JP3731228B2 - Audio signal transmitting / receiving apparatus and reception volume control method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an audio signal transmission / reception device and a reception volume control method for controlling a reception volume according to the level of ambient noise obtained from an input signal.
[0002]
[Prior art]
Recently, mobile phone devices have been widely used. Since this portable telephone device is often used outdoors, it is often difficult to hear a call due to ambient background noise (hereinafter referred to as ambient noise). This is because the minimum audible value of the listener increases due to the masking effect due to noise, and the intelligibility and intelligibility of the received speech deteriorate. On the other hand, it is necessary to suppress noise or increase the speaker's voice volume on the transmitting side, increase the reproduction volume on the receiving side, and close acoustic coupling between the speaker and the telephone as a whole.
[0003]
For this reason, recent portable telephone devices have automatically controlled the reception volume according to the ambient noise.
[0004]
[Problems to be solved by the invention]
By the way, conventionally, when the level of the received sound volume is controlled by the portable telephone device, it is made variable at the same speed when it is increased or decreased. For this reason, the user's ears may be noisy when the noise level has suddenly become quiet, that is, when the ambient noise level suddenly changes from a high level to a low level. At some level, I was temporarily paralyzed, making it difficult to hear the conversation.
[0005]
Also, if ambient noise fluctuates near the noise threshold level that changes the volume level, the volume may fluctuate accordingly.
[0006]
The present invention has been made in view of the above circumstances, and an audio signal transmission / reception device and a reception volume that can prevent deterioration of the listening state of the user and fluctuation of the reception volume with respect to changes in ambient noise. The purpose is to provide a control method.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, an audio signal transmitting / receiving apparatus according to the present invention is an audio signal transmitting / receiving apparatus that controls a reception volume according to ambient noise obtained from an input signal, and detects the level of the ambient noise. Ambient noise level detection means, and control means for variably controlling the increase / decrease speed of the received sound volume according to the vertical change of the ambient noise level, the control means when the ambient noise level increases, the received sound volume The Automatically When the ambient noise level decreases and increases, Automatically Decrease and decrease the rate of decrease compared to the rate of increase in the received sound volume.
[0008]
The control means responds to a plurality of different threshold levels when increasing or decreasing the reception volume. The above control means is The amount of change in ambient noise level is The reception volume is changed after exceeding a predetermined value.
[0009]
In order to solve the above-mentioned problem, the received sound volume control method according to the present invention is a received sound volume control method for controlling the received sound volume in accordance with the ambient noise obtained from the input signal, and detects the ambient noise level. An ambient noise level detecting step, and a control step for variably controlling the rate of increase / decrease of the received sound volume according to a change in the ambient noise level up and down, and the control step is performed when the ambient noise level increases. The Automatically When the ambient noise level decreases and increases, Automatically Decrease and decrease the rate of decrease compared to the rate of increase in the received sound volume.
[0010]
The control process responds to a plurality of different threshold levels when increasing or decreasing the reception volume. The above control process is The amount of change in ambient noise level is The reception volume is changed after exceeding a predetermined value.
[0011]
Specifically, when the surroundings suddenly becomes noisy, the reception volume is increased rapidly, and when the surroundings suddenly becomes quiet, the reception volume is decreased slowly to prevent deterioration of the listening state of the user. In addition, the threshold level is set separately for when the received volume is increased and when the received volume is decreased, and a fluctuation of a small noise level near the threshold level is provided with a dead band, thereby preventing fluctuations in the received volume.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an audio signal transmitting / receiving apparatus and an incoming sound volume control method according to the present invention will be described below. In this embodiment, as shown in FIG. 1, the voice input from the transmission microphone 2 is converted into an RF signal, and this RF signal is transmitted to the other party via the base station. This is a mobile phone device 1 that can receive and reproduce an RF signal transmitted through a station and let the user hear it through a speaker 13.
[0013]
In particular, the cellular phone device 1 includes an ambient noise level detection circuit 8 that detects the ambient noise level so as to control the reception volume in accordance with the ambient noise level obtained from the input signal, and an upper and lower level of the ambient noise level. A reception volume control unit 9 that variably controls the increase / decrease speed of the reception volume according to the change is provided.
[0014]
An input signal converted into an electrical signal by the microphone 2 for transmission of the cellular phone device 1 is converted into digital data by an analog / digital (A / D) converter 3 and supplied to a speech encoder 4.
[0015]
The voice encoder 4 compresses and encodes digital data. As the speech encoder 4, for example, a vector sum excited linear prediction (VSELP) encoder that compresses speech signals with high efficiency by digital signal processing can be used.
[0016]
Data compressed and encoded by the audio encoder 4 is supplied to the baseband signal processing circuit 5, where a synchronization signal, an error correction code, and the like are added or framing processing is performed. Output data from the baseband signal processing circuit 5 is supplied to the RF transmitting / receiving circuit 6, modulated to a required frequency, and transmitted from the antenna 7.
[0017]
The digital data output from the A / D converter 3 is also supplied to the ambient noise level detection circuit 8. The ambient noise level detection circuit 8 detects the level of ambient noise included in the input signal. Specifically, the signal level of the ambient noise section detected by the ambient noise section detector (not shown) is detected.
[0018]
The ambient noise level detected by the ambient noise level detection circuit 8 is supplied to the received sound volume control unit 9. Based on the ambient noise level, the reception volume control unit 9 adjusts the gain of a variable gain amplifier 12 that amplifies reception audio, which will be described later, converted into an analog signal by the digital / analog (D / A) converter 11, Control the listening volume. Here, when changing the increase / decrease speed of the received sound volume by changing the ambient noise level up and down, the received sound volume control unit 9 increases the increase speed of the received sound volume and decreases the decrease speed. Details will be described later.
[0019]
Here, the reception volume is a volume when reproducing a signal from a call partner transmitted via the base station. The signal from the other party is received by the antenna 7 and supplied to the RF transmission / reception circuit 6. The input signal from the other party demodulated to the baseband by the RF transmission / reception circuit 6 is supplied to the baseband signal processing circuit 5 and subjected to predetermined signal processing. The signal from the baseband signal processing circuit 5 is supplied to the speech decoder 10. As this audio decoder 10, for example, a VSELP decoder can be used. The audio signal decoded by the audio decoder 10 is converted into an analog audio signal by the D / A converter 11.
[0020]
The analog audio signal from the D / A converter 11 is supplied to the variable gain amplifier 12. Since the gain of the variable gain amplifier 12 is adjusted by the reception volume control unit 9 as described above, the reproduction volume emitted from the speaker 13, that is, the reception volume is controlled according to the ambient noise.
[0021]
Here, it is a condition that the section in which the ambient noise level detection circuit 8 detects the noise level is a noise section. It is determined whether the corresponding frame is voiced sound or noise. If it is determined that the frame is noise, the ambient noise level is detected.
[0022]
The reception volume control unit 9 increases the increase rate of the reception volume level when the ambient noise level detected by the ambient noise level detection circuit 8 becomes high (ie, when the ambient noise level changes upward). When the value becomes low (decreases or changes downward), the rate of decrease in the received sound volume level is slowed down.
[0023]
This is because the user's ears are temporarily paralyzed with the previous noise level so that the received content will not be missed even if the surroundings suddenly become noisy, and when the surroundings suddenly becomes quiet. This is in order to prevent missed listening to the content of the incoming call.
[0024]
In this case, the received sound volume control method performed by the received sound volume control unit 9 will be described with reference to FIG. In FIG. 2, there are six volume levels that can be set, for example, and when the ambient noise level increases with respect to the volume level (user volume level, user_vol) set in advance by the user, user_vol, user_vol + 6 dB, user_vol An example of changing the volume level to +12 dB is shown. Here, the change of the volume in one step is 6 dB, and when changing the volume, the volume level is changed in 2 dB steps so as to change smoothly. Further, UP_LIMIT and DOWN_LIMIT in the figure respectively represent the maximum volume and the minimum volume of the system (as a mobile phone device), and the volume is not controlled beyond this range. That is, user_vol does not fall below DOWN_LIMIT, and the values of user_vol, user_vol + 6 dB, and user_vol + 12 dB are limited by the value of UP_LIMIT.
[0025]
FIG. 3 shows the relationship between the threshold level of ambient noise and the received sound volume. There are separate threshold levels for volume up and volume down. That is, it has threshold levels NoiseTH2 and NoiseTH3 when the volume is increased, and has threshold levels NoiseTH2d and NoiseTH3d when the volume is decreased.
[0026]
That is, the mobile phone device 1 has a threshold level separately when the volume is increased and when the volume is decreased.
[0027]
Here, each threshold level has a magnitude relationship of NoiseTH3>NoiseTH3d>NoiseTH2> NoiseTH2d.
[0028]
The received sound volume control unit 9 controls the received sound volume so as to be one of user_vol, user_vol + 6 dB, and user_vol + 12 dB with these threshold levels as a boundary according to the ambient noise level.
[0029]
The detailed operation of the reception volume control unit 9 will be described with reference to the flowcharts of FIGS.
[0030]
First, in step S1 of FIG. 4, the reception volume control unit 9 supplies the ambient noise level level detected by the ambient noise level detection circuit 8 to a 3-tap media filter. This is a process for improving the reliability of the ambient noise level. The filtering performed by the median filter is a process of rearranging the values in the filter tap in ascending order and outputting an intermediate value therein.
[0031]
Next, as shown in step S2, the received sound volume control unit 9 calculates the ambient noise level prev_lvl previously detected by the ambient noise level detection circuit 8 and the change amount diff_lvl of the ambient noise level level newly detected.
[0032]
Next, the received sound volume control unit 9 determines whether or not the amount of change diff_lvl of the ambient noise level level exceeds the threshold level Diff_level, as shown in step S3. If YES, that is, if it is determined that the change amount diff_lvl exceeds the threshold level Diff_level, the process proceeds to step S4. If NO, that is, if it is determined that the change amount diff_lvl does not exceed the threshold level Diff_level, the process proceeds to step S5.
[0033]
In step S4, in response to the determination (YES) in step S3, it is determined whether or not the newly detected ambient noise level level has increased from the previously detected ambient noise level prev_lvl. If YES, that is, if it is determined that the newly detected ambient noise level level is higher than the previously detected ambient noise level prev_lvl, the process proceeds to step S13 shown in FIG. 5, and NO, that is, the newly detected ambient noise level level is the previously detected ambient noise level. If it is determined that the noise level has not increased from prev_lvl, the process proceeds to step S6.
[0034]
On the other hand, in step S5, in response to the determination (NO) in step S3, it is determined whether or not the user setting volume has changed. If YES, that is, if it is determined that the user setting volume has changed, the process proceeds to step S4. If NO, that is, if it is determined that the user setting volume has not changed, the process proceeds to step S7.
[0035]
In step S6, it is determined whether the ambient noise level level newly detected in response to the determination (NO) in step S4 is lower than the previously detected ambient noise level prev_lvl. If YES, that is, if it is determined that the newly detected ambient noise level level is lower than the previously detected ambient noise level prev_lvl, the process proceeds to step S8, and NO, that is, the newly detected ambient noise level level is greater than the previously detected ambient noise level prev_lvl. If it is determined that it has not decreased, the process proceeds to step S7.
[0036]
In step S7, the value target_volume, which is the target of volume control, remains the previous target value prev_target_volume in response to the determination (NO) in step S5 and the determination (NO) in step S6, and the volume is not changed. After step S7, the process proceeds to step S18.
[0037]
In step S8, it is determined whether the ambient noise level level newly detected in response to the determination (YES) in step S6 is equal to or lower than the noise threshold level NoiseTH2d shown in FIG. If YES, that is, if it is determined that the newly detected ambient noise level level is less than or equal to the noise threshold level NoiseTH2d, the process proceeds to step S9. If NO, that is, if the newly detected ambient noise level level is determined to be greater than the noise threshold level NoiseTH2d, step S10 is performed. Proceed to
[0038]
In step S9, in response to the determination (YES) in step S8, the value target_volume that is the target of volume control is set to the user setting value user_vol. After step S9, the process proceeds to step S18.
[0039]
In step S10, it is determined whether or not the ambient noise level level newly detected in response to the determination (NO) in step S8 is equal to or lower than the noise threshold level NoiseTH3d shown in FIG. If YES, that is, if it is determined that the newly detected ambient noise level level is less than or equal to the noise threshold level NoiseTH3d, the process proceeds to step S11. If NO, that is, if the newly detected ambient noise level level is determined to be greater than the noise threshold level NoiseTH3d, step S12 is performed. Proceed to
[0040]
In step S11, in response to the determination (YES) in step S10, a value target_volume that is a target of volume control is set to user_vol + 6 dB. After step S11, the process proceeds to step S18.
[0041]
In step S12, in response to the determination (NO) in step S10, a value target_volume that is a target of volume control is set to user_vol + 12 dB. After step S12, the process proceeds to step S18.
[0042]
Next, in step S13 shown in FIG. 5, it is determined whether or not the ambient noise level level newly detected in response to the determination (YES) in step S4 is equal to or lower than the noise threshold level NoiseTH2 shown in FIG. If YES, that is, if it is determined that the newly detected ambient noise level level is less than or equal to the noise threshold level NoiseTH2, the process proceeds to step S14. If NO, that is, if the newly detected ambient noise level level is determined to be greater than the noise threshold level NoiseTH2, step S15 is performed. Proceed to In step S14, in response to the determination (YES) in step S13, the value target_volume that is the target of volume control is set to the user setting value user_vol. After step S14, the process proceeds to step S18.
[0043]
In step S15, it is determined whether or not the ambient noise level level newly detected in response to the determination (NO) in step S13 is equal to or lower than the noise threshold level NoiseTH3 shown in FIG. If YES, that is, if it is determined that the newly detected ambient noise level level is equal to or lower than the noise threshold level NoiseTH3, the process proceeds to step S16. If NO, that is, if it is determined that the newly detected ambient noise level level is greater than the noise threshold level NoiseTH3, step S17 is performed. Proceed to
[0044]
In step S16, in response to the determination (YES) in step S15, a value target_volume that is a target of volume control is set to user_vol + 6 dB. After step S16, the process proceeds to step S18.
[0045]
In step S17, in response to the determination (NO) in step S15, a value target_volume that is a target of volume control is set to user_vol + 12 dB. After step S17, the process proceeds to step S18.
[0046]
In step S18, it is determined whether or not the value target_volume that is the target of volume control is greater than the maximum volume level UP_LIMIT shown in FIG. If YES, that is, if it is determined that the value target_volume targeted for volume control is larger than the maximum volume level UP_LIMIT, the process proceeds to step S19. If NO, ie, the value target_volume targeted for volume control is determined to be less than the maximum volume level UP_LIMIT, step S20 is performed. Proceed to
[0047]
In step S19, in response to the determination (YES) in step S18, the value target_volume that is the target of volume control is limited by the maximum volume UP_LIMIT. After step S19, the process proceeds to step S20.
[0048]
In step S20, it is determined whether the value target_volume that is the target of volume control after the determination (NO) in step S18 and step S19 is smaller than the minimum volume level DOWN_LIMIT shown in FIG. If YES, that is, if it is determined that the value target_volume targeted for volume control is smaller than the minimum volume level DOWN_LIMIT, the process proceeds to step S21. If NO, ie, the value target_volume targeted for volume control is determined to be greater than or equal to the minimum volume level DOWN_LIMIT, step S22 is performed. Proceed to
[0049]
In step S21, in response to the determination (YES) in step S20, the value target_volume that is the target of volume control is limited by the minimum volume DOWN_LIMIT. After step S21, the process proceeds to step S22.
[0050]
In step S22, after the determination (NO) in step S20 and after step S21, the current level level is stored as a previously detected value prev_lvl.
[0051]
Thereafter, as shown in step S23, the volume control unit 9 stores the value target_volume that is the target of the control as the previous value prev_target_volume.
[0052]
Next, the volume control unit 9 stores the current user setting value user_vol as prev_user_vol as shown in step S24. After step S24, the process proceeds to step S25 in FIG.
[0053]
Then, the reception volume control unit 9 stores the volume level Setting_VOL that is actually set in the mobile phone device 1 as Setting_VOL_Prev as shown in step S25 of FIG.
[0054]
Next, as shown in step S26, the reception volume control unit 9 determines whether or not the value target_volume that is the target of volume control is smaller than the volume level Setting_VOL that is actually set. If YES, that is, if it is determined that the value target_volume that is the target of volume control is smaller than the volume level Setting_VOL that is actually set, the process proceeds to step S31, and NO, that is, the value target_volume that is the target of volume control is greater than or equal to the volume level Setting_VOL that is actually set. If it is determined that there is, the process proceeds to step S27.
[0055]
In step S27, in response to the determination (NO) in step S26, it is determined whether or not the value target_volume that is the target of volume control is greater than the volume level Setting_VOL that is actually set. If YES, that is, if it is determined that the target value volume_control target volume is larger than the volume level Setting_VOL that is actually set, the process proceeds to step S28. If it is determined that there is, the process proceeds to step S37 shown in FIG.
[0056]
In step S28, in response to the determination (YES) in step S27, the volume of the received sound volume is increased by +2 dBstep to the volume level Setting_VOL. When step S28 ends, the process proceeds to step S29.
[0057]
In step S29, it is determined whether or not the volume level Setting_VOL obtained by the control in step S28 exceeds the maximum volume UP_LIMIT. If YES, ie, the volume level Setting_VOL exceeds the maximum volume UP_LIMIT, the process proceeds to step S30. If NO, ie, it is determined that the volume level Setting_VOL does not exceed the maximum volume UP_LIMIT, the process proceeds to step S37.
[0058]
In step S30, the volume level Setting_VOL is limited by the maximum volume UP_LIMIT in response to the determination (YES) in step S29. When step S30 ends, the process proceeds to step S37.
[0059]
On the other hand, in step S31, in response to the determination (YES) in step S26, it is determined whether or not the value counter for determining the volume down speed when the volume is lowered is zero. If YES, that is, if it is determined that the value counter that determines the volume down speed is 0, the process proceeds to step S33. If NO, that is, if the value counter that determines the volume down speed is not 0, the process proceeds to step S32.
[0060]
In step S32, the value counter that determines the volume down speed is decremented in response to the determination (NO) in step S31. When step S32 ends, the process proceeds to step S35.
[0061]
In step S33, in response to the determination (YES) in step S31, the volume of the reception volume is decreased by adding -2dBstep to the volume level Setting_VOL. When step S33 ends, the process proceeds to step S34.
[0062]
In step S34, the value counter that determines the volume down speed is reset to the initial value InitCount. The volume down speed decreases as the initial value InitCount of the value counter that determines the volume down speed is large. When step S34 ends, the process proceeds to step S35.
[0063]
In step S35, it is determined whether or not the volume level Setting_VOL is smaller than the minimum volume DOWN_LIMIT. If YES, that is, if it is determined that the volume level Setting_VOL is smaller than the minimum volume DOWN_LIMIT, the process proceeds to step S36. If NO, that is, if it is determined that the volume level Setting_VOL is greater than or equal to the minimum volume DOWN_LIMIT, the process proceeds to step S37.
[0064]
In step S36, the volume level Setting_VOL is limited by the minimum volume DOWN_LIMIT in response to the determination (YES) in step S35. When step S36 ends, the process proceeds to step S37.
[0065]
In step S37 of FIG. 7, the received sound volume control unit 9 determines whether or not the value target_volume that is a control target is equal to the volume level Setting_VOL. If YES, that is, if it is determined that the control target value target_volume is not equal to the volume level Setting_VOL, the process proceeds to step S38, and the system volume is set to Setting_VOL. On the other hand, if NO, that is, if it is determined that the control target value target_volume is equal to the volume level Setting_VOL, the process ends without changing the volume level.
[0066]
In step S39, it is determined whether or not the value target_volume as a control target is equal to the volume level Setting_VOL_Prev. If YES, that is, if it is determined that the control target value target_volume is not equal to the volume level Setting_VOL_Prev, the process proceeds to step S38, and the system volume is set to Setting_VOL. On the other hand, if NO, that is, if it is determined that the control target value target_volume is equal to the volume level Setting_VOL_Prev, the process ends without changing the volume level.
[0067]
As described above, the volume control unit 9 controls the reception volume based on the threshold level of FIG. 3 according to the ambient noise level.
[0068]
The speed at the time of volume up is determined by the volume control cycle + delay of the processing by the median filter, and the speed at the time of volume down is determined by (the volume control cycle × counter value) + the processing delay by the median filter.
[0069]
In this way, the cellular phone device 1 increases the received sound volume level quickly when the ambient noise level detected by the ambient noise level detection circuit 8 is increased by the received sound volume control unit 9, and the ambient noise level decreases. If the surroundings suddenly become noisy, the listening level will not be missed, and when the surroundings suddenly becomes quiet, the user's ear will be noisy. Even if you are temporarily paralyzed at the level, you won't miss the content you just received.
[0070]
Further, the cellular phone device 1 has a threshold level separately when the volume is increased and decreased, and further provides a dead zone for a small change in noise level near the threshold level, thereby making the received volume fluctuate. prevent.
[0071]
【The invention's effect】
In the audio signal transmitting / receiving apparatus according to the present invention, when the ambient noise level detected by the ambient noise level detecting means is increased by the control means, the received volume level is quickly increased, and when the ambient noise level is lowered, the received volume is received. Since the level is lowered slowly, even if the surroundings suddenly become noisy, the user will not miss the received content. In addition, even if the surroundings suddenly become quiet, the user is not allowed to hear the content of the incoming call.
[0072]
In addition, when the volume is increased and decreased, a threshold level is separately provided, and a small noise level change in the vicinity of the threshold level is provided with a dead band, thereby preventing a sense of fluctuation of the received volume.
[0073]
Further, the received sound volume control method according to the present invention increases the received sound volume level quickly when the ambient noise level detected in the ambient noise level detecting step is increased by the control process, and the above-mentioned when the ambient noise level is decreased. Since the listening volume level is lowered slowly, even if the surroundings suddenly become noisy, the user will not miss the listening content. In addition, even if the surroundings suddenly become quiet, the user is not allowed to hear the content of the incoming call.
[0074]
In addition, when the volume is increased and decreased, a threshold level is separately provided, and a small noise level change in the vicinity of the threshold level is provided with a dead band, thereby preventing a sense of fluctuation of the received volume.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of a mobile phone device according to an embodiment of a voice transmitting / receiving device and a received sound volume control method according to the present invention.
FIG. 2 is a schematic diagram for schematically explaining the operation of the volume control unit in which the change is variable between when the received volume is raised and when it is lowered.
FIG. 3 is a schematic diagram showing a relationship between a threshold level of ambient noise and a received sound volume when schematically shown in FIG. 2;
FIG. 4 is a flowchart for explaining in detail the operation of the volume control unit in which the change is variable between when the received volume is raised and when it is lowered.
FIG. 5 is a flowchart for explaining in detail the operation of the volume control unit in which the change is variable between when the received volume is raised and when it is lowered.
FIG. 6 is a flowchart for explaining in detail the operation of the volume control unit in which the change is variable between when the received volume is raised and when it is lowered.
FIG. 7 is a flowchart for explaining in detail the operation of the volume control unit in which the change is variable between when the received volume is raised and when it is lowered.
[Explanation of symbols]
1 Mobile phone device
2 Microphone for transmission
4 Speech encoder
8 Ambient noise level detection circuit
9 Received volume control section
10 Speech decoder

Claims (6)

An audio signal transmitting / receiving apparatus for controlling a reception volume according to ambient noise obtained from an input signal,
Ambient noise level detection means for detecting the ambient noise level;
Control means for variably controlling the increase / decrease speed of the received sound volume according to the vertical change of the ambient noise level,
Said control means when the ambient noise level increases, automatically increase the earpiece volume, when the ambient noise level is reduced automatically decrease the earpiece volume, the rate of increase of the received sound volume An audio signal transmitting / receiving apparatus characterized by slowing down the rate of decrease compared to the above.   2. The audio signal transmitting / receiving apparatus according to claim 1, wherein said control means responds to a plurality of different threshold levels when said reception volume is increased and decreased.   3. The audio signal transmitting / receiving apparatus according to claim 2, wherein the control means changes the received sound volume after a change amount of the ambient noise level exceeds a predetermined value. A reception volume control method for controlling the reception volume according to ambient noise obtained from an input signal,
An ambient noise level detection step for detecting the ambient noise level;
A control step of variably controlling the increase / decrease speed of the received sound volume according to the vertical change of the ambient noise level,
The control process when the ambient noise level increases, automatically increase the earpiece volume, when the ambient noise level is reduced automatically decrease the earpiece volume, the rate of increase of the received sound volume A method for controlling the received sound volume, characterized in that the rate of decrease is made slower than that.   5. The received sound volume control method according to claim 4, wherein the control step responds to a plurality of different threshold levels when the received sound volume is increased and decreased.   6. The received sound volume control method according to claim 5, wherein the received sound volume is changed after the change amount of the ambient noise level exceeds a predetermined value.

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