Gain control in post filtering process using scaling

A second post filter having characteristics similar to those of a first post filter used in the signal path is prepared and a gain of the first post filter is preliminarily presumed from an input and an output of the second post filter. An optimum scaling value when performing a filtering operation in the first post filter is set by using the gain obtained from the input and output of the second post filter, so that an optimum gain when controlling a gain fluctuation caused by the first post filter is set.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a digital signal processing apparatus suitable for 
use in execution of a post filtering process to improve a quality of a 
decoded audio signal in a digital cellular phone. 
2. Description of the Related Art 
A VSELP (Vector Sum Excited Linear Prediction) technique has been used as 
an audio coding system in a digital cellular phone in North America and 
Japan. According to the VSELP system, an adaptive signal is formed from 
pitch information and a past exciting signal vector. A noise signal is 
formed by adding a basic vector. An exciting signal is formed by linearly 
adding the adaptive signal and the noise signal in accordance with a gain 
which is set in accordance with information indicative of a 
sound/soundless state. An audio signal is synthesized from the exciting 
signal by a short period synthesizing filter. A coding is performed by 
comparing the synthesized audio signal and an input audio signal, and 
selecting a code such that an error between them is minimum. 
In the VSELP, therefore, a parameter .alpha. of the shortperiod 
synthesizing filter, an exciting source code I, pitch information L, and 
gains .beta. and .gamma. are transmitted. Upon decoding, the exciting 
signal is synthesized from a long period filtering state based on the 
pitch information L and the past exciting signal, an output of a code book 
based on the exciting source code I, and the gains .beta. and .gamma.. The 
exciting signal is supplied to a predictive synthesizing filter of the 
parameter .alpha. and an audio signal is formed. Further, a post filter is 
used to improve an auditory impression. An auditory distortion is reduced 
by adaptively enhancing a pitch periodic component and enhancing a formant 
component. 
That is, FIG. 1 shows a construction of a conventional decoder of the 
VSELP. In FIG. 1, reference numeral 151 denotes a long period filtering 
state. The long period filtering state 151 outputs a signal b.sub.L (n) 
based on a past exciting vector and the pitch information L from an input 
terminal 161. Reference numeral 152 denotes a code book. The code book 152 
outputs a noise signal c(n) on the basis of the exciting source code I 
from an input terminal 162. 
An output of the long period filtering state 151 is supplied to a 
multiplier 153 for multiplying the gain .beta. from an input terminal 163. 
An output of the code book 152 is supplied to a multiplier 154 to multiply 
the gain .gamma. from an input terminal 164. Outputs of the multipliers 
153 and 154 are supplied to an adder 155. An exciting signal vector ex(n) 
is formed by the adder 155. The exciting signal vector is supplied to a 
short period synthesizing filter 156. 
The parameter .alpha. from an input terminal 165 is set into the short 
period synthesizing filter 156. An audio signal is synthesized by the 
short period synthesizing filter 156. The audio signal is supplied to a 
post filter 157. The post filter 157 adaptively enhances the pitch 
periodic component and enhances the formant component. An output of the 
post filter 157 is taken out from an output terminal 158. 
As mentioned above, according to the coding system like a VSELP, the post 
filter 157 is inserted upon decoding in order to reduce the auditory 
distortion. In case of realizing such a post filter 157 by a fixed point 
arithmetic operation, since a gain fluctuation value of a filtering 
process cannot be known before the filtering, as for a scaling of the 
filtering process, it is necessary to preliminarily set a slightly larger 
margin in consideration of a case where the gain becomes maximum. 
Therefore, when a signal to be filtered that is inputted to the post 
filter 157 is small and a gain of the filtering process is not so large, 
there is a problem such that an enough precision cannot be obtained in the 
filtering process. 
That is, the exciting signal vector ex(n) is a linear sum based on 
sound/soundless information (.beta., .gamma.) of the signal vector b.sub.L 
(n) which is formed on the basis of the pitch information L and the past 
exciting signal vector state and the noise signal c(n) from the code book 
and is expressed by 
EQU e(X)=.beta.b.sub.L (n)+.gamma.c(n) (1) 
By synthesizing it by the short period synthesizing filter 156, a decoded 
audio signal s(n) which is inputted to the post filter 157 is derived. 
By the above equation (1), the exciting signal vector ex(n) is seen as if 
it is proportional to the signal vector b.sub.L (n) and noise signal c(n). 
However, the signal vector b.sub.L (n) and noise signal c(n) mutually 
exert an influence and are not mutually independent. The exciting signal 
vector ex(n) is fed back to a long period filtering state r(n) and, as 
shown in FIG. 2, it is expressed as follows. 
EQU r(n)=r(n+N) (0.ltoreq.n&lt;L.sub.max -N) 
EQU r(L.sub.max -N+n)=ex(n) 
The long period filter output b.sub.L (n) is obtained as follows from the 
pitch information L. 
EQU b.sub.L (n)=r(L.sub.max -L+n) (0.ltoreq.n.ltoreq.N) 
where, 
N: signal vector length 
L.sub.max : past exciting signal vector state b.sub.L (n) is obtained from 
the signal ex(n). The long period filter output b.sub.L (n) and exciting 
signal ex(n) are not proportional. 
If the gain fluctuation value by the filtering process in the post filter 
157 is known before the filtering process, the scaling of the filtering 
process can be set to an optimum value when operating the post filter 157 
by a fixed point arithmetic operation from the gain fluctuation value, and 
a precision can be improved. Since the gain fluctuation occurs by 
transmitting the signal through the post filter 157, it is considered to 
provide a gain control circuit at the post stage of the post filter 157. 
If the gain fluctuation value of the filtering process is known before the 
filtering process, by using the gain fluctuation value of the filter, a 
gain of the gain control circuit at the post stage of the post filter 157 
can be optimally set. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide a digital signal 
processing apparatus which can perform an optimum scaling by previously 
knowing a gain fluctuation value caused by a post filter and can improve a 
precision of the post filter. 
Another object of the invention is to provide a digital signal processing 
apparatus which can optimally set a fluctuation of a gain occurring by a 
post filter by previously knowing the gain fluctuation value caused by the 
post filter. 
According to the invention, there is provided a digital signal processing 
apparatus comprising: first filter means to which a filtering signal is 
supplied; scaling means for performing a scaling process to a filter 
operation in the first filtering means; gain control means for correcting 
a gain fluctuation caused by the first filtering means; second filter 
means which has characteristics similar to those of the first filter means 
and to which a pseudo filtering signal is supplied; and gain operating 
means for obtaining a gain of the second filtering means from an input 
signal to the second filter means and an output signal thereof, wherein a 
scaling value of the scaling means and a gain correction value of the gain 
control means are controlled by using the gain of the second filter means 
obtained by the gain operating means. 
According to the invention, there is provided a digital signal processing 
apparatus comprising: first synthesizing filer means for synthesizing an 
audio signal from an exciting signal; first post filter means for 
filtering an output of the first synthesizing filter means; scaling means 
for performing a scaling process to a filter operation in the first post 
filter means; gain control means for correcting a gain fluctuation 
occurring in the first post filter means; second synthesizing filter means 
which has characteristics similar to those of the first synthesizing 
filter means and synthesizes a pseudo signal from a pseudo exciting 
signal; second post filter means which has characteristics similar to 
those of the first filter means and filters an output of the second 
synthesizing filter means; and gain operating means for obtaining a gain 
of the second post filter means from an input signal to the second post 
filter means and an output signal thereof, wherein a scaling value of the 
scaling means and a gain correction value of the gain control means are 
controlled by using the gain of the second post filter means obtained by 
the gain operating means. 
Another filter having characteristics similar to those of the post filter 
for processing the decoded audio signal is prepared. The gain of the post 
filter for processing the audio signal can be previously presumed by such 
another filter. Therefore, in the case where the filter operation in the 
post filter for processing the audio signal is performed by a fixed point 
arithmetic operation, the optimum scaling can be performed. By using the 
gain obtained as mentioned above, the fluctuation of the gain occurring by 
the post filter can be optimally corrected. 
The above and other objects and features of the present invention will 
become apparent from the following detailed description and the appended 
claims with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the invention will now be described hereinbelow with 
reference to the drawings. FIG. 3 shows a fundamental construction of the 
invention. In FIG. 3, it is now assumed that a filtering process is 
performed in a filter 1 by a fixed point arithmetic operation. If a gain 
fluctuation value caused by the filter 1 is known in this case, a word 
length can be effectively used by a scaling process and a filter operating 
process of a high precision can be performed. Therefore, a scaling value 
calculating circuit 3 and shifting circuits 4 and 5 are provided for 
executing the filtering operation of the filter 1 by scaling processing. 
When a gain fluctuation occurs in the filter 1, if the gain fluctuation in 
the filter 1 is known, a gain which corresponds to the gain fluctuation in 
the filter 1 and is in the reverse direction of that of such a gain is 
added to the post stage of the filter 1, so that the gain fluctuation in 
the filter 1 can be compensated. A gain control circuit 6 is provided in 
order to compensate the gain fluctuation caused by the filter 1 as 
mentioned above. 
The gain of the filter 1 can be obtained from the input signal to the 
filter 1 and an output signal thereof. However, when an adaptive type 
process in which its coefficients are not fixed is performed in the filter 
1, such an arithmetic operation can be derived only after the filter 
operation was completed in the filter 1. On the other hand, in order to 
perform the optimum scaling, it is necessary to presume the gain of the 
filter 1 before the filter operation in the filter 1 is completed. 
For this purpose, a filter 11 having characteristics similar to those of 
the filter 1 is prepared and the gain of the filter 1 is presumed by using 
the filter 11 before the filter operation in the filter 1 is finished. By 
using the gain presumed by using the filter 11, the optimum scaling can be 
performed in case of executing the scaling process to the arithmetic 
operation of the filter 1. The optimum gain compensation can be also 
performed in case of performing a gain compensation by the gain control 
circuit 6. 
That is, in FIG. 3, a signal s.sub.1 (n) to be filtered is supplied to an 
input terminal 2. The signal s.sub.1 (n) from the input terminal 2 is 
shifted by the shifting circuit 4 on the basis of a scaling value from the 
scaling value calculating circuit 3. An output of the shifting circuit 4 
is supplied to the post filter 1. A filtering operation is performed to 
the signal s.sub.1 (n) by the filter 1. An output of the filter 1 is 
supplied to the shifting circuit 5. The shifting circuit 5 shifts the bits 
in correspondence to the shift amount in the shifting circuit 4 in the 
reverse direction of the shifting direction. An output s.sub.2 (n) of the 
shifting circuit 5 is supplied to the gain control circuit 6. The gain 
fluctuation in the filter 1 is compensated by the gain control circuit 6. 
An output s.sub.3 (n) of the gain control circuit 6 is outputted from an 
output terminal 7. 
A pseudo filtering signal p.sub.-- s.sub.1 (n) is supplied to an input 
terminal 12. The pseudo filtering signal p.sub.-- s.sub.1 (n) is supplied 
to the filer 11 and to a gain calculating circuit 13. An output p.sub.-- 
s.sub.2 (n) of the filter 11 is supplied to the gain calculating circuit 
13. 
The gain calculating circuit 13 calculates a gain fluctuation G by the 
filter 11 as 
EQU G=.alpha..sub.2 (n)/.alpha..sub.1 (n) 
from a value .alpha..sub.1 (n) that is proportional to the input signal 
vector p.sub.-- s.sub.1 (n) of the filter 11 and a value amplitude 
.alpha..sub.2 (n) that is proportional to the output signal vector 
p.sub.-- s.sub.2 (n) of the filter 11. 
The filter 11 has characteristics similar to those of the post filter 1. 
Therefore, the gain of the filter 11 obtained by the gain calculating 
circuit 13 corresponds to the gain of the filter 1. Therefore, by using 
the gain obtained by the gain calculating circuit 13, the gain 
corresponding to the gain of the post filter 1 can be preliminarily 
obtained prior to performing the filter operation in the filter 1. 
The gain obtained by the gain calculating circuit 13 is supplied to the 
scaling value calculating circuit 3. Thus, the scaling when the filtering 
operation is performed in the filter 1 is optimally carried out. That is, 
as the gain obtained by the gain calculating circuit 13 is larger, a 
scaling value K is set to a smaller value. 
The gain obtained by the gain calculating circuit 13 is also supplied to 
the gain control circuit 6. The gain fluctuation of the filter 1 is 
compensated by the gain control circuit 6. That is, a gain is multiplied 
by the gain control circuit 6 so as to compensate the gain obtained by the 
gain calculating circuit 13. 
As mentioned above, when the gain fluctuation by the filter 1 is not known, 
by preparing the filter 11 with characteristics similar to those of the 
filter 1, the gain corresponding to the gain by the filter 1 can be 
preliminarily presumed. On the basis of such a fundamental principle, a 
construction of a decoder of an audio signal which can perform the optimum 
scaling for the post filter and can compensate the gain fluctuation 
occurring by the post filter will now be examined. 
A construction in a case where an audio signal can be synthesized by a 
synthesizing filter for the exciting signal is shown in FIG. 4. As a 
synthesizing filter 22, a linear predictive coefficient filter (LPC) as 
shown in FIG. 5, a partial autocorrelation (COR) coefficient filter as 
shown in FIG. 6, a linear spectral pair (LSP) coefficient filter as shown 
in FIG. 7, or the like is used. 
According to such a construction, as shown in FIG. 4, by providing the 
synthesizing filter 22 with an exciting signal ex.sub.11 (n), an audio 
signal s.sub.11 (n) is derived. A quality of audio signal is improved by 
providing a post filter 24 to the audio signal s.sub.11 (n). 
In case of performing a filter operation of the post filter 24 by a fixed 
point arithmetic operation, if a gain fluctuation value occurring by the 
post filter 24 is known, a word length can be effectively used by the 
scaling process. The filtering operating process with a high precision can 
be performed. Therefore, a scaling value calculating circuit 25 and 
shifting circuits 23 and 26 are provided in order to perform the filtering 
operation of the post filter 24 by executing the scaling process to it. 
When the gain fluctuation occurs by the post filter 24, so long as the gain 
fluctuation in the post filter 24 is known, the gain which corresponds to 
the gain fluctuation of the post filter 24 and is in the reverse direction 
of that of such a gain is added to the post stage of the post filter 24, 
so that the gain fluctuation in the post filter 24 can be compensated. A 
gain control circuit 27 is provided to compensate the gain fluctuation 
occurring by the post filter 24. 
A synthesizing filter 32 having characteristics similar to those of the 
synthesizing filter 22 and a post filter 33 having characteristics similar 
to those of the post filter 24 are provided to presume the gain in the 
post filter 24. A pseudo exciting signal p.sub.-- ex.sub.11 (n) is 
supplied to the synthesizing filter 32, thereby obtaining a pseudo audio 
signal p.sub.-- s.sub.11 (n). The pseudo audio signal p.sub.-- s.sub.11 
(n) is supplied to the post filter 33 and a gain of the post filter 33 is 
obtained by using an input signal and an output signal of the post filter 
33, thereby presuming the gain of the post filter 24. By using the gain 
presumed by using the post filter 33, the optimum scaling can be carried 
out in case of executing the operation of the post filter 24 by the 
scaling process. The optimum gain compensation can be also performed when 
executing the gain compensation by the gain control circuit 27. 
That is, in FIG. 4, the exciting signal ex.sub.11 (n) is supplied to an 
input terminal 21. The exciting signal ex.sub.11 (n) from the input 
terminal 21 is supplied to the synthesizing filter 22. The audio signal 
s.sub.11 (n) is synthesized from the exciting signal ex.sub.11 (n) by the 
synthesizing filter 11. The signal s.sub.11 (n) is supplied to the 
shifting circuit 23. The signal s.sub.11 (n) synthesized by the 
synthesizing filter 22 is shifted by the shifting circuit 23 on the basis 
of the scaling value from the scaling value calculating circuit 25. An 
output of the shifting circuit 23 is supplied to the post filter 24. A 
filtering operation is performed to the signal s.sub.11 (n) from the 
synthesizing filter 22 by the post filter 24. The filtering operation in 
the post filter 24 is realized by a fixed point arithmetic operation. The 
output of the post filter 24 is supplied to the shifting circuit 26. The 
shifting circuit 26 shifts the signal in correspondence to the shift 
amount in the shifting circuit 23 in the reverse direction of the shifting 
direction. An output s.sub.12 (n) of the shifting circuit 26 is supplied 
to the gain control circuit 27. The gain control circuit 27 is used to 
compensate the gain fluctuation caused by the post filter 24. An output 
s.sub.13 (n) of the gain control circuit 27 is outputted from an output 
terminal 28. 
The pseudo exciting circuit p.sub.-- ex.sub.11 (n) is supplied from an 
input terminal 31. The pseudo exciting signal p.sub.-- ex.sub.11 (n) is 
supplied to the synthesizing filter 32. The synthesizing filter 32 has a 
construction similar to that of the synthesizing filter 22. The pseudo 
audio signal p.sub.-- s.sub.11 (n) is synthesized by the synthesizing 
filter 32. An output of the synthesizing filter 32 is supplied to the post 
filter 33 and to a gain calculating circuit 34. An output of the post 
filter 33 is supplied to the gain calculating circuit 34. The post filter 
33 has characteristics similar to those of the post filter 24. 
The gain calculating circuit 34 calculates a gain fluctuation by the filter 
33 from a value that is proportional to the input signal vector p.sub.-- 
s.sub.11 (n) of the post filter 33 and a value that is proportional to an 
output signal vector p.sub.-- s.sub.12 (n) of the filter 33. 
The synthesizing filter 32 has characteristics similar to those of the 
synthesizing filter 22. The post filter 33 has characteristics similar to 
those of the post filter 24. Therefore, the gain of the post filter 33 
obtained by the gain calculating circuit 34 corresponds to the gain of the 
post filter 24. 
The gain obtained by the gain calculating circuit 34 is supplied to the 
scaling value calculating circuit 25. Thus, the scaling when the filtering 
operation is performed in the post filter 24 is optimally carried out. The 
gain obtained by the gain calculating circuit 34 is supplied to the gain 
control circuit 27. The gain fluctuation of the post filter 24 is 
compensated by the gain control circuit 27. 
A case where an exciting signal vector is expressed by a linear sum of an 
output of an impulse sequence generator based on the pitch information and 
sound/soundless information of an output of a white noise generator will 
now be examined. 
As shown in FIG. 8, it is assumed that an exciting signal ex.sub.21 (n) is 
shown by a linear sum 
EQU ex.sub.21 (n)=.beta.b.sub.L (n)+.gamma.c(n) 
of the sound/soundless information (.beta., .gamma.) of an impulse signal 
h.sub.L21 (n) of an impulse sequence generator 41 for generating an 
impulse sequence based on the pitch information L and a noise signal 
c.sub.21 (n) from a white noise generator 42. An audio signal s.sub.21 (n) 
is derived by providing a synthesizing filter 46 for the exciting signal 
ex.sub.21 (n) which is obtained as mentioned above. A quality of the audio 
signal is improved by providing a post filter 49 for the audio signal 
s.sub.21 (n). 
In case of performing a filtering operation of the post filter 49 by a 
fixed point arithmetic operation, if a gain fluctuation value occurring by 
the post filter 49 is known, a word length can be effectively used by a 
scaling process and a filtering operating process with a high precision 
can be carried out. A scaling value calculating circuit 48 and shifting 
circuits 47 and 50 are provided to execute a filtering operation of the 
post filter 49 by executing a scaling process. 
When a gain fluctuation occurs by the post filter 49, if the gain 
fluctuation in the post filter 49 is known, a gain which corresponds to 
the gain fluctuation in the post filter 49 and is in the reverse direction 
of that of such a gain is added to the post stage of the post filter 49, 
so that the gain fluctuation in the post filter 49 can be compensated. A 
gain control circuit 51 is provided to compensate the gain fluctuation 
occurring by the post filter 49 as mentioned above. 
A synthesizing filter 61 having characteristics similar to those of the 
synthesizing filter 46 and a post filter 63 having characteristics similar 
to those of the post filter 49 are provided in order to presume the gain 
in the post filter 49. A pseudo audio signal p.sub.-- s.sub.21 (n) is 
obtained by supplying a pseudo exciting signal p.sub.-- es.sub.21 (n) to 
the synthesizing filter 61. The pseudo audio signal p.sub.-- s.sub.21 (n) 
is supplied to the filter 63 and a gain of the filter 63 is obtained by 
using an input signal and an output signal of the filter 63, thereby 
presuming the gain of the post filter 49. By using the gain presumed by 
using the filter 63, the optimum scaling can be performed when executing 
an arithmetic operation of the post filter 49 by the scaling process. The 
optimum gain compensation can be carried out in case of performing the 
gain compensation by the gain control circuit 51. 
That is, in FIG. 8, the impulse sequence signal h.sub.L21 (n) based on the 
pitch information L is generated from the impulse sequence generator 41. 
An output of the impulse sequence generator 41 is supplied to a multiplier 
43 for multiplying the gain .beta. indicative of the sound/soundless 
information. An output of the multiplier 43 is supplied to an adder 45. 
The white noise generator 42 generates the noise signal c.sub.21 (n). An 
output of the white noise generator 42 is supplied to a multiplier 44 for 
multiplying the gain .gamma. indicative of the sound/soundless 
information. An output of the multiplier 44 is supplied to the adder 45. 
An exciting signal vector ex.sub.21 (n) is formed by the adder 45. The 
exciting signal vector ex.sub.21 (n) is expressed by 
EQU ex.sub.21 (n)=.beta.h.sub.L21 (n)+.gamma.c.sub.21 (n) 
The exciting signal vector ex.sub.21 (n) is supplied to the synthesizing 
filter 46. An audio signal is synthesized by the synthesizing filter 46. 
The signal s.sub.21 (n) synthesized by the synthesizing filter 46 is 
supplied to the shifting circuit 47. The audio signal synthesized by the 
synthesizing filter 46 is shifted by the shifting circuit 47 on the basis 
of the scaling value from the scaling value calculating circuit 48. An 
output of the shifting circuit 47 is supplied to the post filter 49. The 
post filter 49 executes a process to improve a sound quality. A filtering 
operation is performed by the post filter 49 to the signal from the 
synthesizing filter 46. The filtering operation in the post filter 49 is 
realized by a fixed point arithmetic operation. An output of the post 
filter 49 is supplied to the shifting circuit 50. The shifting circuit 50 
shifts the bits in accordance with the shift amount in the shifting 
circuit 47 in the reverse direction of the shifting direction. An output 
s.sub.22 (n) of the shifting circuit 50 is supplied to the gain control 
circuit 51. The gain control circuit 51 compensates the gain fluctuation 
occurring by the post filter 49. An output s.sub.23 (n) of the gain 
control circuit 51 is outputted from an output terminal 52. 
The synthesizing filter 61 has a construction similar to that of the 
synthesizing filter 46. The pseudo exciting signal p.sub.-- ex.sub.21 (n) 
is supplied from an input terminal 62 to the synthesizing filter 61. The 
pseudo audio signal p.sub.-- s.sub.21 (n) is synthesized by the 
synthesizing filter 61. The pseudo audio signal p.sub.-- s.sub.21 (n) is 
supplied to the post filter 63 and to a gain calculating circuit 64. An 
output of the post filter 63 is supplied to the gain calculating circuit 
64. The post filter 63 has characteristics similar to those of the post 
filter 49. 
The gain calculating circuit 64 calculates the gain by the filter 63 from a 
value that is proportional to the input signal vector p.sub.-- s.sub.21 
(n) of the post filter 63 and a value that is proportional to an output 
signal vector p.sub.-- s.sub.22 (n) of the post filter 63. 
The synthesizing filter 61 has characteristics similar to those of the 
synthesizing filter 46. The post filter 63 has characteristics similar to 
those of the post filter 49. Therefore, the gain of the post filter 63 
obtained by the gain calculating circuit 64 corresponds to the gain of the 
post filter 49. 
The gain obtained by the gain calculating circuit 64 is supplied to the 
scaling value calculating circuit 48. Thus, the scaling when the filtering 
operation is performed in the post filter 49 is optimally carried out. The 
gain obtained by the gain calculating circuit 64 is supplied to the gain 
control circuit 51. The gain fluctuation of the post filter 49 is 
compensated by the gain control circuit 51. 
An example in the case where the exciting signal vector is expressed by a 
linear sum of the sound/soundless information of the past exciting signal 
vector state based on the pitch information and the noise signal will now 
be examined. 
As shown in FIG. 9, it is now assumed that an exciting signal ex.sub.31 (n) 
is shown by a linear sum 
EQU ex.sub.31 (n)=.beta.b.sub.L31 (n)+.gamma.c.sub.31 (n) 
of the sound/soundless information (.beta., .gamma.) of a signal vector 
b.sub.L31 (n) that is formed by the pitch information L and the past 
exciting signal vector state and a noise signal c.sub.31 (n) from a white 
noise generator 72. An audio signal s.sub.31 (n) is obtained by providing 
a synthesizing filter 76 for the exciting signal ex.sub.31 (n) which is 
obtained as mentioned above. A quality of the audio signal is improved by 
providing a post filter 79 for the audio signal s.sub.31 (n). 
In case of performing a filtering operation of the post filter 79 by a 
fixed point arithmetic operation, if a gain fluctuation value occurring by 
the post filter 79 is known, a word length can be effectively used by the 
scaling process and a filtering operating process of a high precision can 
be performed. Therefore, a scaling value calculating circuit 78 and 
shifting circuits 77 and 80 are provided to execute the filtering 
operation of the post filter 79 by performing the scaling process. 
When the gain fluctuation occurs by the post filter 79, if the gain 
fluctuation of the post filter 79 is known, a gain which corresponds to 
the gain fluctuation in the post filter 79 and is in the reverse direction 
of that of such a gain is added to the post stage of the post filter 79, 
so that the gain fluctuation in the post filter 79 can be compensated. A 
gain control circuit 81 is provided to compensate the gain fluctuation 
occurring by the post filter 79 as mentioned above. 
A synthesizing filter 91 having characteristics similar to those of the 
synthesizing filter 76 and a post filter 93 having characteristics similar 
to those of the post filter 79 are provided to presume the gain in the 
post filter 79. A pseudo audio signal p.sub.-- s.sub.31 (n) is obtained by 
supplying a pseudo exciting signal p.sub.-- ex.sub.31 (n) to the 
synthesizing filter 91. The pseudo audio signal p.sub.-- s.sub.31 (n) is 
supplied to the post filter 93 and a gain of the post filter 93 is 
obtained by using an input signal and an output signal of the post filter 
93, thereby presuming the gain of the post filter 79. By using the gain 
presumed by using the post filter 93, the optimum scaling can be performed 
in case of executing the operation of the post filter 79 by the scaling 
process. The optimum gain compensation can be carried out in case of 
performing the gain compensation by the gain control circuit 81. 
That is, in FIG. 9, a signal b.sub.L31 is generated from a signal generator 
71 on the basis of the pitch information L and the past exciting signal 
vector state. The signal b.sub.L31 is supplied to a multiplier 73 for 
multiplying the gain .beta. indicative of the sound/soundless information. 
An output of the multiplier 73 is supplied to an adder 75. 
The noise signal c.sub.31 (n) is generated from the white noise generator 
72. The noise signal is supplied to a multiplier 74 to be multiplied with 
the gain .gamma. indicative of the sound/soundless information. An output 
of the multiplier 74 is supplied to the adder 75. 
The exciting signal vector ex.sub.31 (n) is formed by the adder 75. The 
exciting signal vector ex.sub.31 (n) is expressed as 
EQU ex.sub.31 (n)=.beta.b.sub.L31 (n)+.gamma.c.sub.31 (n) 
The exciting signal vector ex.sub.31 (n) is supplied to the synthesizing 
filter 76. An audio signal s.sub.31 (n) is synthesized by the synthesizing 
filter 76. 
The audio signal s.sub.31 (n) synthesized by the synthesizing filter 76 is 
supplied to the shifting circuit 77. The audio signal synthesized by the 
synthesizing filter 76 is shifted by the shifting circuit 77 on the basis 
of the scaling value from the scaling value calculating circuit 78. An 
output of the shifting circuit 77 is supplied to the post filter 79. The 
post filter 79 executes a process to improve the sound quality. A 
filtering operation is performed by the post filter 79 to the signal from 
the synthesizing filter 76. The filtering operation in the post filter 79 
is realized by a fixed point arithmetic operation. An output of the post 
filter 79 is supplied to the shifting circuit 80. The shifting circuit 80 
shifts the bits in accordance with the shift amount in the shifting 
circuit 77 in the reverse direction of the shifting direction. An output 
s.sub.32 (n) of the shifting circuit 80 is supplied to the gain control 
circuit 81. The gain control circuit 81 compensates the gain fluctuation 
occurring by the post filter 79. An output s.sub.33 (n) of the gain 
control circuit 81 is outputted from an output terminal 82. 
The synthesizing filter 91 has a construction similar to that of the 
synthesizing filter 76. The pseudo exciting signal p.sub.-- ex.sub.31 (n) 
is supplied from an input terminal 92 to the synthesizing filter 91. The 
pseudo audio signal p.sub.s.sub.31 (n) is synthesized by the synthesizing 
filter 91. The pseudo audio signal p.sub.-- s.sub.31 (n) is supplied to 
the post filter 93 and to a gain calculating circuit 94. An output of the 
post filter 93 is supplied to the gain calculating circuit 94. The post 
filter 93 has characteristics similar to those of the post filter 79. 
The gain calculating circuit 94 calculates a gain by the post filter 93 
from a value that is proportional to the input signal vector p.sub.-- 
s.sub.31 (n) of the post filter 93 and a value that is proportional to an 
output signal vector p.sub.-- s.sub.32 (n) of the post filter 93. 
The synthesizing filter 91 has characteristics similar to those of the 
synthesizing filter 76. The post filter 93 has characteristics similar to 
those of the post filter 79. Therefore, the gain of the post filter 93 
obtained by the gain calculating circuit 94 corresponds to the gain of the 
post filter 79. 
The gain obtained by the gain calculating circuit 94 is supplied to the 
scaling value calculating circuit 78. Thus, the scaling when the filtering 
operation is performed by the post filter 79 is optimally executed. The 
gain obtained by the gain calculating circuit 94 is supplied to the gain 
control circuit 81, so that the gain fluctuation of the post filter 79 is 
compensated. 
In FIG. 9, it is desirable that the pseudo exciting signal p.sub.-- 
ex.sub.31 (n) which is supplied to the synthesizing filter 91 is similar 
to the exciting signal ex.sub.31 (n) which is supplied to the synthesizing 
filter 76. However, the exciting signal ex.sub.31 (n) is shown by a linear 
sum 
EQU ex.sub.31 (n)=.beta.b.sub.L31 (n)+.gamma.c.sub.31 (n) 
of the sound/soundless information (.beta., .gamma.) of the signal vector 
b.sub.L31 (n) which is formed by the pitch information L and the past 
exciting signal vector state and the noise signal c.sub.31 (n) from the 
white noise generator 42. It is difficult to form the pseudo exciting 
signal p.sub.-- ex.sub.31 (n) similar to the exciting signal ex.sub.31 
(n). 
As shown in FIG. 10, therefore, it is considered to use an impulse sequence 
signal h.sub.L41 (n) based on the pitch information L as a pseudo exciting 
signal p.sub.-- ex.sub.41 (n). 
That is, in FIG. 10, the impulse sequence signal h.sub.L41 (n) based on the 
pitch information L is generated from an impulse sequence generator 101. 
The impulse sequence signal h.sub.L41 (n) is supplied to the synthesizing 
filter 91. An output of the synthesizing filter 91 is supplied to the post 
filter 93 and to the gain calculating circuit 94. An output of the post 
filter 93 is supplied to the gain calculating circuit 94. 
The gain calculating circuit 94 calculates a gain of the post filter 93 
from a value that is proportional to the input signal vector of the post 
filter 93 and a value that is proportional to the output signal vector of 
the post filter 93. The gain obtained by the gain calculating circuit 94 
is supplied to the scaling value calculating circuit 78. Thus, the scaling 
when the filtering operation is performed by the post filter 79 is 
optimally executed. The gain obtained by the gain calculating circuit 94 
is supplied to the gain control circuit 81. The gain fluctuation of the 
post filter 79 is compensated by the gain control circuit 81. 
As shown in FIG. 11, as a pseudo exciting signal p.sub.-- ex.sub.51 (n), it 
is considered to use a linear sum of the sound/soundless information 
(.beta., .gamma.) of an impulse sequence signal p.sub.-- h.sub.L51 (n) 
based on the pitch information L and a noise signal p.sub.-- c.sub.51 (n). 
That is, in FIG. 11, the impulse sequence signal p.sub.-- h.sub.L51 (n) 
based on the pitch information L is generated from an impulse sequence 
generator 111. The impulse sequence signal p.sub.-- h.sub.L51 (n) is 
supplied to a multiplier 113 for being multiplied with the gain .beta.. An 
output of the multiplier 113 is supplied to an adder 115. A noise signal 
p.sub.-- c.sub.52 (n) is generated from a white noise generator 112. A 
noise signal p.sub.-- c.sub.52 (n) is supplied to a multiplier 114 for 
being multiplied with the gain .gamma.. An output of the multiplier 114 is 
supplied to the multiplier 115. 
A linear sum of the sound/soundless information (.beta., .gamma.) of the 
impulse signal p.sub.-- h.sub.L51 (n) based on the pitch information L and 
the noise signal p.sub.-- c.sub.52 (n) is obtained by the adder 115 and 
the pseudo exciting signal p.sub.-- ex.sub.51 (n) is obtained by 
EQU p.sub.-- ex.sub.51 (n)=.beta..multidot.(p.sub.-- h.sub.L51 
(n))+.gamma..multidot.(p.sub.-- c.sub.52 (n)) 
The pseudo exciting signal p.sub.-- ex.sub.51 (n) formed as mentioned above 
is supplied to the synthesizing filter 91. 
An output of the synthesizing filter 91 is supplied to the post filter 93 
and to the gain calculating circuit 94. An output of the post filer 93 is 
supplied to the gain calculating circuit 94. The gain of the post filter 
93 is calculated by the gain calculating circuit 94. The gain obtained by 
the gain calculating circuit 94 is supplied to the scaling value 
calculating circuit 78. Thus, the scaling when the filtering operation is 
performed in the post filter 79 is optimally executed. The gain obtained 
by the gain calculating circuit 94 is supplied to the gain control circuit 
81. The gain fluctuation of the post filter 79 is compensated by the gain 
control circuit 81. 
It is considered that the pitch position of the impulse sequence of the 
pitch period based on the pitch information P is synchronized with the 
pitch position of the exciting signal vector. The pitch position can be 
coarsely known by searching the peak of the exciting signal vector. 
Further, as shown in FIG. 12, it is considered that a signal b.sub.L61 
which is based on the pitch information L and the past exciting signal 
vector state and is generated from the signal generator 71 is used as a 
pseudo exciting signal p.sub.-- ex.sub.61 (n). 
That is, in FIG. 12, the signal b.sub.L61 (n) which is based on the pitch 
information L and the past exciting signal vector state and is generated 
from the signal generator 71 is supplied as a pseudo exciting signal 
p.sub.-- ex.sub.61 (n) to the synthesizing filter 91. An output of the 
synthesizing filter 91 is supplied to the post filter 93 and to the gain 
calculating circuit 94. An output of the post filter 93 is supplied to the 
gain calculating circuit 94. The gain of the filter 93 is calculated by 
the gain calculating circuit 94. The gain obtained by the gain calculating 
circuit 94 is supplied to the scaling value calculating circuit 78. Thus, 
the scaling when the filtering operation is executed in the post filter 79 
is optimally performed. The gain obtained by the gain calculating circuit 
94 is supplied to the gain control circuit 81. The gain fluctuation of the 
post filter 79 is compensated by the gain control circuit 81. 
FIG. 13 shows an example in which a decoder of the VSELP is realized in 
consideration of the above study. In FIG. 13, reference numeral 121 
denotes a long period filtering state. The past exciting vector is 
supplied to the long period filtering state 121 and the received pitch 
information L is also supplied to an input terminal 120. The long period 
filtering state 121 forms the signal b.sub.L61 (n) based on the received 
pitch information L and the past exciting signal vector state. The formed 
signal b.sub.L61 (n) is supplied to a multiplier 122. 
Reference numeral 123 denotes a code book. The exciting source code I 
received is supplied from an input terminal 124 to the code book 123. A 
basic vector is added by the code book 123 on the basis of the exciting 
source code I and a noise signal c.sub.61 (n) is formed. The noise signal 
c.sub.61 (n) is supplied to a multiplier 125. 
The gain .beta. received is supplied from an input terminal 126 to the 
multiplier 122. The gain .gamma. received is supplied from an input 
terminal 127 to the multiplier 125. The gain .beta. is multiplied with the 
signal b.sub.L61 (n) by the multiplier 122. The gain .gamma. is multiplied 
with the noise signal c.sub.61 (n) by the multiplier 125. 
Outputs of the multipliers 122 and 125 are supplied to an adder 130. An 
exciting signal vector ex.sub.61 (n) is formed by the adder 130. The 
exciting signal vector ex.sub.61 (n) is expressed by 
EQU ex.sub.61 (n)=.beta.b.sub.L61 (n)+.gamma.c.sub.61 (n) 
The exciting signal vector ex.sub.61 (n) is supplied to a short period 
synthesizing filter 131 and is fed back to the long period filtering state 
121. 
The parameter .alpha. is supplied from a terminal 128 to the short period 
synthesizing filter 131. The audio signal is synthesized by the short 
period synthesizing filter 131. The synthesized audio signal is supplied 
to a shifting circuit 132. The shifting circuit 132 shifts the synthesized 
audio signal on the basis of the scaling value from a scaling value 
calculating circuit 133. An output of the shifting circuit 133 is supplied 
to a post filter 134. 
The post filter 134 executes a process to improve a sound quality. The post 
filer 134 executes the filtering operation to the signal from the short 
period synthesizing filter 131. The filtering operation in the post filter 
134 is realized by a fixed point arithmetic operation. An output of the 
post filter 134 is supplied to a shifting circuit 135. The shifting 
circuit 135 shifts the input signal in accordance with the shift amount in 
the shifting circuit 132 in the reverse direction of the shifting 
direction. An output of the shifting circuit 135 is supplied to a gain 
control circuit 136. The gain control circuit 136 compensates a gain 
fluctuation caused by the post filter 134. An output of the gain control 
circuit 136 is outputted as a decoding signal from an output terminal 137. 
The signal b.sub.L61 (n) from the long period filtering state 121 is 
supplied as a pseudo exciting signal p.sub.-- ex.sub.61 (n) to a short 
period synthesizing filter 141. The parameter .alpha. is supplied from the 
terminal 128 to the short period synthesizing filter 141. The short period 
synthesizing filter 141 is constructed in a manner similar to the short 
period synthesizing filter 127. 
An output p.sub.-- s.sub.61 (n) of the short period synthesizing filter 141 
is supplied to a post filter 142 and to a gain calculating circuit 143. An 
output of the post filter 142 is supplied to the gain calculating circuit 
143. The post filter 142 has characteristics similar to those of the post 
filter 134. 
The input signal vector p.sub.-- s.sub.61 (n) of the post filter 142 and an 
output signal vector p.sub.-- s.sub.62 (n) of the post filter 142 are 
supplied to the gain calculating circuit 143. The gain calculating circuit 
143 calculates a gain by the post filter 142 from a value that is 
proportional to the input signal vector p.sub.-- s.sub.61 (n) of the post 
filter 142 and a value that is proportional to the output signal vector 
p.sub.-- s.sub.62 (n) of the post filter 142. 
The gain obtained by the gain calculating circuit 143 is supplied to the 
scaling value calculating circuit 133. Thus, the scaling when the 
filtering operation is executed in the post filter 134 is optimally 
performed. The gain obtained by the gain calculating circuit 143 is 
supplied to the gain control circuit 136. The gain fluctuation caused by 
the post filter 134 is compensated by the gain control circuit 136. 
In the above example, although the case of mainly using the VSELP as a 
compressing system has been described, the invention can be also similarly 
applied to the other compressing systems. 
According to the invention, another post filter having characteristics 
similar to those of the post filter for processing the demodulated audio 
signal is prepared and the gain of the post filter to process the audio 
signal can be previously presumed by the post filer. Therefore, the 
optimum scaling can be executed when the filtering operation in the post 
filter is performed by the fixed point arithmetic operation. By using the 
gain obtained as mentioned above, the gain fluctuation occurring by the 
post filter can be optimally corrected. 
The present invention is not limited to the foregoing embodiments but many 
modifications and variations are possible within the spirit and scope of 
the appended claims of the invention.