According to this structure, the coded short-term phase amplitude 
characteristic is decoded, and the phase amplitude characteristic is 
positively added to the excitation signal. As a result, it is possible to 
synthesize speech of a high quality with a good reproducibility of the 
phase characteristic of the excitation signal. 
In a third aspect of the present invention, there is provided a speech 
coding and decoding method comprising a coding process and a decoding 
process: 
the coding process including the steps of: coding a linear prediction 
parameter by the linear prediction analysis of an input speech signal; 
selecting a codevector for generating optimum synthesized speech from an 
adaptive codebook and a random codebook; and coding and transmitting the 
excitation signal; and 
the decoding process including the steps of: generating an excitation 
signal and a decoded linear prediction parameter signal on the basis of 
the received signal; and synthesizing the excitation signal and the 
decoded linear prediction parameter signal by a synthesis filter so as to 
generate an output speech signal. The coding process further includes the 
steps of: quantizing and coding the phase amplitude characteristic which 
is obtained by analyzing the linear prediction residual signal of an input 
speech signal; and adding a short-term phase amplitude characteristic to 
the excitation signal, and the decoding process further includes the steps 
of: decoding the coded phase amplitude characteristic; and adding the 
decoded phase amplitude characteristic to the excitation signal so as to 
generate the output speech signal. 
According to this structure, the short-term phase amplitude characteristic 
of an excitation signal is quantized in the coding process, and the coded 
phase amplitude characteristic is decoded in the decoding process, so that 
the phase amplitude characteristic is positively added to the excitation 
signal. As a result, it is possible to transmit speech of a high quality 
with a good reproducibility of the phase characteristic of the excitation 
signal. 
In a fourth aspect of the present invention, there is provided a phase 
amplitude characteristic extracting apparatus for extracting the 
short-term phase amplitude characteristic of a signal, comprising: a phase 
amplitude characteristic codebook which stores a plurality of short-term 
phase amplitude characteristics of signals; a phase amplitude 
characteristic removing filter for removing a phase amplitude 
characteristic; a residual signal generating means for generating a 
residual signal by removing the phase amplitude characteristic stored in 
the phase amplitude characteristic codebook from the input signal the 
phase amplitude characteristic removing filter; a pulse approximate means 
or a pulse signal representation means for generating a pulse approximated 
signal or a pulse signal representation signal by reducing the residual 
signal to a small number of pulses; a trial signal generating means for 
generating a trial signal by adding each removed phase amplitude 
characteristic to the pulse approximated signal; and a selecting and 
outputting means for selecting the phase amplitude characteristic which 
minimizes the distortion between the trial signal and the input signal, 
from the phase amplitude characteristic codebook and outputting the 
selected phase amplitude characteristic. 
According to this structure, a residual signal is obtained by removing each 
of the phase amplitude characteristics stored in the phase amplitude 
characteristic codebook from an input signal by inverse filters, and each 
residual signal is reduced to a small number of pulses. Each of the 
removed phase amplitude characteristics is added to the approximate 
signal, and the phase amplitude characteristic which minimizes the 
distortion between this signal and the input signal is selected from the 
codebook. In this way, the short-term phase amplitude characteristic of 
the signal is obtained. As a result, for example, when the short-term 
phase amplitude characteristic of the linear prediction residual signal of 
a speech is obtained, it is not necessary to extract the pitch period and 
the pitch position, thereby preventing an error in the extraction of the 
phase amplitude characteristic. 
The above and other objects, features and advantages of the present 
invention will become clear from the following description of the 
preferred embodiments thereof, taken in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
A speech coding and decoding apparatus according to the present invention 
will be explained with reference to the accompanying drawings. 
FIG. 1 is a block diagram of a first embodiment of a speech coding and 
decoding apparatus according to the present invention. The same elements 
as those shown in FIG. 7 are provided with the same reference numerals and 
explanation thereof will be omitted. 
This embodiment is characterized by the following newly added elements: 
phase amplitude characteristic analysis means 28 for analyzing a phase 
amplitude characteristic, phase amplitude characteristic coding means 29 
for coding a phase amplitude characteristic, phase amplitude 
characteristic adding filters 80, 32 for adding a phase amplitude 
characteristic, and phase amplitude characteristic decoding means 31 for 
decoding phase amplitude characteristic. 
In the coding portion 1, the phase amplitude characteristic analysis means 
28 generates a linear prediction residual signal by using the input speech 
5 and the linear prediction parameter which is input from the linear 
prediction parameter coding means 8, obtains the short-term phase 
amplitude characteristic of the linear prediction residual signal as a 
filter coefficient by using, for example, a conventional method of 
obtaining the short-term phase amplitude characteristic of a linear 
prediction residual signal of speech, and outputs the filter coefficient 
to the phase amplitude characteristic coding means 29. The phase amplitude 
characteristic coding means 29 quantizes the filter coefficient and 
outputs the corresponding code to the multiplexing means 3, and the 
quantized filter coefficient to the phase amplitude characteristic adding 
filter 30. 
The phase amplitude characteristic adding filter 30 adds the phase 
amplitude characteristic by using the quantized filter coefficient to the 
excitation signal which is obtained by multiplying the adaptive vector 
which is output from the adaptive codebook 10 by the excitation gain 
.beta. and multiplying the random vector which is output from the random 
codebook 11 by the excitation gain .gamma., and adding the products, and 
outputs the thus-obtained excitation signal to the synthesis filter 9. The 
synthesis filter 9 generates synthesized speech by using the quantized 
linear prediction parameter which is input from the linear prediction 
parameter coding means 8 and the excitation signal with the phase 
amplitude characteristic added thereto. 
The optimum code searching means 12 evaluates the perceptual weighted 
distortion of a residual signal between the synthesized speech and the 
input speech 5, obtains the adaptive code L, the random code I and the 
excitation gains .beta. and .gamma. which minimize the distortion, and 
outputs the adaptive code L and the random code I to the multiplexing 
means 3 and the excitation gains .beta. and .gamma. to the excitation gain 
coding means 13. The excitation gain coding means 13 quantizes the 
excitation gains .beta. and .gamma. and outputs those codes to the 
multiplexing means 3. 
On the basis of these results, the multiplexing means 3 supplies the code 
which corresponds to the quantized linear prediction parameter, the code 
which corresponds to the quantized filter coefficient of the phase 
amplitude characteristic adding filter 30, and the codes which correspond 
to the adaptive code L, the random code I and the excitation gains .beta. 
and .gamma. to a transmission path. 
The above-described operation is characteristic of the coding portion 1 of 
a speech coding and decoding apparatus of this embodiment. 
The operation of the decoding portion 2 will now be explained. 
The separating means 4 which receives the outputs from the multiplexing 
means 3 separates the outputs and transmits the supplied adaptive code L 
to the adaptive codebook 14, the random code I to the random codebook 15, 
the codes of the excitation gains .beta. and .gamma. to the excitation 
gain decoding means 18, the code of the filter coefficient of the phase 
amplitude characteristic adding filter 30 to the phase amplitude 
characteristic decoding means 31, and the code of the linear prediction 
parameter to the linear prediction parameter decoding means 17. 
The phase amplitude characteristic decoding means 31 decodes the filter 
coefficient which corresponds to the code of the filter coefficient of the 
phase amplitude characteristic adding filters 30 and outputs the decoded 
filter coefficient to the phase amplitude characteristic adding filter 32. 
The phase amplitude characteristic adding filter 32 adds the phase 
amplitude characteristic obtained using decoded quantized filter 
coefficient to the excitation signal which is obtained by multiplying the 
adaptive vector which is output from the adaptive codebook 14 by the 
excitation gain .beta. output from the excitation gain decoding means 18 
and multiplying the random vector which is output from the random codebook 
15 by the excitation gain .gamma. output from the excitation gain decoding 
means 18, and adding the products, and outputs the thus-obtained 
excitation signal to the synthesis filter 18. The synthesis filter 18 
generates synthesized speech by using the linear prediction parameter 
which is input from the linear prediction parameter decoding means 17 and 
the excitation signal with the phase amplitude characteristic added 
thereto, and outputs the synthesized speech. 
The above-described operation is characteristic of the decoding portion 2 
of a speech coding and decoding apparatus of this embodiment. 
According to this embodiment, it is possible to enhance the reproducibility 
of an excitation signal and to improve the quality of synthesized speech 
by coding the short-term phase amplitude characteristic of a linear 
prediction residual signal and addling it to the excitation signal. 
Second Embodiment 
Another embodiment of a speech coding and decoding apparatus according to 
the present invention will be explained with reference to the accompanying 
drawings. 
FIG. 2 is a block diagram of a second embodiment of a speech coding and 
decoding apparatus according to the present invention. The same elements 
as those shown in FIG. 1 are provided with the same reference numerals and 
explanation thereof will be omitted. 
In this embodiment, the following elements are newly added to the first 
embodiment: pitch extracting means 33 for extracting a pitch period, pitch 
coding means for coding an extracted pitch period, pulse random codebooks 
35, 37, and pitch decoding means 38. 
The operation of this embodiment will now be explained with priority given 
to the newly added elements. 
In the coding portion 1, the pitch extracting means 33 extracts the pitch 
period of the input speech 5 by a known method and outputs the extracted 
pitch period to the pitch coding means 34. The pitch coding means 34 
quantizes the pitch period and outputs the corresponding code to the 
multiplexing means 3 and the quantized pitch period to the pulse random 
codebook 35. 
The pulse random codebook 35 generates a plurality of excitation vectors 
consisting of a pulse train of the quantized pitch period in which, for 
example, the positions of the head pulses are different, and stores them 
as at least a part of the random vectors in the codebook 35. FIG. 3 shows 
an example of the excitation vector consisting of a pulse train of the 
pitch period, and FIG. 4 shows an example of the excitation vectors stored 
in the pulse random codebook 35. And the pulse random codebook 35 outputs 
the random vector which corresponds to the random code I input from the 
optimum code searching means 12. 
The phase amplitude characteristic adding filter 30 adds the phase 
amplitude characteristic obtained using the quantized filter coefficient 
input from the phase amplitude characteristic coding means 29 to the 
excitation signal which is obtained by multiplying the adaptive vector 
which is output from the adaptive codebook 10 by the excitation gain 
.beta. and multiplying the random vector which is output from the pulse 
random codebook 35 by the excitation gain .gamma., and adding the 
products, and outputs the thus-obtained excitation signal to the synthesis 
filter 9. The synthesis filter 9 generates synthesized speech by using the 
quantized linear prediction parameter which is input from the linear 
prediction parameter coding means 8 and the excitation signal with the 
phase amplitude characteristic added thereto. 
The optimum code searching means 12 evaluates the perceptual weighted 
distortion of a residual signal between the synthesized speech and the 
input speech 5, obtains the adaptive code L, the random code I and the 
excitation gains .beta. and .gamma. which minimize the distortion, and 
outputs the adaptive code L and the random code I to the multiplexing 
means 3 and the excitation gains .beta. and .gamma. to the excitation gain 
coding means 13. The excitation gain coding means 13 quantizes the 
excitation gains .beta. and .gamma. and outputs those codes to the 
multiplexing means 3. 
On the basis of these results, the multiplexing means 3 supplies the code 
which corresponds to the quantized linear prediction parameter, the code 
which corresponds to the quantized filter coefficient of the phase 
amplitude characteristic adding filter 30 and the codes which correspond 
to the adaptive code L, the quantized pitch period, the random code I and 
the excitation gains .beta. and .gamma. to a transmission path. 
The schematic structure of the coding portion 1 of the second embodiment of 
the speech coding and decoding apparatus has been described above. 
The operation of the decoding portion 2 will now be explained. 
The separating means 4 which receives the outputs from the multiplexing 
means 3 separates the outputs and transmits the supplied adaptive code L 
to the adaptive codebook 14, the code of the pitch period to the pitch 
decoding means 36, the random code I to the random codebook 37, the codes 
of the excitation gains .beta. and .gamma. to the excitation gain decoding 
means 16, the code of the filter coefficient of the phase amplitude 
characteristic adding filter 30 to the phase amplitude characteristic 
decoding means 31, and the code of the linear prediction parameter to the 
linear prediction parameter decoding means 17. 
The pitch decoding means 36 decodes the pitch period which corresponds to 
the code of the pitch period and outputs the decoded pitch period to the 
pulse random codebook 37. The pulse random codebook 37 stores the 
excitation vector consisting of a pulse train of the decoded pitch period 
in the codebook 37 in the same way as the random codebook The pulse random 
codebook 37 outputs the random vector which corresponds to the random code 
I. 
The phase amplitude characteristic adding filter 32 adds the phase 
amplitude characteristic by using the filter coefficient input from the 
phase amplitude characteristic decoding means 31 to the excitation signal 
which is obtained by multiplying the adaptive vector which is output from 
the adaptive codebook 14 by the excitation gain .beta. and multiplying the 
random vector which is output from the pulse random codebook 37 by the 
excitation gain .gamma., and adding the products, and outputs the 
thus-obtained excitation signal to the synthesis filter 18. The synthesis 
filter 18 outputs an output speech 8 by using the linear prediction 
parameter which is input from the linear prediction parameter decoding 
means 17 and the excitation signal with the phase amplitude characteristic 
added thereto. 
As has been described above, according to the second embodiment, a pulse 
train of a pitch period is used for a random vector, and a phase amplitude 
characteristic is added to the random vector. In this manner, it is 
possible to generate an appropriate excitation signal from only a random 
vector. Consequently, even if an adaptive vector does not work, it is 
possible to produce an excitation signal with good reproducibility and to 
improve the quality of synthesized speech. 
In this embodiment, the pulse train may be obtained from an adaptive code. 
In this case, the pitch extracting means 33, the pitch coding means 34 and 
the pitch decoding means in FIG. 2 are eliminated, and the pulse interval 
of the pulse train which is used as a random vector is obtained from the 
adaptive code. At this time, since it is not necessary to transmit the 
information of the pitch period with respect to the pulse interval, it is 
possible to reduce the amount of information transmitted. In addition, 
since the reproducibility of an excitation signal is good even if the 
adaptive vector does not work, it is possible to improve the quality of 
synthesized speech. 
Third Embodiment 
An embodiment of a phase amplitude characteristic extracting apparatus for 
extracting the short-term phase amplitude characteristic of a signal 
according to the present invention will be explained with reference to the 
accompanying drawings. 
FIG. 5 is a block diagram of the structure of an apparatus for obtaining a 
phase amplitude characteristic. This apparatus is used to obtain the 
short-term phase amplitude characteristic of a linear prediction residual 
signal. 
The following elements are newly added to the conventional apparatus shown 
in FIG. 9: a phase amplitude characteristic codebook 108, a phase 
amplitude characteristic removing filter 109 for removing the 
characteristic of a phase amplitude, pulse approximate means 110 for 
approximating or representing a residual signal by some pulses, a phase 
amplitude characteristic adding filter 111 for adding the characteristic 
of a phase amplitude, a synthesis filter 112 for synthesizing a speech 
form a linear prediction parameter and an excitation signal, and optimum 
phase amplitude characteristic searching means 113 for searching an 
optimum phase amplitude characteristic. 
The operation of the apparatus will be explained with priority given to the 
characteristic structure thereof. 
The linear prediction parameter analysis means 103 analyzes input speech 
101 so as to extract the linear prediction parameter and outputs the 
extracted linear prediction parameter to the linear predictive inverse 
filter 104 and the synthesis filter 112. The linear predictive inverse 
filter 104 generates a linear prediction residual signal from the input 
speech 101 by using the linear prediction parameter, and outputs the 
linear prediction residual signal to the phase amplitude characteristic 
removing filter 109. 
A plurality of phase amplitude characteristics are stored in the phase 
amplitude characteristic codebook as, for example, filter coefficients, 
and the phase amplitude characteristic codebook outputs the filter 
coefficient of the phase amplitude characteristic which corresponds to the 
code input from the optimum phase amplitude characteristic searching means 
113 to the phase amplitude characteristic removing filter 109 and the 
phase amplitude characteristic adding filter 111. The phase amplitude 
characteristic removing filter 109 generates a residual signal by removing 
the phase amplitude characteristic from the linear prediction parameter 
signal by using the filter coefficient, and outputs the residual signal to 
the pulse approximate means 110. The pulse approximate means 110 generates 
a pulse signal representation residual signal by reducing the residual 
signal to zero except for N samples having the largest amplitude, for 
example, and outputs the pulse signal representation residual signal to 
the phase amplitude characteristic adding filter 111. 
FIG. 6 shows an example of representation. FIG. 6 shows the process of 
generating a residual signal from a linear prediction residual signal by 
removing the phase amplitude characteristic, and then reducing the 
residual signal to a pulse so as to generate a pulse signal representation 
residual signal. 
The phase amplitude characteristic adding filter 111 then adds the phase 
amplitude characteristic to the pulse signal representation residual 
signal by using the filter coefficient so as to produce an excitation 
signal and outputs the excitation signal to the synthesis filter 112. The 
synthesis filter 112 generates synthesized speech by using the linear 
prediction parameter and the excitation signal. 
The optimum phase amplitude characteristic searching means 113 evaluates 
the perceptual weighted distortion of the residual signal between the 
synthesized speech and the input speech 101, selects the filter 
coefficient corresponding to the phase amplitude characteristic which 
minimizes the distortion from the phase amplitude characteristic codebook 
108, and outputs the selected filter coefficient as the phase amplitude 
characteristic 102. 
According to this embodiment, a codebook which stores a plurality of 
short-term phase amplitude characteristic of a signal is provided, a trial 
signal is generated by using each phase amplitude characteristic in the 
codebook and the phase amplitude characteristic which minimizes the 
distortion between an input signal and the trial signal is selected from 
the codebook. In this manner, it is possible to extract the phase 
amplitude characteristic without an error and without the need for pitch 
extraction or pitch position extraction when the short-term phase 
amplitude characteristic of a linear prediction residual signal of speech 
is obtained. 
While there has been described what are at present considered to be 
preferred embodiments of the invention, it will be understood that various 
modifications may be made thereto, and it is intended that the appended 
claims cover all such modifications as fall within the true spirit and 
scope of the invention.