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DESCRIPTION JPH10285083

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DESCRIPTION JPH10285083
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
voice communication apparatus having a hands-free calling function, and more particularly to an
improvement of a system for suppressing acoustic echo generated during the hands-free calling.
[0002]
2. Description of the Related Art In the past, for example, some wireless telephones used in
automobiles have a handsfree call mode in addition to a handset call mode. In the hands-free
communication mode, a speaker for receiving speech and a microphone for transmitting speech
are installed on a dashboard or the like, and a call is made using these speaker and microphone
as a handset. Using this hands-free call mode, the speaker can talk without having a handset, and
can talk without being in one-handed operation during a call, which is effective in terms of safety
during driving.
[0003]
However, during such a hands-free call, the received voice from the speaker may wrap around
the microphone and be sent to the other party as an acoustic echo. In order to prevent the
deterioration of the speech quality due to the acoustic echo, an echo canceler is required to
cancel the incoming voice that has come around. In particular, in a digital radio telephone
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1
apparatus that digitizes and transmits voice, the signal delay caused by the codec etc. is large,
and the influence of the acoustic echo that is looped in is large, so the acoustic echo is used to
maintain the speech quality. An echo canceller that cancels out is essential.
[0004]
As a conventional example of the echo canceller, an adaptive filter is used to estimate the
relationship between the received signal and the echo mixed in the transmission input signal, and
pseudo echo is calculated from the received signal that is the input of the echo canceller using
the above estimated contents. It is known to generate and subtract this from the transmit input
signal.
[0005]
Furthermore, when the echo can not be canceled even by this subtraction processing, a method is
also adopted in which the echo is suppressed by non-linear means.
A center clipper is known as this type of echo suppression function. The center clipper forcibly
sets the output from its own circuit to 0 when the absolute value of the level of the input signal
(residual signal) is equal to or less than the threshold (clip level), and in the other cases, the input
signal (residual signal) Function to output as it is.
[0006]
FIG. 7 shows the time variation characteristic of the transmission signal output level in a
conventional echo canceller using a center clipper. In the same figure, from time t0 'at which the
single talk (listening) state is switched to, echo cancellation for the transmission signal which is
the input signal is started, and the output signal level TS gradually decreases. Thereafter, the
output signal level TS further decreases, and the center clip operates from time t1 'divided by the
preset clip level CL0 to time t2' when the double talk state is brought about. During this time, the
output signal level TS is Forced to be 0. Thereafter, echo cancellation is started again from time
t3 'at which the single talk state returns, and the transmission signal output signal level TS
gradually decreases, and the transmission signal output signal level TS divides the clip level CL0
from time t4' to the center. The clip operation resumes.
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[0007]
A conventional echo canceller using such a center clipper can effectively suppress echo and
noise, but when the input signal level to the center clipper falls below a certain level, the
transmission output is completely eliminated To make the call discontinuous. If you are listening
to a call from the other party, especially when there is in-car noise such as running noise, when
the other party's call is interrupted, the center clip operates to suppress not only the echo signal
but also the in-car noise, and the voice of the other party suddenly It became silent, which made
the user feel uncomfortable due to the discontinuity of the call.
[0008]
As described above, in the radio telephone apparatus equipped with the conventional echo
canceller, the center clipper is always activated when only the other party talks, that is, in the
single talk state, the input signal The center clip operation suppresses the signal when the level
of the signal) is smaller than a certain clip level, so the transmission signal is interrupted each
time the signal level of the residual signal cuts the clip level, and the other party suddenly Is
silent and there is a problem that the discontinuance of the call causes discomfort. This
incongruity was clearly recognized when the in-vehicle noise such as the traveling noise was
relatively high, and the deterioration of the speech quality became remarkable. Also, even if the
call repeats the single talk state and the double talk state in a short time, the change of the call
state causes the center clipper to work frequently, resulting in the generation of unpleasant click
sound in the call signal and the call also after the call The quality was to be reduced.
[0009]
The present invention is a voice communication apparatus capable of eliminating the above
problems, reducing the discontinuity of a call signal and the generation of click sound, and
achieving a stable echo cancellation function while maintaining an extremely natural call that
does not give discomfort to the other party. Intended to provide.
[0010]
In order to achieve the above object, the invention according to claim 1 comprises a reproduction
means for reproducing a reception signal, a transmission means for generating a transmission
signal from a transmission input signal, and said reception signal In a voice communication
apparatus having an echo cancellation means for suppressing an acoustic echo signal generated
by being input to a transmission means and outputting a residual signal, the level of the
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reception signal and the residual outputted from the echo cancellation means Call state
determination means for determining a call state based on a signal level; and signal clip means
for removing the residual signal when the average level of the constant period of the residual
signal is smaller than a set clip level; And signal clip control means for variably setting the clip
level used by the signal clip means based on the determination result of the call state
determination means. And wherein the Rukoto.
[0011]
According to the invention of claim 2, in the invention of claim 1, a transmission attenuator is
further provided, which is inserted in the transmission signal path to provide the residual signal
with an attenuation based on a call state, and the signal clip control The means variably sets the
clip level based on the call state determined by the call state determination means and the
attenuation amount of the transmission attenuator.
[0012]
The invention of claim 3 is the invention according to claim 2, further comprising attenuation
amount indicating means for outputting a control signal representing the attenuation amount of
the transmission attenuator when the residual signal is attenuated. The signal clip control means
stores from the storage means a storage means for storing a clip level corresponding to the
attenuation amount of the transmission attenuator, and a clip level corresponding to the control
signal outputted from the attenuation amount indication means. And clip level selection means
for outputting.
[0013]
Furthermore, according to the invention of claim 4, in the invention of claim 3, the clip level
stored in the storage means becomes a smaller value with respect to the attenuation amount of
the transmission attenuator as the attenuation amount becomes larger. Are associated with each
other.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION A voice communication apparatus according
to the present invention will be described in detail below with reference to the accompanying
drawings, taking a wireless telephone apparatus as an example.
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FIG. 1 is a block diagram showing a configuration of an echo canceller of a radio telephone
apparatus according to an embodiment of the present invention and its peripheral circuits.
FIG. 2 is a diagram showing an entire configuration of a radio telephone apparatus including the
circuit portion shown in FIG.
[0015]
Here, the digital radio telephone apparatus shown in FIG. 2 will be described first.
The transmission system of this radio telephone apparatus includes a microphone 111 as a
transmitter, a speech coder (SPCOD) 112, an error correction coder (CHCOD) 113, a digital
modulator (MOD) 114, a multiplier 115, a power amplifier (PA) 116, a high frequency switch
(SW) 117, and an antenna 118.
[0016]
The speech encoder 112 encodes the transmission signal output from the microphone 111.
Further, the error correction encoder 113 performs error correction encoding on the digitized
transmission signal output from the speech encoder 112 and the digitized control signal output
from the control circuit 130.
[0017]
The digital modulator 114 generates a modulation signal according to the digitized transmission
signal output from the error correction encoder 113.
The multiplier 115 mixes this modulation signal with the local oscillation signal output from the
frequency synthesizer 131, thereby frequency-converting the modulation signal into a radio
frequency signal. The power amplifier 116 amplifies the wireless transmission signal output from
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the multiplier 115 to a predetermined transmission power.
[0018]
The high frequency switch 117 becomes conductive only during a transmission time slot
designated by the control circuit 130, supplies a radio transmission signal output from the power
amplifier 116 during this period to the antenna 118, and is not shown from the antenna 118.
Send to the base station.
[0019]
On the other hand, the reception system comprises a receiver (RX) 121, a digital demodulator
(DEMOD) 122, an error correction decoder (CHDEC) 123, an audio decoder (SPDEC) 124, and a
speaker 125 as a receiver. Ru.
[0020]
The receiver 121 performs a mixing operation to directly frequency convert the received high
frequency signal received by the antenna 118 and the high frequency switch 117 into an
intermediate frequency signal or a baseband signal.
The digital demodulator 122 performs bit synchronization and frame synchronization with the
received digital signal output from the receiver 121, that is, word synchronization, and further
performs digital demodulation.
Here, the synchronization signal obtained by word synchronization is supplied to the control
circuit 130.
[0021]
The error correction decoder 123 performs error correction decoding on the digital demodulated
signal output from the digital demodulator 122. A signal obtained by this error correction
decoding is composed of a digital reception signal and digital control information, and the digital
reception signal is input to the speech decoder 124, and the digital control information is for
various control such as channel setting and speech. It is input to the control circuit 130. The
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speech decoder 124 decodes the digital reception signal. Then, the received signal that has been
decoded into an analog signal is output from the speaker 125 as loud sound.
[0022]
The control system comprises a control circuit (CONT) 130 and a frequency synthesizer (SYN)
131 etc. The frequency synthesizer 131 generates a local oscillation frequency corresponding to
each channel frequency of control, communication and synchronization designated by the
control circuit 130 Output to each part. The control circuit 130 carries out a series of control
related to device initialization, incoming / outgoing calls and calls. The power supply circuit 140
is a circuit that supplies stable power of the digital radio telephone apparatus from the DC power
supply of the automobile.
[0023]
Next, the circuit of FIG. 1 will be described. This circuit corresponds to the microphone 111 side
of the speech coder 112 of the digital radio telephone apparatus shown in FIG. 2 and the speaker
125 side of the speech decoder 124. In the figure, 1 is a D / A converter, 2 is a reception
amplifier, 3 is a speaker, 4 is a microphone, 5 is a transmission amplifier, 6 is an A / D converter,
7 is an adaptive filter (ADF), 8 is 9 is a tap coefficient updating unit, 10 is a double talk detection
circuit, 11 is a reception attenuator, 12 is a transmission attenuator, 13 is a voice switch control
circuit, 14 is a center clip control circuit, and 15 is a center clip circuit. is there. In this circuit, the
adaptive filter 7 and the subtracter 8 constitute an echo canceller main body 16.
[0024]
Next, the operation of this circuit will be described. In this circuit, in the hands-free
communication mode, the reception signal RS is applied to the voice switch control circuit 13
and, after passing through the reception attenuator 11 which is on / off controlled by the voice
switch control circuit 13, the double talk detection circuit 10, tap The coefficient updating unit 9,
the adaptive filter 7, and the D / A converter 1 are supplied.
[0025]
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The reception signal supplied to the D / A converter 1 is converted to an analog signal, amplified
by the reception amplifier 2, and then output as sound by the speaker 3. Here, a part of the
reception voice output from the speaker 3 may wrap around to the microphone 4 through the
echo path EP and may be input to the transmission system as an audio signal. In this case, the
input signal is amplified by the transmission amplifier 5 and converted into a digital signal by the
A / D converter 6 to generate an acoustic echo signal ES. The acoustic echo signal ES is supplied
to the + input terminal of the subtractor 8 of the echo canceller main body 16.
[0026]
On the other hand, the adaptive filter 7 simulates the acoustic characteristic of the echo path
from the speaker 3 to the microphone 4 to generate a pseudo echo signal ESS, which is supplied
to the-input terminal of the subtractor 8. The generation of the pseudo echo ESS in the adaptive
filter 7 is performed by the following method. First, the adaptive filter 7 adds a plurality of delay
circuits connected in series, a plurality of amplifiers for amplifying each output of the delay
circuits, and an output of each amplifier, and the addition result is added to the pseudo echo
signal ESS. And an adder that outputs the
[0027]
In the adaptive filter 7, the received speech signal after passing through the reception attenuator
11 is taken in by the first delay circuit connected in series as described above, and sequentially
sent to the next stage delay circuit. . At this time, the input voice signal for the first delay circuit
and the output signal of each delay circuit including the first delay circuit are supplied to the
above adder through the amplifier corresponding to each of these amplifiers. At this time, the
acoustic characteristics of the simulated echo path EP can be changed by changing the
amplification factor of each amplifier.
[0028]
The control of the change of the amplification factor of each amplifier is performed by the
control of the tap coefficient updating unit 9. That is, the tap coefficient update unit 9 is
configured to include at least a coefficient operation unit and a coefficient memory, and the
coefficient memory holds amplification coefficients to be supplied to the respective amplifiers. In
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the tap coefficient update unit 9, the coefficient operation unit generates pseudo echo signal
generation operation so that the residual signal is always 0 based on the reception voice signal
RS and the output of the subtracter 8 (residual signal). The amplification factor is adaptively
changed on the coefficient memory, and the amplification factor is read from the coefficient
memory and supplied to the respective amplifiers. By this adaptive operation, the acoustic
characteristic of the used space, that is, the echo path is estimated, and the generation of the
pseudo echo signal ESS is performed based on the estimation result and the reception voice
signal RS.
[0029]
The subtractor 8 performs arithmetic processing for subtracting the pseudo echo signal ESS from
the acoustic echo signal ES, thereby canceling the echo signal ES and realizing an echo
cancellation function.
[0030]
The double talk detection circuit 10 compares the levels of the reception signal RS and the
transmission signal (residual signal output from the subtracter 8 of the echo canceller body 16
and passed through the transmission attenuator 12), and according to the comparison result, A
double-talk state in which both the transmitting side and the receiving side have voice input or a
single-talk state in which a voice input exists in only one of the transmitting side and the
receiving side is detected. Output to 9
[0031]
When the tap coefficient updating unit 9 recognizes that switching to the double talk state has
been made based on the communication state detection result DTS, the amplification coefficient
immediately before that is stored in the coefficient memory, and thereafter the single talk state is
established. At the moment of returning to (4), the amplification factor stored in the abovementioned coefficient memory is read out and supplied to the adaptive filter 7.
In addition to the double talk state, the echo path estimation operation in the adaptive filter 7 is
stopped also when, for example, the handsfree call mode is switched to the handset call mode by
an instruction from the operation unit (not shown).
Also at this time, the tap coefficient updating unit 9 stores the amplification coefficient
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immediately before the stop of the echo path estimation operation on the coefficient memory.
[0032]
The voice switch control circuit 13 detects and compares the levels of the reception signal RS
and the transmission signal (residual signal ES output from the subtracter 8 of the echo canceller
main body 16), and Or, turn on 12 and perform control to turn off the attenuators 11 or 12 on
the high-level signal side. By this control, for example, in the reception state in which only the
reception signal in the single talk state exists, the transmission attenuation 12 is inserted, and in
the double talk state in which both the transmission state and the reception signal and the
transmission signal exist, reception is received. The attenuator 11 is inserted. As described above,
the voice switch control circuit 13 performs automatic level control in which attenuation is given
to the signal path side where there is no signal, thereby preventing phenomena such as howling
and sound breaking.
[0033]
Further, the voice switch control circuit 13 has a function of controlling the inserted loss amount
according to the line condition and the like. This function detects the reception electric field level
based on the reception signal at the receiver 121 (see FIG. 2), for example, and increases or
decreases the loss amount of the corresponding attenuator 11 or 12 according to the reception
electric field level. To be realized. The voice switch control circuit 13 supplies a control signal
(voice switch level VSL) representing the variable attenuation amount of the attenuator 11 or 12
to be controlled to the center clip control circuit 14 in the variable control of the insertion loss
amount.
[0034]
The center clip control circuit 14 performs centering based on the voice switch level VSL given
from the voice switch control circuit 13, the call state detection result DTS given from the double
talk detection circuit 10, and the residual signal ES outputted from the transmission attenuator
12. The clip circuit 15 is driven, and when the transmission signal (residual signal ES) is below
the clip level, the signal is removed, and when it is above the clip level, control is made to pass
the signal as it is.
[0035]
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FIG. 3 is a block diagram showing a functional configuration of the center clip control circuit 14.
The center clip control circuit 14 includes a residual signal level calculator 141, a clip level
selector 142, a comparator 143, and an AND circuit 144.
[0036]
The residual signal level calculator 141 receives the residual signal ES output from the subtracter
8 for echo cancellation and passed through the transmission attenuator 12.
Here, the residual signal level calculation unit 141 calculates an average level of a predetermined
time (for example, several tens to several hundreds of ms) of the residual signal ES to be input,
and inputs the calculation result to the comparator 143.
[0037]
The voice switch level VSL output from the voice switch control circuit 13 is input to the clip
level selection unit 142.
The clip level selection unit 142 has a plurality of clip levels (1 to N) respectively corresponding
to the voice switch level VSL used for variable control of the insertion loss amount to the
transmission attenuator 12 in the voice switch control circuit 13 described above. It is stored in
advance as a coefficient value. This coefficient value has, for example, a characteristic as shown
in FIG. 4, with the fixed attenuation of the transmission attenuator 12 as a parameter, and from
the voice switch control circuit 13 representing the variable attenuation of the transmission
attenuator 12. It takes a value corresponding to the voice switch level VSL.
[0038]
When the voice switch level VSL is input from the voice switch control circuit 13, the clip level
selection unit 142 selects a clip level corresponding to the input voice switch level VSL from
among the storage data, and the comparator 143 is selected. input.
[0039]
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As described above, the clip level selection unit 142 has a function of variably setting the clip
level in accordance with the voice switch level VSL input from the voice switch control circuit 13.
Here, referring to the characteristic diagram shown in FIG. 4, it can be seen that the clipping level
becomes a lower value as the attenuation amount of the transmission attenuator 12 increases.
[0040]
Next, the comparator 143 compares the clip level selected by the clip level selection unit 142
according to the voice switch level VSL and the residual signal level (average level within a fixed
period) calculated by the residual signal level calculation unit 141. Are compared, and the
comparison result is input to the AND circuit 144.
[0041]
The communication state detection result DTS from the double talk detection circuit 10 is input
to the AND circuit 144 together with the comparison result from the comparator 143.
Here, when the speech state detection result DTS inputted from the double talk detection circuit
10 is in the single talk state and the comparison result inputted from the comparator 143 is
smaller in residual level than the clip level, the AND circuit 144 The center clip control signal CS
is output to the clip circuit 15.
[0042]
The center clip circuit 15 clips the residual signal based on the center clip control signal CS. FIG.
5 shows a general example of the input / output characteristics of the residual signal due to the
function of the center clip in the center clip circuit 15. As shown in FIG. According to this
characteristic diagram, in the center clip region to which the center clip control signal CS is
applied, the output is suppressed to 0 even if the residual signal is input to the center clip circuit
15, and the center clip control signal CS is applied. It can be seen that the input residual signal is
output at the same level outside the unintended center clip region.
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[0043]
An example of the time variation characteristic of the transmission signal output level
accompanying the signal clip operation in the center clip circuit 15 based on the above control of
the center clip control circuit 14 is shown in FIG. In this example, the attenuation amount for the
transmission attenuator 12 is set to a value larger than 0 by the variable control of the insertion
attenuation amount in the voice switch control circuit 13, and the center clip control circuit 14
sets the clip level selection unit At 142, a clip level CL having a value smaller than the clip level
CL0 corresponding to the voice switch level VSL when the attenuation amount is 0 is set
corresponding to the voice switch level VSL with respect to the set attenuation amount of the
transmission attenuation 12 The case is assumed.
[0044]
In FIG. 6, at time t0, it is in a single talk (receive) state, the transmission attenuator 12 has been
activated, and echo cancellation is performed, and the output signal level TS of the transmission
signal gradually decreases. Even if the output signal level TS becomes lower than the clip level
CL0 (corresponding to the clip level in the conventional example) when the transmission
attenuation amount is 0, the clip level CL is already equal to the attenuation amount of the
transmission attenuation 12 Since it is low, the center clip does not work here, and the center
clip operates only when the output signal level TS becomes the clip level CL at time t1.
[0045]
The center clip is in operation from time t1 to time t2. At time t2, the call enters a double talk
state, the center clip is stopped, and the attenuation amount of the transmission attenuator 12
also becomes zero. The double talk state continues from time t2 to time t3 and enters the single
talk (listening) state at time t3. Here, the transmission attenuation 12 works again to perform
echo cancellation. Thereafter, the output signal level TS of the transmission signal gradually
decreases as echo cancellation continues, and the output signal level TS divides the clip level CL0
(corresponding to the clip level in the conventional example) when the transmission attenuation
amount is 0. Further, the center clip operates again at time t4 at which the clip level CL, which
has a lower value by the amount corresponding to the attenuation amount in the transmission
attenuator 12 at this time, is divided.
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[0046]
Compared with the function of the center clip in the center clip circuit 15 of the present
invention (see FIG. 6) and the function of the center clip in the conventional example (see FIG. 7),
the clip level is fixed to CL0 in the conventional example. On the other hand, in the present
invention, the clip level can be varied according to the attenuation amount of the transmission
attenuation 12. In this example, the clipping level is equal to the attenuation amount of the
transmission attenuation 12 with respect to the clip level CL0. Only low clip level is set to CL. In
this way, operating the clip at a clip level CL that is lower by an amount corresponding to the
attenuation amount in the transmission attenuator 12 causes the far-end speaker to have a lower
speaking voice (listening voice) of the other speaker. This means that the level is reached and
then interrupted, which can reduce the sense of discomfort that the received voice is suddenly
interrupted.
[0047]
As described above, in the center clip control circuit 14 of the present invention, the clip level is
made variable according to the attenuation amount of the transmission attenuation 12, so that
the center clip works on the residual signal level ES of the echo canceller output. The point is
fixed.
[0048]
In addition, when talking is performed in a state where the ambient noise is large, the average
level of the residual signal level ES steadily increases.
As a result, the residual signal level ES becomes larger than the clip level, and the center clip
control is not performed despite the single talk state, and the residual signal level ES becomes the
transmission signal as it is, and the far end speaker is not good. I will not give nature.
[0049]
Furthermore, when the double talk state and the single talk state are repeated within a relatively
short time, the average level of the residual signal level ES is unlikely to be small, and the
residual signal level ES continues to be larger than the clip level. The center clip control is not
performed, and the residual signal level ES becomes the transmission signal as it is, and the
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generation of the click sound is prevented.
[0050]
As described above, according to the present invention, the clip level is variably set according to
the call state with respect to signal clip means for clipping the residual signal after echo
cancellation according to the clip level. In addition, since the clip is operated according to the
result of comparison between the variably set clip level and the average level of the residual
signal within a fixed period, the clip level is fixed and the absolute value of the residual signal is
Compared with the case where the clip is operated by comparison, it is possible to reduce the
sense of discomfort that the call is suddenly interrupted.
[0051]
Further, the present invention further includes a transmission attenuator that is inserted into the
transmission signal path and that attenuates the residual signal based on the communication
state, and the attenuation amount of the transmission attenuator is set when the clip level is
variably set. It may be added to the parameter.
As described above, when the in-vehicle noise during traveling or the like is relatively high by
variably setting the clip level based on the average level of the residual signal for a certain
period, the communication state determination result, and the attenuation amount of the
transmission attenuator. It is possible to reduce click noise in situations where the call signal is
discontinuous or when single talk and double talk are frequently switched, and to realize a stable
echo cancellation function while maintaining an extremely natural call that does not give
discomfort to the other party .
[0052]
Brief description of the drawings
[0053]
1 is a block diagram showing the configuration of the echo canceller of the radio telephone
apparatus according to the present invention and its peripheral circuits.
[0054]
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2 is an overall configuration diagram of a wireless telephone apparatus including the circuit
portion shown in FIG.
[0055]
3 is a block diagram of a center clip control circuit of the radio telephone apparatus according to
the present invention.
[0056]
4 is a diagram showing the relationship between the clip level and the voice switch level used in
the center clip control circuit.
[0057]
5 is a diagram showing the input and output characteristics of the residual signal in the center
clip circuit.
[0058]
6 is a diagram showing the time variation characteristic of the transmission power level in the
radio telephone apparatus according to the present invention.
[0059]
7 is a diagram showing the time variation characteristics of the transmission power level in the
conventional wireless telephone device.
[0060]
Explanation of sign
[0061]
Reference Signs List 1 D / A converter 2 reception amplifier 3 speaker 4 microphone 5
transmission amplifier 6 A / D converter 7 adaptive filter (ADF) 8 subtractor 9 tap coefficient
updating unit 10 double talk detection circuit 11 reception attenuator 12 transmission
Attenuator 13 voice switch control circuit 14 center clip control circuit 141 residual signal level
calculation unit 142 clip level selection unit 143 comparator 144 AND circuit 15 center clip
circuit 16 echo canceller 111 microphone 112 speech coder (SPCOD) 113 error Correction
encoder (CHCOD) 114 Digital modulator (MOD) 115 Multiplier 116 Power amplifier (PA) 117
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High frequency switch (SW) 118 Antenna 121 Receiver (RX) 122 Digital demodulator (DEMOD)
123 Error correction decoder (CHDEC) 124 Voice decoder (SPDEC) 125 speakers 130 control
circuit (CONT) 131 Frequency synthesizer (SYN) 140 power supply circuit
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