Adaptive codebook

In a CELP system, a coder and a decoder have identical codebooks, and the amount of data to be transmitted is compressed by transmission and reception of codebook indexes. Past excitation signals are stored in a memory and used as an adaptive codebook to improve the speech quality. The coder and the decoder each comprise memory means for storing index data for at least one frame, and means for generating an adaptive codebook afresh by initialization to zero for each frame when generating an excitation signal according to stored indexes.

BACKGROUND OF THE INVENTION 
The present invention relates to adaptive codebooks for signal generation 
according to indexes. More specifically, the present invention relates to 
speech coding techniques using communication systems or radio 
communication systems based on packet exchange network, particularly to 
adaptive codebooks used for emphasizing pitch components. 
Many communication systems such as cellular communication systems or 
personal communication systems are based on radio channels for data 
communication. In such data communication, the radio channel is affected 
by some error sources such as multi-path fading. Such error sources may 
give rise to a problem of frame missing. By the term "missing" is meant 
total or partial destruction of the group of bits transmitted to the 
receiver. By the term "frame" is meant a fixed number of bits dealt with 
as an entity for communication in a communication system. 
In the event of perfect missing of the bits of one frame, the receiver no 
longer has any bit for interpretation. In such an occasion, the receiver 
may generate a meaningless result. When the received frame is destroyed 
and thus unreliable, the receiver may generate an extremely distorted 
result. Increasing demand for the radio system capacity has given rise to 
the necessity of utmost utilization of the radio system bandwidth capable 
of being utilized. One method for improving the system bandwidth utility 
efficiency is to use signal compression techniques. In a radio system for 
transmitting a speech signal, speech compression (or speech coding) 
techniques may be used to this end. Such a speech coding technique is 
implemented by a synthesized speech coder based on analysis, such as 
well-known Code Excited Linear Prediction speech coder. 
The problem of packet missing in a packet exchange network adopting a 
speech coding system is very analogous to the frame missing in the case of 
radio communication. Specifically, in the event of packet missing, the 
receiver, that is, speech decoder may no longer be able to receive frame 
or receive a frame with a missing of a considerable number of bits. In 
either case, the speech decoder presents essentially the same problem; 
that is, the speech decoder should synthesize speech in spite of missing 
of compressed speech data. Both the "frame missing" and "packet missing" 
concern the problem in communication channel (or network) to bring about 
missing of transmitted bits. In the following description, the term "frame 
missing" may be regarded to be a synonym of the packet missing. 
A CELP speech coder uses an excitation signal codebook for coding an 
original speech signal. The excitation signals are used for "exciting" a 
linear prediction (LPC) filter for synthesizing a speech signal (or some 
precursor thereto). The synthesized speech signal is compared with the 
signal to be coded. A codebook index which is most identical with the 
original signal is transmitted to the CELP decoder. Communication of other 
type data may be made in dependence on the type of the CELP system. For 
the brevity of description, in the present specification the indexes and 
data obtained as a result of code correction or like process on the 
indexes are thus generally described as "index data". 
In the prior art CELP coder, excitation signals are generated with a 
structure as shown in FIG. 3, as is well known in, for instance, "Vector 
Sum Excited Linear Prediction (VSELP) Speech Coding for Japan Digital 
Cellular", RCS90-26, (TRREDCE) Technical Research Reports of the Institute 
of Electronics and Data Communication Engineers of Japan. 
FIG. 3 is a block diagram illustrating the excitation signal generation 
described in the reports, i.e., a summary of typical excitation signal 
generation. Referring to the Figure, a multiplier 302 adjusts the output 
signal level of a fixed codebook 301 by multiplying the signal by a gain 
Gc. Another multiplier 304 adjusts the output signal level of an adaptive 
codebook 303 by multiplying the signal by a gain Gp. An adder 305 adds 
together the two level adjusted signals to generate an excitation signal. 
The excitation signal thus generated is fed back to the adaptive codebook 
to realize reproduction of the pitch lag of speech. Generally, the 
transfer function of the adaptive codebook is given as: 
EQU P1(z)=GpZ.sup.-P, 
where p is the group delay, i.e., pitch lag. In the excitation signal 
generation, the CELP speech coder makes a retrieval for the best identical 
index to the input speech signal. In FIG. 3, the best identical indexes in 
the current frame are labeled I fcb curr and I acb curr, and the gains 
obtained as a result of conversion of the indexes I Gc curr and I Gp curr 
concerning the gain are labeled Gc curr and Gp curr. The CELP speech 
decoder receives the most identical data from the CELP speech coder and, 
like the coder, generates an excitation signal. However, generation of an 
error in the transmission line due to multi-pulse fading or the like, 
results in frame missing and deterioration of the speech quality. 
Heretofore, "a method of improving the performance of coding systems" which 
is disclosed in Japanese Laid-Open Patent Publication 8-227300, has been 
well known as a method of improving the performance of coding systems 
against frame missing. 
FIG. 4 shows a prior art radio communication system disclosed in this 
Laid-Open Patent Publication. 
Referring to the Figure, the illustrated ratio communication system 
comprises a G.728 speech coder 401, a decoder pre-processor 403 and a 
G.728 speech decoder 404. 
The G.728 speech coder 401 codes input speech, and transmits coded speech 
signal thus obtained to a communication channel 402. The coded speech 
signal is affected by some error sources such as multi-fading as it is 
passed through the communication channel 402, and received as coded speech 
signal with frame missing in the decoder pre-processor 403. The decoder 
pre-processor 403 "decodes" missing-frame-free coded speech signal in a 
range necessary for the generation of an excitation signal which is also 
generated in the coder. When frame missing is recognized, a "decoded" 
excitation signal of the preceding frame is externally inserted throughout 
the period of the missing frame. The externally inserted excitation signal 
is coded by using the best codebook identity that can be utilized, and is 
made so by executing a series of codebook "retrievals". Particularly, a 
codebook vector which is most identical with each vector of the externally 
inserted excitation signal is selected. The pre-processor discriminates 
the index that represents the best codebook, and generates the coded 
speech signal based on this index. Using this correction signal, the 
decoder can approximate the externally inserted excitation signal from the 
pre-processor, thus minimizing the advantages of destroyed frames in the 
reconstituted speech signal. 
FIG. 5 is a flow chart concerning the operation of the decoder 
pre-processor. In this example, the CELP speech coder is used, and a 
target signal is selected as being an excitation signal, which is 
constituted by external insertion of an excitation signal represented by 
coded signal corresponding to the preceding frame. The pre-processor 
"decodes" missing-frame-free coded speech signal in a range necessary for 
the excitation signal generation. In other words, it executes the same 
codebook lookup as executed in the excitation signal generator 405 in the 
decoder. This means that the pre-processor 403 includes the same codebook 
as that present in both the coder and decoder. When a missing frame is 
recognized, the pre-processor 403 externally inserts the decoded 
excitation signal corresponding to the preceding frame inserted in the 
missing frame period. Subsequently, the (best identical) codebook index 
representing the externally inserted excitation signal is generated by 
executing codebook retrieval. 
With reference to FIG. 4, the pre-processor 403, receiving each frame from 
the communication channel 402 (step 500), checks whether the coded speech 
signal corresponding to the received frame has been destroyed (step 501). 
The check may be made by using a usual error detection signal. When the 
pre-processor 403 determines that the given frame has not been destroyed 
(step 502), it supplies the coded speech signal without correction to the 
decoder 404 (step 503). The pre-processor 403 executes codebook lookup for 
each codebook index contained in the given frame and, as a result, 
generates and stores an excitation signal (step 504). This process is 
essentially the same as executed by the excitation signal generator 405 in 
the decoder 404 shown in FIG. 3. The stored data is preserved for being 
used in the next frame process (when it is found that the next frame is a 
missing frame). 
When the pre-processor 403 recognizes in the step 502 that the given frame 
has been destroyed, it executes the steps 505 to 507. In the step 505, the 
pre-processor 403 corrects the coded speech signal. Specifically, in this 
step the pre-processor 403 executes external insertion of the excitation 
signal of the preceding frame (i.e., the signal decoded and stored in the 
step 500) as a corrected signal corresponding to the pertinent frame. 
In the next step 506, the pre-processor 403 executes the "coding" of the 
externally inserted excitation signal. Specifically, the pre-processor 403 
executes codebook retrieval for the best identical codebook entry with the 
externally inserted signal. Codebook is retrieved for each vector of the 
missing frame and the entry which is the best identical with the part 
corresponding to the externally inserted excitation signal. The reference 
of the best identity may be based on the mean square error measure or 
other error references well known to the person skilled in the art. 
Finally, in the step 507 the pre-processor 403 replaces the missing frame 
part of the coded speech signal with the codebook index generated in the 
step 506. Using this codebook index; the decoder can generate an 
excitation signal which approximates the externally inserted excitation 
signal generated in the step 505, thus permitting improvement of the 
performance of the coding system. After the pre-processor 403 has 
transmitted the coded speech signal to the decoder in the step 503 (and 
generated the excitation signal in the step 504), or after it has 
corrected the coded speech signal in the steps 505 to 507, the control 
routine returns to the step 500 to receive the next frame. 
In the technique as described above, in the event of the occurrence of a 
transmission line error on the communication channel, the internal states 
of the adaptive codebooks of the coder and decoder may fail to be 
identical. The occurrence of such identify failure may result in abnormal 
sound generation and deterioration of the speech quality when the decoder 
executes decoding by receiving the index transmitted from the coder, even 
though retrieval for the best identical index is made on the coder side. 
This is so because of the fact that the adaptive codebook has a feedback 
constitution that an adaptive codebook is generated by using the 
excitation signal of the preceding frame. Due to an error occurring during 
voiced speech, the internal state of the adaptive codebook of the decoder 
becomes different from that of the adaptive codebook of the coder. When 
the signal level is reduced in such a case as when a non-voice state is 
brought about, the signal level of the adaptive codebook internal state is 
also reduced, so that an error occurring on the transmission line of 
course has less adverse advantages. An error occurring on the transmission 
line during a voiced speech signal period, however, has advantages 
continuous to a non-voice period due to feedback loop. During the period 
until the non-voice period sets in after occurrence of a transmission line 
error, the index combination may lead to generation of abnormal noise and 
extreme deterioration of the speech quality. 
SUMMARY OF THE INVENTION 
An object of the present invention, therefore, is to improve the speech 
quality by reducing abnormal sound due to identity failure of the internal 
states of adaptive codebooks of the coder and decoder, in which abnormal 
sound may occur even in the absence of any transmission error after 
occurrence of a previous identify failure due to a transmission line 
error. 
According to an aspect of the present invention, there is provided an 
adaptive codebook, in a coder of CELP for coding a speech signal or audio 
signal to index data, and also in a decoder for decoding the index data to 
the speech signal or audio signal, a codebook for signal generation 
according to the index, comprising memory means for storing index data 
transmitted and received between the coder and the decoder for at least 
one frame, index data stored in the memory means being used to generate an 
excitation signal, signal series thus generated being used as an adaptive 
codebook. 
The excitation signal generation based on index data for at least one frame 
stored in the memory means is caused after clearing last excitation signal 
memory contents in the memory means. 
According to another aspect of the present invention, there is provided an 
adaptive codebook in a coder of CELP for coding a speech signal or audio 
signal to index data, and also in a decoder for decoding the index data to 
the speech signal or audio signal comprising: index memory means for 
providing fixed codebook index preceding by i frames, adaptive codebook 
index and gain index; a fixed codebook for providing a signal series 
according to the data of fixed codebook index preceding by i frames; an 
excitation signal memory means for providing a signal series according to 
the data of adaptive codebook index preceding by i frames; an excitation 
signal generator for generating signal of at least one frame by using the 
outputs of the fixed codebook and excitation signal memory and the gain 
index; and an adaptive codebook for producing an adaptive codebook on the 
basis of the output of the excitation signal memory, wherein the data in 
the excitation signal memory means is updated according to the excitation 
signal. 
All the data in the excitation signal memory means is set to zero or to 
known data before the commencement of the excitation signal production of 
the current frame. 
The excitation signal generator generates the signal on the basis of the 
summed signal of the gain controlled outputs of the fixed codebook and 
excitation signal memory. The index memory is constituted by a RAM, the 
fixed codebook is constituted by a ROM in which a noise signal series has 
been written, and the excitation signal memory is constituted by a RAM. 
According to other aspect of the present invention, there is provided a 
coder having the foregoing adaptive codebook comprising: an adaptive 
codebook for producing an excitation vector signal corresponding to pitch 
vector including a component dependent on the periodicity of the speech 
signal; a fixed codebook for producing an excitation output vector 
corresponding to codevector of a non-periodic component; multiplier for 
multiplying the excitation vector signal and excitation output vector by 
respective gains; an adder for generating an excitation signal of the 
current frame by adding together the two product outputs of the 
multipliers; a synthesizing filter for generating a reproduced signal 
based on the excitation signal of the current frame; a subtracter, 
responsive to the reproduced signal, for producing an error between the 
reproduced signal and an input signal; and an error power evaluator for 
controlling and scanning the outputs of the adaptive and fixed codebooks 
and the gains of the multipliers for each frame, and producing an 
excitation signal corresponding to a minimum error to be the optimal 
excitation signal. 
According to still other aspect of the present invention, there is provided 
a decoder having the foregoing adaptive codebook comprising: an adaptive 
codebook for producing an excitation vector signal corresponding to pitch 
vector including a component dependent on the periodicity of the speech 
signal; a fixed codebook for producing an excitation output vector 
corresponding to codevector of a non-periodic component; multiplier for 
multiplying the excitation vector signal and excitation output vector by 
respective gains; an adder for generating an excitation signal of the 
current frame by adding together the two product outputs of the 
multipliers; a synthesizing filter for generating a reproduced signal 
based on the excitation signal of the current frame; and a post-filter, 
cascade connected to the output of the synthesizing filter, for generating 
a reconstituted speech signal. 
According to other aspect of the present invention, there is provided an 
adaptive codebook, in a CELP system as a speech coding system, a coder and 
a decoder have identical codebooks, and the amount of data to be 
transmitted is compressed by transmission and reception of codebook 
indexes, past excitation signals are stored in a memory and used as an 
adaptive codebook, the coder and the decoder each comprise memory means 
for storing index data for at least one frame, and means for generating an 
adaptive codebook afresh by initialization to zero for each frame when 
generating an excitation signal according to stored indexes. 
In the adaptive codebook for signal generation based on indexes according 
to the present invention, the excitation signal generating means produces 
the adaptive code width afresh from index data in a certain past period of 
time. Thus, when no error occurs for several continuous frames after 
occurrence of a transmission line error, it is possible to make the 
internal states of the adaptive codebooks of the coder and decoder 
identical. 
Other objects and features will be clarified from the following description 
with reference to attached drawings.

PREFERRED EMBODIMENTS OF THE INVENTION 
An embodiment of the present invention will now be described with reference 
to the drawings. Referring to FIG. 1, the best form of the present 
invention comprises a fixed codebook, an excitation signal memory means, 
gain control means for controlling the levels of the output signals of the 
fixed codebook and the excitation signal memory means, a synthesizing 
means for combining the gain controlled signals, and an excitation signal 
memory means for receiving a resultant excitation signal output of the 
synthesizing means. Index memory means supplies necessary past frame 
indexes to the fixed codebook, the excitation signal memory means and the 
individual gain control means. The internal state data of the excitation 
signal memory means is supplied to the adaptive codebook after generation 
of at least one preceding frame excitation signal. 
The fixed codebook, the adaptive codebook, the excitation memory means and 
the index memory means are formed as memory means. The fixed codebook is 
desirably constituted by a ROM. The adaptive codebook, the excitation 
memory means and the index memory means are desirably constituted by RAMs 
as tentative memory means. The gain control means is desirably constituted 
by a multiplier. The synthesizing means is desirably constituted by an 
adder. 
The fixed codebook is not particularly limitative, and it may contain noise 
signal series, pulse signal series, etc. stored in it. 
The operation of the embodiment of the present invention will now be 
described in detail with reference to the drawings. 
Adaptive codebook generating means 100 comprises an index memory means 101, 
a fixed codebook 102, an excitation signal memory means 104, gain control 
means 103 and 105 and a synthesizing means 106. 
The index memory means 101 supplies fixed codebook index I fcb prev i 
preceding by i frames to the fixed codebook 102, adaptive codebook index I 
acb prev i to the excitation signal memory means 104, gain indexes I Gc 
prev i and I Gp prev i to the gain control means 103 and 105. The gain 
indexes are converted to gains Gc Prev i and Gp prev i by table lookup of 
the gain codebook. The index memory means may store data obtained as a 
result of conversion of coded speech signal (i.e., index) according to 
error correction code and also data obtained as a result of table 
conversion and various other data conversions, as well as the best 
identical index. 
The fixed codebook 102 provides a signal series through table lookup 
according to the data of fixed codebook index I fcb prev i preceding by 1 
frames, supplied from the index memory means 101. The excitation signal 
memory means 104 provides a signal series through table lookup according 
to the data of adaptive codebook index I acb prev i preceding by 1 frames, 
supplied from the index memory means 101. One of important feature of the 
present invention resides in that, before the commencement of the 
excitation signal generation of the current frame, all the data in the 
excitation signal memory means is set to zero or to known data (for 
instance, data of a certain fixed pattern), and then signal of at least 
one frame is generated by using past index. 
The gain controller 103 gain controls the output signal of the fixed 
codebook 102, and the gain controller 105 gain controls the output signal 
of the excitation signal memory means 104. The synthesizing means 106 
combines the two gain controlled signals, and thus generates a resultant 
excitation signal. The excitation signal memory means 104 receives this 
excitation signal, and updates its internal state. 
Likewise, excitation signals are generated by using indexes previous by 
(i-1) frames to one frame, received from the index memory means 101. 
The updated internal state data of the excitation signal memory means 104 
is supplied to the adaptive codebook 107 and used as a codebook of the 
current frame. 
The embodiment of the invention will now be described with reference to the 
drawings. Referring to FIG. 1, the embodiment of the present invention 
comprises a fixed codebook, an excitation signal memory, multipliers for 
controlling the levels of the output signals of the fixed codebook and the 
excitation signal memory, and an adder for combining the gain controlled 
signals, the output of the adder being fed back to the excitation signal 
memory. The index memory outputs the past index to the fixed codebook, 
excitation signal memory and respective multipliers. The internal state 
data of the excitation signal memory is supplied to the adaptive codebook 
after generation of excitation signal of at least one preceding frame. 
The operation of the embodiment of the present invention will now be 
described with reference to FIG. 1. Referring to FIG. 1, the adaptive 
codebook generator 100 comprises the index memory 101, fixed codebok 102, 
excitation signal memory 104, multipliers 103 and 105 and adder 106. 
The index memory 101 is constituted by a RAM, and stores indexes of 10 
frames. Of indexes read out from the index memory, fixed codebook index I 
fcb prev i preceding by i frames is supplied to the fixed codebook 102, 
the adaptive codebook index I acb prev i is supplied to the excitation 
signal memory 104, and gain indexes I Gc prev i and Gp prev i are supplied 
to the multipliers 103 and 105, respectively. The gain indexes are 
converted to gains Gc prev i and Gp prev i by table lookup of the gain 
codebook. 
The fixed codebook 102 is constituted by a ROM, in which a noise signal 
series has been written. The noise signal series is supplied by table 
lookup based on data of fixed codebook index I fcb prev i preceding by i 
frames, supplied from the index memory 101. The excitation signal memory 
104 is constituted by a RAM, and the signal series is supplied by table 
lookup based on data of adaptive codebook index I acb prev i, supplied 
from the index memory 101. The excitation signal memory 104, when starting 
the generation of the excitation signal of the current frame, sets all the 
data in the excitation signal memory 104 to zero, and then generates at 
least one frame signal by using past index. 
The multiplier 103 gain controls the output signal (i.e., noise signal 
series) of the fixed codebook 102, and the multiplier 105 gain controls 
the output signal (i.e., signal series) of the excitation signal memory 
104. The adder 106 generates the excitation signal by adding together the 
two gain controlled signals. The excitation signal memory 104 updates its 
internal state by receiving the excitation signal. Excitation signals are 
generated likewise by using indexes preceding by (i-1) frames to one 
frame, received from the index memory 101. 
Data of the excitation signal memory internal state updated in the above 
way, is supplied as codebook of the current frame to the adaptive codebook 
107. 
FIG. 2 is a block diagram showing a coder embodying the invention. 
Referring to FIG. 2, reference numeral 201 designates block diagram 
showing a coder, and 202 a block diagram showing a decoder. 
The coder 201 includes two different codebooks, i.e., an adaptive codebook 
204 and a fixed codebook 205. Multipliers 206 and 207 multiply excitation 
vector signal (i.e, pitch vector) and excitation output vector (i.e., 
codevector) supplied from the adaptive codebook 204 and the fixed codebook 
205 by respective gains (i.e., pitch gain and code gain), and an adder 208 
generates the excitation signal of the current frame by adding together 
the two product outputs of the multipliers. The pitch vector from the 
adaptive codebook 204 includes a component dependent on the periodicity of 
the speech signal, and the codevector from the fixed codebook 205 contains 
a non-periodic component. A vector is selected and provided by each 
codevector, which is constituted by a plurality of vector patterns. The 
adaptive codebook 204 is of the type for signal generation according to 
indexes as shown in FIG. 1 and described earlier, and supplies the past 
excitation signal generated in the adaptive codebook generator 203 to the 
adaptive codebook 204. 
The excitation signal of the current frame is supplied to a weight 
multiplication synthesizing filter 209 and subjected for short period 
prediction in a linear prediction or like process to generate a reproduced 
signal. A subtracter 210 receives the reproduced signal and determines an 
error thereof for an acoustical weight multiplication processed input 
signal. This error is supplied to an error power evaluator 211. 
The error power evaluator 211 controls and scans the outputs of the 
adaptive and fixed codebooks 204 and 205 and the gains of the multipliers 
206 and 207 for each frame, and determines an excitation signal 
corresponding to a minimum error to be the optimal excitation signal. 
The decoder 202 can be realized by omitting the subtracter 210 and the 
error power evaluator 211 from the construction of the coder 201 and 
replacing the weight application synthesizing filter with a synthesizing 
filter free from weight application. A post-filter 218 is cascade 
connected to the output of the synthesizing filter to generate a 
reconstituted speech signal for the purpose of improving the sound quality 
of the decoder. 
The coder 201 transmits the pitch and code vector parameters supplied from 
the adaptive and fixed codevectors 204 and 205 at the time of the optimum 
excitation signal determination, the gain parameters for multiplication in 
the multipliers 206 and 207 and filter coefficients before weight 
application process in the weight application synthesizing filter 209, as 
coded index data of the input signal, to the decoder 202. The decoder 202, 
receiving these index data, operates an adaptive and a fixed codebook 213 
and 214 in the decoder 202 corresponding to the coder 201, multipliers 215 
and 216 for gain multiplying the vectors from the codebooks, a 
synthesizing filter 218 based on a short period prediction process and a 
post-filter 219 according to the received parameters and filter 
coefficients (generated from the index data), thus obtaining a 
reconstituted speech signal which best approximates the input speech. 
In applications to radio communication systems, transmission errors may 
occur on the communication channel, on which the index data are 
transmitted from the coder 201 to the decoder 202, due to multi-fading or 
like problems. The transfer function of the prior art adaptive codebook is 
P1(z)=CpZ.sup.-P. As is seen from this equation, once an error occurs, its 
detrimental effects are subsequently continued. In the equation, P is the 
group delay, i.e., pitch lag. Occurrence of a transmission line error may 
result in failure of identity of the internal states of the adaptive 
codebooks of the coder 201 and the decoder 202. In the occasion that the 
input speech to the coder at the time of the error occurrence is 
non-voiced signal or sole background noise, the error has less adverse 
detrimental effects. However, in such occasion as a sudden pitch lag 
change during voiced speech, the error has very great adverse effects, 
thus resulting in great departure of the contents of the adaptive 
codebooks of the coder and the decoder from each other. When decoding is 
made in such different states of the codebooks, the reconstituted speech 
signal of the decoder may contain noise even in the execution of retrieval 
for the best identical code in the coder. 
When the adaptive codebook according to the present invention is used, the 
disadvantages of the error are determined by the number of preceding frame 
indexes that have used an excitation signal generation anew. In the 
embodiment, the adaptive codebook is generated by using indexes for 10 
frames. This means that the disadvantages of the error are continued for 
only 10 frames. Thus, in the event of communication line error occurrence, 
it is possible to reduce generation of abnormal sound due to failure of 
identity of the internal states of the adaptive codebooks of the coder and 
the decoder. 
A first advantage of the present invention is attributable to generation of 
the adaptive codebook, i.e., the excitation signal, anew from index data 
of at least one frame. By so doing, when several frames subsequent 
transmission line error occurrence have passed in the error-free state, it 
becomes possible to make the internal states of the adaptive codebooks of 
the coder and the decoder to be identical. With the coder and decoder 
adaptive codebook internal states made identical again subsequent to the 
lapse of the number of index storage frames after the occurrence of a 
transmission line error, unlike the existing adaptive codebook, the 
adverse effects of the error will not continue up to a non-voice period 
after the error occurrence. It is thus possible to reduce the probability 
of abnormal sound generation the co failure of identity of the coder and 
decoder codebooks. This advantage is particularly pronounced in the 
occasion of transmission error generation during voiced speech period. 
This is so because the adaptive codebook does not constitute a perfect 
feedback loop but the excitation signal is generated according to index 
data for a certain period of time. 
A second advantage of the present invention is that it is possible to 
reduce the memory capacity necessary for holding the adaptive codebook 
internal state data at all times. This means that it is possible to reduce 
memory, which need be otherwise provided in the base station for such 
purposes as speech coding of several channels. This advantage permits 
memory provision as a single digital signal processor (DSP) chip (that is, 
it permits processing without provision of any external memory but with 
the sole DSP internal memory). 
Changes in construction will occur to those skilled in the art and various 
apparently different modifications and embodiments may be made without 
departing from the scope of the present invention. The matter set forth in 
the foregoing description and accompanying drawings is offered by way of 
illustration only. It is therefore intended that the foregoing description 
be regarded as illustrative rather than limiting.