JPH0946268A - Digital voice communication device - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To make it easier to hear noise after decoding, thereby improving the communication quality. A long-term predictor 63 in a speech encoding circuit 16
The path selector 67 is provided between the short-term predictor 64 and the short-term predictor 64, and the pitch component and the auditory sense by the long-term predictor 63 and the short-term predictor 64 are detected in the sound period according to the judgment result of the sound / silence judging unit 17. Encoding is performed with consideration given to each, and in the meantime, during the silent period, the long-term predictor 63 is removed from the path so that the short-term predictor 64 performs encoding considering only the sense of hearing. In addition, a variable LPF is provided in the preceding stage of the voice encoding circuit 14.
15 is provided, and the cutoff frequency of the variable LPF 15 is set high during the voiced period, while the cutoff frequency of the variable LPF 15 is set low during the voiceless period according to the determination result of the voice / silent determination unit 17. It is the one.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital voice communication device used in an environment having a relatively high ambient noise level, such as a digital automobile telephone device, a portable telephone device, and a digital cordless telephone.

[0002]

2. Description of the Related Art Generally, in order to increase the number of communication channels in a digital car telephone device or a mobile telephone device,
Low bit rate voice encoding is performed in a voice encoding device.

FIG. 5 is a schematic configuration diagram of a speech encoding apparatus which has been conventionally used. In the figure, the transmitted voice is converted into an electric signal by the microphone 101, then sampled at a predetermined sampling period in the analog / digital (A / D) converter 102 and converted into a digital signal, and then the voice encoding circuit. Department (ENC) 1
03 is input. In the encoding circuit unit 103, the voice encoding processing of the digital transmission voice signal is performed,
The transmission data obtained by this voice coding is subjected to an error correction coding process by an error correction code decoding circuit (CH-CO).
D) 104 is supplied.

On the other hand, the transmission data output from the error correction code decoding circuit 104 is the voice decoding circuit unit (DEC).
A voice decoding process is performed at 105, whereby the digital received voice signal is reproduced. This digital received voice signal is converted into an analog signal by the digital / analog (D / A) converter 106, and then the speaker 10
A loud voice is output from 7.

FIG. 6 is a block diagram showing the internal configuration of the speech coding circuit section 103. In the figure, 61 is a drive signal section (codebook section) in which past drive signal data is stored, and the drive signal data read from this drive signal section 61 is gain-controlled by a gain control section 62. Then, it is sequentially input to the long-term predictor 63 and the short-term predictor 64.

In the long-term predictor 63, the drive signal section 61 is used.
On the basis of the drive signal data read from, the data representing the pitch component, that is, the pitch period and the pitch gain are reproduced. That is, pitch prediction is performed. Further, the short-term predictor 64 reproduces a short-term prediction coefficient representing the weighting of the auditory sense. The prediction signals output from the predictors 63 and 64 are input to the subtractor 65, and the subtractor 65 detects an error from the input audio signal. The power error minimization unit 66 obtains the code and gain of the drive signal that minimizes the error. Then, the short-term prediction coefficient, pitch period, pitch gain, drive signal code, and optimum gain obtained by the above processing are sent to the error correction code decoding circuit 104 as transmission data. The error E is expressed by the following equation.

[0007]

[Equation 1]

However, β and M are pitch gain,
It represents the pitch period, and v (n) and W (n) represent the drive signal and the coefficient of the perceptual weighting filter, respectively.
The coefficient of the short-term predictor 64 is used as the value of W (n).

By the way, a mobile voice communication device such as a mobile phone device is not limited to a relatively quiet environment such as in an office or a stopped vehicle, but may be in a surrounding area such as a station yard or a running vehicle. It is often used in a noisy environment. However, as described above, the conventional digital voice communication device performs voice coding processing by using a human voice generation system as a model. For this reason, if various miscellaneous noises are input in a state where the transmitted voice is not being uttered, such as during reception, this noise is also encoded and transmitted by the same processing as the voice signal. . Therefore, on the other end of the call, the decoded noise becomes very difficult to hear, which is one of the causes of deterioration of the call quality.

[0010]

As described above, in the conventional digital voice communication apparatus, since the noise signal is encoded exactly the same as the voice signal, the noise after decoding becomes very difficult to hear. However, there is a problem that this causes deterioration of the call quality.

The present invention has been made in view of such circumstances, and an object thereof is to provide a digital voice communication device which makes it easier to hear noise after decoding and thereby improves the call quality. To do.

[0012]

In order to achieve the above object, the digital voice communication device of the present invention has a predetermined threshold level for the input voice signal in addition to a voice coding section for coding the input voice signal. A voice / non-voice determining unit that determines whether the input audio signal is voiced or non-voiced by comparing the value with a value, and an encoding control unit are provided. The encoding control unit causes the speech encoding unit to perform long-term prediction and short-term prediction, respectively, during the period when the sound / silence determining unit determines that the sound is present, and the sound is determined to be silent. During the period, the speech coding unit is controlled so as to omit the long-term prediction.

Therefore, during the transmission period in which the transmission voice signal is being input, the encoding process in consideration of the pitch component by the long-term predictor is performed as usual, and the input of the transmission voice signal is interrupted. During the reception period, the pitch component is not predicted by the long-term predictor, and only the encoding process is performed by the short-term predictor in consideration of only the sense of hearing. For this reason, the noise after decoding is not affected by the pitch component prediction, so that the receiving side can hear the noise with a high sense of naturalness, and the speech quality is improved.

In a digital voice communication apparatus of another invention, a low-pass filter is provided in front of the voice encoding unit and a control means for the low-pass filter is provided, and the filter control means allows the voice / silence determining means to operate. The characteristics of the low-pass filter are variably controlled according to the determination result.

For example, during the period in which the sound / silence determining means determines that there is sound, the cutoff frequency of the low-pass filter is set to a sufficiently high first value, and it is determined that there is no sound. The cut-off frequency of the low-pass filter is set to a second value lower than the first value during the above period. In another example, the input voice signal is input to the low-pass filter and the filtered voice signal is supplied to the voice encoding unit during the period when the voice / non-voice determining unit determines that the voice is not present. During the period when it is determined that there is sound, the input voice signal is bypassed without being input to the low pass filter and supplied to the voice encoding unit.

By doing so, the ambient noise can be sufficiently suppressed by the low-pass filter and supplied to the encoding process, and the noise after decoding can be made more audible. it can. On the other hand, since the transmission voice signal can be subjected to the encoding process without being suppressed by the low pass filter, the quality of the communication voice can be kept high.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit block diagram showing an example of the configuration of a digital portable telephone device which is one of the embodiments of the present invention. In the figure, a radio carrier signal sent from a base station (not shown) via a radio channel is received by an antenna 1 and then an antenna duplexer (DU).
P) 2 and is input to the receiving circuit (RX) 3, where it is mixed with the receiving local oscillation signal output from the frequency synthesizer (SYN) 4 and frequency-converted into a receiving intermediate frequency signal. The received intermediate frequency signal is sampled by an A / D converter (not shown) and then input to the digital demodulator (DEM) 6.

The digital demodulator 6 carries out digital demodulation processing after establishing frame synchronization and bit synchronization for the digital received intermediate frequency signal. The baseband digital demodulated signal obtained by this demodulation processing is input to a time division multiple access circuit (TDMA) 8 where the time slot destined for itself is separated and extracted for each transmission frame. The information on the frame synchronization and the bit synchronization obtained by the digital demodulator 6 is input to the control circuit 18.

The digital demodulated signal output from the TDMA circuit 8 is subsequently subjected to an error correction code decoding circuit (CH-
COD) 9 and undergoes error correction decoding processing. Then, this error-correction-decoded digital demodulated signal is input to a voice decoding circuit (DEC) 10 and subjected to voice decoding processing, whereby a digital reception signal is reproduced. The digital reception signal is converted into an analog reception signal by the D / A converter 11 and then supplied to the speaker 12 via a voice amplifier (not shown), and the speaker 12 outputs the signal in a loud voice.

On the other hand, the voice transmitted by the speaker is the microphone 1
After being picked up by 3 and converted into an electric signal, it is inputted to the A / D converter 14 and sampled at a predetermined sampling period by the A / D converter 14 and converted into a digital transmission signal. This digital transmission signal is input to a voiced / non-voiced determination unit 17 which will be described later, passes through a variable low pass filter (variable LPF) 15, and then to a voice encoding circuit (COD) 16 to perform voice encoding. To be done.

The encoded voice data output from the voice encoding circuit 16 is input to the error correction code decoding circuit (CH-COD) 9 together with the control signal output from the control circuit 18, and the error correction encoding is performed here. To be done. Then, this error correction coded digital transmission signal is input to the TDMA circuit 8. In this TDMA circuit 8, a transmission frame corresponding to the time division multiple access (TDMA) system is generated, and a process for inserting the digital transmission signal in the time slot assigned to the own device in this transmission frame is performed. . The digital transmission signal output from the TDMA circuit 8 is subsequently transmitted to the digital modulator (M
OD) 7.

In the digital modulator 7, a transmission intermediate frequency signal digitally modulated by the digital transmission signal is generated, and the transmission intermediate frequency signal is not shown in D.
After being converted into an analog signal by the / A converter, it is input to the transmission circuit (TX) 5. As a digital modulation method, for example, π / 4 shift QPSK (π / 4 shift)
ed quadrature phase shift keying) method is used.

In the transmission circuit 5, the modulated transmission intermediate frequency signal is mixed with the transmission local oscillation signal output from the frequency synthesizer 4, and thereby converted into a radio carrier frequency corresponding to the speech channel. Then, this radio modulated wave signal is controlled to a predetermined transmission power level by the transmission power amplifier, and then transmitted from the antenna 1 to the base station (not shown) via the antenna duplexer 2.

Reference numeral 19 denotes an operation panel section. The operation panel section 19 includes a key input section having a transmission key, a dial key, and various function keys, and a liquid crystal display (LC).
D) and a display unit having a light emitting diode (LED).

By the way, the voiced / non-voiced determination section 17 is constructed as follows. FIG. 2 is a circuit block diagram showing the configuration. That is, the sound / silence determination unit 17 includes a peak value calculation circuit 71, a determination unit 72, and a variable threshold value generation circuit 73.

The peak value calculation circuit 71 calculates the peak value (power value) of the input audio signal for each predetermined section (for example, 20 msec), and supplies the calculated peak value to the decision unit 72.

The variable threshold value generating circuit 73 first determines whether or not the level of the input audio signal is below a predetermined level, and when it is above the predetermined level, generates a first threshold value. On the other hand, when the level of the input audio signal is equal to or lower than the predetermined level, the second threshold value that changes according to the noise level input at this time is generated.

The judging device 72 is the peak value calculating circuit 71.
The peak value of the input audio signal output from the above is compared with the first and second threshold values output from the variable threshold value generating circuit 73. And the peak value is the first and second
If the peak value is less than the first and second threshold values, it is determined to be silent. That is, the peak value of the input audio signal is P (n)
, And the threshold value at that time is T (n), when P (n) ≥ T (n), and when P (n) <T (n), the judgment is made as if there is no sound. And the judgment result
The silent flag HS is transmitted to the variable LPF 15 and the voice encoding circuit 16.

Next, the variable LPF 15 is constructed as follows. FIG. 3 is a circuit block diagram showing the configuration. That is, the variable LPF 15 is a first-order feedback type filter, and includes an input gain variable device 51, an adder 52, a feedback gain variable device 53, a buffer 54, a gain value memory controller 55, and a gain value memory 56. ing.

When it is determined that the voice is present based on the voice / non-voice flag HS output from the voice / non-voice determining unit 17, the gain value memory controller 55 sends the input gain variable device 51 to the input gain variable device 51. 1 or a gain value Ng1 (Ng ≦ 1) close to 1 is given, and 0 or a gain value Fg1 close to 0 (Fg = 1−Fg)
Ng) is given. As a result, the variable LPF 15 is set to have a high cutoff frequency or pass the input audio signal. As a result, the input speech signal is not suppressed and is used for speech coding processing.

On the other hand, when the voiced / silent flag HS indicates silence, the gain value memory controller 55 gives the input gain variable device 51 a gain value Ng2 (Ng≤1) smaller than 1. At the same time, the feedback gain variable device 53 is provided with a gain value Fg2 (Fg = 1-Ng) which is about 1/2 of the gain value Fg1 in the presence of voice. As a result, the variable LPF 15 is brought into a state of operating at a cutoff frequency lower than that in the presence of voice, whereby high-frequency components of ambient noise are suppressed.

The gain value memory controller 55 gradually changes each gain value when there is a change from the presence of sound to the absence of sound. As a result, the sense of switching that occurs in the received sound when the sound changes from the sound to the silence is reduced.

On the other hand, the voice encoding circuit 16 is constructed as follows. FIG. 4 is a circuit block diagram showing the configuration. In the figure, the same parts as those in FIG. 6 are designated by the same reference numerals and detailed description thereof will be omitted.

The voice encoding circuit 16 of this embodiment is
The route selector 6 is provided between the long-term predictor 63 and the short-term predictor 64.
7 are provided. The path selector 67 is composed of, for example, a semiconductor switch, and a path of an input audio signal from the gain control section 62 to the short-term predictor 64 in accordance with the sound / silence flag HS output from the sound / silence determination section 17. To the path through the long-term predictor 63, or the long-term predictor 6
Alternately switch to a route that bypasses 3.

Next, the operation of the apparatus configured as described above will be described. First, in the state where the speaker is transmitting, since the transmitted voice signal is input, the sound / silence determination unit 17 determines that the sound is “sound”, and that is the sound / silence flag HS.
The variable LPF 15 and the voice encoding circuit 16 are notified by

Then, in the variable LPF 15, the gain value memory controller 55 gives the input gain variable device 51 and the feedback gain variable device 53 a gain value for setting a high cutoff frequency. Therefore, the input transmission voice signal is directly passed through the variable LPF 15 without being band-limited and supplied to the voice encoding circuit 16. Therefore, the transmitted voice signal is supplied to the encoding process while sufficiently including the high frequency part and maintaining high sound quality.

Further, in the voice encoding circuit 16, upon receiving the notification of the presence of the voice, the route selector 67 selects a route including the long-term predictor 63. Therefore, in the speech coding circuit 16, the long-term predictor 63 and the short-term predictor 64 respectively perform coding in consideration of both the pitch component and the auditory perception of the transmitted speech signal, and as a result, the short-term prediction coefficient. , The pitch period, the pitch gain, the drive signal code and the optimum gain are generated and supplied to the error correction code decoding circuit 9. That is, the necessary encoding is performed for the audio signal.

On the other hand, when the speaker listens to the received voice from the other speaker, the transmitted voice signal disappears and only ambient noise is input. Therefore, the sound / silence determination unit 17 determines that the sound is “silence”, and the sound / silence flag HS is used to notify the variable LPF 15 and the voice encoding circuit 16 of that fact.

Therefore, in the variable LPF 15, the gain value memory controller 55 gives the input gain variable device 51 and the feedback gain variable device 53 a gain value for setting the cutoff frequency low. Therefore, the input ambient noise has only the low-frequency component, which is relatively easy for the other speaker to hear because the mid-high frequency component is suppressed.

Upon receipt of the notification of silence, the speech coding circuit 16 causes the route selector 67 to operate the long-term predictor 6.
A route that bypasses 3 is selected. Therefore, in the speech coding circuit 16, the prediction of the pitch component by the long-term predictor 63 is omitted, and only the prediction by the short-term predictor 64 is performed. That is, the encoding is performed in consideration of only the sensation of ambient noise. Then, of the short-term prediction coefficient, pitch period, pitch gain, drive signal code, and optimum gain forming the transmission data, the pitch period and pitch gain are prevented from being decoded on the decoding side in consideration of the pitch component. Pseudo information for performing is inserted and transmitted.

As described above, in this embodiment, the path selector 67 is provided between the long-term predictor 63 and the short-term predictor 64 in the speech coding circuit 16 so that the judgment result of the sound / silence judging section 17 is determined. Correspondingly, during the voiced period, the long-term predictor 63 and the short-term predictor 64 perform encoding in consideration of the pitch component and the auditory sense, respectively, while in the silent period, the long-term predictor 63 is removed from the path and the short-term predictor is used. The encoding is performed in consideration of only the auditory sense of 64.

Therefore, according to this embodiment, it is possible to perform appropriate encoding in the voiced period in consideration of the pitch component and the audibility of the input transmission voice, while in the silent period, the audibility of the ambient noise is heard. It is possible to perform encoding taking only into consideration. For this reason, it is possible to improve the call quality in the silent period as compared with the conventional device that performs the coding prediction by both the long-term predictor 63 and the short-term predictor 64 for both the voiced period and the silent period.

Further, in this embodiment, the variable LPF 15 is provided in the preceding stage of the speech encoding circuit 16, and the variable LPF 15 is provided in the sound period according to the judgment result of the sound / silence judging section 17.
The cutoff frequency of the variable LPF 15 is set high while the cutoff frequency of the variable LPF 15 is set low during the silent period.

For this reason, no adverse effect is exerted on the transmitted voice signal of the speaker input in the voiced period, and the ambient noise input in the silent period is relatively heard by suppressing the middle and high frequency components. It is possible to use only low-frequency components that are easy.
Therefore, even when the talker is talking in an environment with a large amount of ambient noise, the other speaker can talk in a relatively easy state.

Further, at that time, since the cutoff frequency of the variable LPF 15 is gradually changed at the time of transition from the voiced period to the silence period, the receiver side is able to perform the transition from the voiced period to the silence period. The problem that an unnatural switch feeling occurs does not occur.

Further, in this embodiment, the sound / silence judging section 17 is adapted to vary the judgment threshold value according to the input level of the ambient noise during the silent period. Therefore, for example, even if the ambient noise level is high, it is possible to accurately determine the voiced / silent field.

The present invention is not limited to the embodiments described above. For example, in the above embodiment, the voice / silence determining unit 17 is configured to change the threshold value according to the input level of the ambient noise, but the threshold value is not necessarily variable and may be fixed. Absent.

Further, in the above-described embodiment, the long-term predictor 63 is included in the path in the voice coding circuit 16 in the voice coding period, and the long-term predictor 63 is bypassed in the silent period. According to the judgment result of the sound / silence judgment unit 17, the long-term predictor 63 is set to the operation state during the sound period, the processing operation of the long-term predictor 63 is stopped during the silent period, and the input signal is passed as it is. It may be configured so as to allow it. In this way, the route selector 67 and the bypass circuit can be eliminated.

Further, in the voice encoding circuit 16, a drive signal section for a silent period in which a pulse train having a random pitch period is accumulated is prepared, and when the sound / silence judging section 17 judges that there is no sound. May select the drive signal section for the silent period and perform the encoding process using the random pulse train read from the drive signal section. With this configuration, the sound / silence determination unit 1
It is not necessary to remove the long-term predictor 63 from the signal path or to turn on / off the operating state of the long-term predictor 63 according to the determination result of 7.

On the other hand, in the above-described embodiment, the cutoff frequency of the variable LPF 15 is variably controlled according to the judgment result of the sound / silence judging section 17, but the fixed LPF is fixed.
By providing a changeover switch on at least one of the input side and the output side and switching the changeover switch according to the judgment result of the sound / silence judgment unit 17, the fixed LPF is inserted in the transmission signal path during the silent period. In the voiced period, the fixed LPF may be removed from the transmission signal path to directly connect the transmission signal path. By doing so, it becomes possible to use an inexpensive LPF with a fixed cutoff frequency.

In the above embodiment, the case where the insertion / bypass of the long-term predictor 63 in the speech encoding circuit 16 and the switching of the characteristics of the variable LPF 15 are performed respectively have been described. It may be configured to perform only one of them.

Further, the present invention can be applied not only to the digital form telephone device but also to various other digital radio telephones such as a digital automobile telephone device and a digital cordless telephone, and can also be applied to a digital wired telephone device. Is.

In addition, the configuration of the voice encoding circuit, the low-pass filter for suppressing ambient noise, the configuration of the voiced / non-voiced determination unit, etc. can be variously modified and implemented without departing from the scope of the present invention. Needless to say.

[0054]

As described above in detail, according to the present invention, in addition to the voice coder for encoding the input voice signal, the level of the input voice signal is compared with a predetermined threshold value and the input voice signal is detected. A sound / silence judging means for judging whether there is sound or no sound;
And encoding control means. The encoding control unit causes the speech encoding unit to perform long-term prediction and short-term prediction, respectively, during the period when the sound / silence determining unit determines that the sound is present, and the sound is determined to be silent. During the period, control is performed so that the speech encoding unit may omit long-term prediction.

According to another aspect of the invention, a low-pass filter is provided in front of the speech coder and a control means for the low-pass filter is provided, and the filter control means allows the sound / silence determining means to respond to the determination result. The characteristic of the low pass filter is variably controlled. Therefore, according to these inventions, it is possible to provide a digital voice communication device that makes it easier to hear noise after decoding and thereby improves the call quality.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing an example of a configuration of a digital mobile phone device which is one of the embodiments of the present invention.

FIG. 2 is a block diagram showing the voice / voice of the digital mobile phone device shown in FIG.
FIG. 3 is a circuit block diagram showing the configuration of a silence determination unit.

FIG. 3 is a variable L of the digital mobile phone device shown in FIG.
The circuit block diagram which shows the structure of PF.

FIG. 4 is a circuit block diagram showing a configuration of a voice encoding circuit of the digital mobile phone device shown in FIG.

FIG. 5 is a block diagram showing a schematic configuration of a conventional speech encoding device.

6 is a circuit block diagram showing a configuration of a speech coding circuit unit of the speech coding apparatus shown in FIG.

[Explanation of symbols]

 1 ... Antenna 2 ... Antenna duplexer (DUP) 3 ... Reception circuit (RX) 4 ... Frequency synthesizer (SYN) 5 ... Transmission circuit (TX) 6 ... Digital demodulator (DEM) 7 ... Digital modulator (MOD) 8 ... Time division multiple access circuit (TDMA) 9 ... Error correction code decoding circuit (CH-COD) 10 ... Voice decoding circuit (DEC) 11 ... D / A converter 12 ... Speaker 13 ... Microphone 14 ... A / D converter 15 ... Variable low-pass filter (variable LPF) 16 ... Speech coding circuit (COD) 17 ... Voice / silence determination unit 18 ... Control circuit 19 ... Operation panel unit 51 ... Input gain variable unit 52 ... Adder 53 ... Feedback gain variable 54 ... Buffer 55 ... Gain value memory controller 56 ... Gain value memory 61 ... Drive signal section (codebook section) 62 ... Gain control section 63 ... Long-term prediction 64 ... Short-term predictor 65 ... subtractor 66 ... power error minimizing section 71 ... peak value calculating circuit 72 ... determiner 73 ... variable threshold generating circuit HS ... speech / silence flag

Claims (7)

[Claims] 1. A speech encoding unit performs a long-term prediction and a short-term prediction based on past drive signal data stored in a drive signal unit to generate a prediction signal, and generates an input speech signal and this prediction signal. In a digital voice communication device that obtains various coding parameters that minimize the error of the input voice signal and uses them as transmission data, compares the level of the input voice signal with a predetermined threshold value Voice /
The silence determining means and the voice coding unit are caused to perform long-term prediction and short-term prediction respectively during the period when it is determined to be voiced by the voice / soundless determination means, while the voice coding unit is set to perform the long-term prediction and the short-term prediction, respectively. A digital voice communication device, comprising: a voice control unit for controlling a voice encoding unit to omit long-term prediction. 2. The voiced / unvoiced determination means has a function of estimating a noise level from an input voice signal and varying a threshold value based on the estimated value.
The described digital voice communication device. 3. A code for causing a digital voice communication device of a communication partner to omit a decoding process corresponding to long-term prediction, when the long-term prediction is omitted under the control of the coding control means. 2. The digital voice communication device according to claim 1, wherein the digitalization communication parameter is transmitted. 4. A digital voice communication device for encoding an input voice signal by a voice encoding unit and transmitting the encoded voice signal, the low voice signal being provided in a front stage of the voice encoding unit for filtering and outputting the input voice signal. A pass / pass filter is used to compare the level of the input audio signal with a predetermined threshold to determine whether the input audio signal is voiced or not.
And a filter control unit for variably controlling the filtering operation of the low-pass filter with respect to the input audio signal according to the determination result of the sound / silence determination unit. Digital voice communication device. 5. The filter control means sets the cutoff frequency of the low-pass filter to a predetermined first value during the period when the sound / silence judging means judges that there is sound, and the silence control means 4. The digital voice communication device according to claim 3, wherein the cutoff frequency of the low-pass filter is set to a second value lower than the first value during the period determined to be. 6. The filter control means outputs the input audio signal filtered by the low pass filter during a period when the sound / silence judging means judges that the sound is silent.
4. The digital voice communication device according to claim 3, wherein the input voice signal bypassing the low pass filter is output during a period when it is determined that the voice is present. 7. A voice encoding unit performs long-term prediction and short-term prediction based on past drive signal data stored in the drive signal unit to generate a prediction signal, and generates an input voice signal and this prediction signal. In a digital voice communication device that obtains various encoding parameters that minimize the error of (1) and uses this as transmission data, the input voice signal is filtered and output in the preceding stage of the voice encoding unit. The low-pass filter and the level of the input audio signal are compared with a predetermined threshold to determine whether the input audio signal is voiced or silenced.
Silence determination means, filter control means for variably controlling the filtering operation of the low-pass filter with respect to the input audio signal in accordance with the determination result of the voice / silence determination means, the voice / silence determination The speech coding unit is caused to perform long-term prediction and short-term prediction respectively during the period when it is determined to be voiced by the means, while the speech coding unit is allowed to omit the long-term prediction during the period where it is determined to be silent. And a coding control means for controlling the digital voice communication device.