JPH11220345A - Automatic gain controller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the unnecessary gain control against the ending-of-word parts and the background noises of voices and also to secure the maximum gain. SOLUTION: It is decided based on the input signal level calculated by a CPU whether the gain control should be executed. If the input signal level is less than threshold Th11, a flag 2 is set at 1 and the gain control is not executed by the CPU. If the input signal level is more than threshold Th12, the flag 2 is set at 0 and the gain control is executed. If the Th11 is equal to or less than the input signal level and also the input signal level is equal to or less than the Th12, it is decided that the input signal level is reduced compared with the value larger than the Th12 as long as the flag is set at 0. Thus, no gain control is executed. If the flag is set at 1 when the Th11 is equal to or less than the input signal level and also the input signal level is equal to or less than the TH12, it is decided that the input signal level is increased compared with the value smaller than the TH11. Thus, the gain control is executed. Both Th11 and Th12 can also be variably set.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control apparatus, and more particularly to an automatic gain control apparatus for voice used for adjusting a voice level in a telephone system, a voice memo, and the like.

[0002]

2. Description of the Related Art Conventionally, in a system for handling a voice signal such as a telephone system or a voice memo, an automatic gain control device for voice has been used which automatically amplifies a voice having a low input level to an optimum level. FIG. 14 is a functional block diagram showing an example of a main part of the conventional automatic gain control device 1. As shown in FIG. In the automatic gain controller 1, the input signal level calculator 2
, An input signal level is calculated, the input signal level is compared with a predetermined threshold value by the operation determining unit 3, and the gain control unit 4 is turned on / off according to the comparison result. When the input signal level is larger than the threshold, the gain control unit 4
N, the gain in the amplifying unit 6 is controlled by the gain control unit 4 so that the output signal level calculated by the output signal level calculating unit 5 becomes constant. On the other hand, if the input signal level is equal to or less than the threshold, the gain control unit 4 is turned off and the gain control is stopped, thereby preventing unnecessary amplification of the background noise in the silent section.

[0003]

However, in the conventional automatic gain control device 1, when it is desired to perform gain control on a smaller input signal, it is necessary to reduce a threshold value for determining ON / OFF of the gain control. . In this case, the gain always tends to be amplified in an input signal section having a low level such as an end part. In addition, the possibility that the automatic gain control device 1 malfunctions (erroneous gain control) increases even for background noise that is larger than the threshold.

On the other hand, in order to prevent a malfunction due to noise and an unnecessary gain increase at the end part, it is necessary to increase the threshold value. In this case, however, the maximum gain of the automatic gain control device 1 cannot be increased.

Further, if the threshold is made variable so that the threshold follows the background noise level, a malfunction due to the background noise can be considerably prevented, but an unnecessary increase in the gain at the end portion cannot be prevented.

SUMMARY OF THE INVENTION Therefore, a main object of the present invention is to provide an automatic gain control device capable of suppressing unnecessary gain control for the ending part of speech and background noise and obtaining a larger maximum gain.

[0007]

In order to achieve the above object, an automatic gain control device according to claim 1 is an automatic gain control device which obtains an output signal by multiplying and amplifying an input signal by gain. Input signal level calculating means for calculating an input signal level based on the input signal, output signal level calculating means for calculating an output signal level based on the output signal, and a gain based on the output signal level and a preset target level. Gain control means for controlling, storage means for respectively storing a first threshold value and a second threshold value larger than the first threshold value, first means for stopping gain control by the gain control means when the input signal level is smaller than the first threshold value, input A second means for performing gain control by the gain control means if the signal level is higher than the second threshold; and an input signal level being higher than the first threshold and lower than the second threshold. In this case, when the input signal level increases from a value smaller than the first threshold value, gain control by the gain control means is executed. On the other hand, when the input signal level decreases from a value larger than the second threshold value, the gain control means performs the gain control. A third means for stopping the gain control is provided.

According to a second aspect of the present invention, in the automatic gain control device according to the first aspect, the input signal level has increased from a value smaller than the first threshold or decreased from a value larger than the second threshold. The third means includes means for determining whether to execute or stop gain control by the gain control means with reference to the flag.

An automatic gain control device according to a third aspect of the present invention is the automatic gain control device according to the second aspect, further comprising threshold value setting means for variably setting the first threshold value and the second threshold value. Means for setting a background noise reference value according to a result of comparison between an input signal level and a background noise reference value, first threshold setting means for setting a first threshold based on the set background noise reference value, and first threshold And a second threshold value setting unit that sets a second threshold value based on the first threshold value set by the setting unit.

According to a fourth aspect of the present invention, in the automatic gain control apparatus according to the third aspect, the first threshold setting means multiplies the first value by multiplying the background noise reference value by a first predetermined value. The second threshold setting means includes means for setting a threshold, and the second threshold setting means includes means for setting a second threshold by adding a second predetermined value to the first threshold.

In the automatic gain control device according to the first aspect,
The gain is controlled by the gain control means based on the output signal level calculated by the output signal level calculation means and a preset target level. If the input signal level calculated by the input signal level calculation means is smaller than the first threshold, the gain control is stopped, and
If it is greater than the threshold, gain control is performed. When the input signal level increases from a value smaller than the first threshold value and becomes equal to or more than the first threshold value and equal to or less than the second threshold value, gain control is performed, while the input signal level decreases from a value larger than the second threshold value. When the value becomes equal to or more than the first threshold value and equal to or less than the second threshold value, the gain control is stopped. At the time of gain control, the gain determined by the gain control means is applied to the input signal and amplified to obtain an output signal.

In the automatic gain control device according to the second aspect,
The history of the input signal level is determined with reference to the flag, and it is determined whether or not to execute gain control.

In this way, two types of thresholds are set for the input signal level, and a hysteresis is provided between the thresholds. That is, a first threshold set lower and a second threshold set higher are prepared, and by using the first threshold as a reference, the minimum level of gain control can be made smaller, and a large maximum gain can be obtained. Is obtained. Also, the second
By using the threshold value as a reference, unnecessary gain control for the ending part and the background noise can be suppressed.

Further, the first threshold value and the second threshold value may be made variable.

In the automatic gain control device according to the third aspect,
In the threshold setting means, first, a background noise reference value is set according to a comparison result between the input signal level and the background noise reference value. Then, the first threshold value setting means sets the first threshold value based on the set background noise reference value.
For example, the first threshold value may be set by multiplying a background noise reference value by a first predetermined value (claim 4). The second threshold value is set by the second threshold value setting device based on the first threshold value set by the first threshold value setting device.
For example, the second threshold may be set by adding a second predetermined value to the first threshold (claim 4).

By making the first threshold value and the second threshold value variable as described above, the first threshold value and the second threshold value that follow the fluctuation of the background noise level are set.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

Referring to FIG. 1, an automatic gain control device 10 according to an embodiment of the present invention includes an A / D converter 12 for digitizing an analog input signal, and the digitized input signal is sent to CPU 14. Is entered. The CPU 14
A flag 1 and a flag 2 are provided. The flag 1 indicates whether or not to perform gain control. The flag 2 indicates that the input signal level in_lev is equal to or greater than the threshold Thl1 and equal to or less than the threshold Thl2. in_l
whether ev has increased from a value smaller than the threshold value Thl1,
This indicates whether the value has decreased from a value greater than the threshold value Thl2. The CPU 14 has a ROM 16 and a RAM 1
8 is connected.

The ROM 16 stores a program for controlling the operation of the automatic gain controller 10 and data set in advance. The data stored in the ROM 16 includes the output signal level out_
Target level to be the target value to match lev, initial value of gain AGC_gain applied to input signal, gain increase per unit time when increasing gain G_up, gain per unit time when decreasing gain Decrease width G_down, threshold value Thl1, threshold value Thl1 for determining whether or not to perform gain control
A larger threshold value Thl2 and the like are included. The RAM 18 temporarily stores data such as calculated values obtained by the calculation in the CPU 14. The data stored in the RAM 18 includes a gain AGC_gain applied to an input signal, an input signal level in_lev, an output signal level out_lev, and the like.

The digitized output signal obtained by the CPU 14 is converted into an analog signal by the D / A converter 20 and output.

The automatic gain control device 1 thus configured
0 automatic gain control is performed by the CPU 14, the ROM 16, and the R
This is executed by the AM 18 and its functional block diagram is shown in FIG.
Shown in

Referring to FIGS. 2 and 3, the operation of the entire automatic gain control device 10 will be described.

First, each value is initialized (step S).
1). At this time, the gain AGC_gain applied to the input signal is set to the initial value, the input signal level in_lev, the output signal level
out_lev is cleared to zero and RAM1
8 is stored. Further, the flag 1 and the flag 2 in the CPU 14 are cleared.

Then, the input signal digitized by the A / D converter 12 is given to the CPU 14, the input signal level in_lev is calculated, and the calculated input signal level in_lev is temporarily stored in the RAM 18 ( Step S3). The input signal level in_lev is calculated by, for example, a sum of squares of 20 consecutive samples extracted at a sampling frequency of 8 KHz. Output signal level described later
The same applies to out_lev. Step S3 corresponds to the processing in the input signal level calculation unit 22.

Next, based on the input signal level in_lev stored in the RAM 18, the CPU 14
It is determined whether or not to perform gain control by the operation shown in (5) (step S5). When it is determined not to perform the gain control, the flag 1 in the CPU 14 is set to “0”,
When it is determined that the gain control is performed, the flag 1 is set to “1”. In step S5, the operation determining unit 24
Corresponds to the ON / OFF determination processing of the gain control unit 26 by

The CPU 14 determines whether or not to perform gain control with reference to the flag 1 (step S7). When performing gain control, the CPU 14 controls the gain so that the output signal level out_lev approaches the target level. AGC_gain is controlled (step S9). Step S7 corresponds to whether or not the gain control unit 26 is turned on.
9 corresponds to a gain control process by the gain control unit 26.

Then, the CPU 14 amplifies and outputs the input signal (step S11). If it is determined in step S7 that the gain control is to be performed, the adjusted gain
The input signal is amplified by AGC_gain. On the other hand, if it is determined in step S7 that the gain control is not to be performed, the process proceeds to step S11, where the gain AGC_gain determined immediately before the stop of the gain control is held as it is, and the gain AG
Amplified by C_gain. As a result, it is possible to prevent unnecessary gain change due to switching of gain control ON / OFF. Step S11 corresponds to an input signal amplification process performed by the amplification unit 28.

Then, the output signal level out_lev is calculated by the CPU 14 and temporarily stored in the RAM 18 (step S13). Step S13 corresponds to output signal level out_lev calculation processing by the output signal level calculation unit 30.

Thereafter, the flow returns to step S3, and the above operation is repeated.

Next, the operation in step S5 will be described in detail with reference to FIG.

First, the CPU 14 reads the input signal level in_lev stored in the RAM 18 (step S2).
1). Step S21 corresponds to the input of the input signal level in_lev from the input signal level calculation unit 22 to the operation determination unit 24.

Then, the input signal level in_lev <threshold Th
11 is determined (step S23). If input signal level in_lev <threshold Thl1, flag 2 =
It is set to "1" (step S25), it is determined not to perform gain control, flag 1 is set to "0" (step S27), and the process ends.

In step S23, the input signal level
If in_lev <threshold Thl1, input signal level in_l
It is determined whether or not ev> threshold value Thl2 (step S2).
9). If the input signal level in_lev> threshold Thl2,
The flag 2 is set to "0" (step S31), it is determined to perform the gain control, the flag 1 is set to "1" (step S33), and the process ends.

On the other hand, in step S29, if the input signal level is not in_lev> threshold Thl2, the threshold Thl1
Since ≦ input signal level in_lev ≦ threshold value Thl2, it is determined with reference to flag 2 whether input signal level in_lev has increased from a value smaller than threshold value Thl1 or decreased from a value larger than threshold value Thl2 (step S3).
5). If flag 2 = “0”, the input signal level in_l
Since ev indicates a state in which it has decreased from a value greater than the threshold value Thl2, it is determined not to perform gain control, and the flag 1
= "0" (step S27).

In step S35, flag 2 =
If “1”, the input signal level in_lev is equal to the threshold value Thl1
Since it indicates a state of increasing from a smaller value, it is determined to perform gain control, and flag 1 is set to "1" (step S33).

As described above, whether or not to perform gain control is determined with reference to the comparison between the input signal level in_lev and the thresholds Thl1 and Thl2 and the flag 2.

Further, the operation in step S9 will be described in detail with reference to FIG.

First, the CPU 14 reads the output signal level out_lev stored in the RAM 18 (step S4).
1). Step S41 corresponds to the input of the output signal level out_lev from the output signal level calculator 30 to the gain controller 26.

Then, it is determined whether or not the output signal level is out_lev <the target level (step S43). If output signal level out_lev <target level, output signal level o
Because ut_lev needs to be larger, AGC_gain =
AGC_gain set to G_up and gain applied to input signal AGC_
The gain is increased (step S45). Step S4
In step 3, if the output signal level is not out_lev <target level, it is determined whether the output signal level is out_lev> target level (step S47). Output signal level out_le
If v> the target level, the output signal level out_lev needs to be reduced, so that AGC_gain = AGC_gain-G_down is set, and the gain AGC_gain applied to the input signal is reduced (step S49). In step S47, unless the output signal level out_lev> the target level, the output signal level out_lev = the target level, so that the gain AGC_gain applied to the input signal is not changed.

The input signal is amplified by the gain AGC_gain set as described above.

By the above operation, the gain control section is set, for example, as shown in FIG. In the graph of FIG. 6, the input signal level in_lev is indicated by a solid line, and the input signal level in_lev is divided into a region 1, a region 2 and a region 3 based on the thresholds Thl1 and Thl2.

In region 1, no gain control is performed. In the area 2, the gain control is performed when the input signal level in_lev is higher than the area 1, and the gain control is not performed when the input signal level in_lev is lower than the area 3. In region 3, gain control is performed.

Therefore, sections a and b are gain control sections.

As described above, in the automatic gain control device 10,
Two types of threshold values Thl1 for the input signal level in_lev,
The threshold value Thl2 is set, and a hysteresis is provided between the threshold value Thl1 and the threshold value Thl2. That is, the threshold value Thl1 set lower and the threshold value Thl2 set higher
By using the threshold value Thl1 as a reference, the minimum level of gain control can be made smaller, and a larger maximum gain can be obtained. Also, the threshold value Thl
By using 2 as a reference, unnecessary gain control for the ending part of speech and background noise can be suppressed. The automatic gain control device 10 is particularly effective when performing automatic gain control on speech mixed with noise.

Next, an automatic gain control device 10a according to another embodiment of the present invention will be described.

The automatic gain control device 10a has the same configuration as the automatic gain control device 10 shown in FIG. In addition, ROM
In 16, the threshold value Thl1 is calculated from the initial value of the background noise reference value Thl_base, the correction width T_up of the background noise reference value Thl_base in the rising direction, the correction width T_down of the background noise reference value Thl_base in the falling direction, and the background noise reference value Thl_base. And a predetermined addition amount b for calculating the threshold value Thl2 from the threshold value Thl1 are further stored. The multiplier a and the addition amount b are respectively set according to the system. Also, the thresholds Thl1 and Thl2 will be stored in the RAM 18.

The automatic gain control by the automatic gain control device 10a is executed by the CPU 14, the ROM 16 and the RAM 18, and its functional block diagram is shown in FIG.

As can be seen by comparing FIG. 7 with FIG. 2, the automatic gain control device 10a further includes a threshold value calculation unit 32 and a switch 34. The threshold calculator 32 calculates the threshold value Thl
1 and threshold Thl2 are not fixed values but input signal levels
Calculate according to in_lev. The switch 34 outputs the input signal itself as an output signal when the input signal is only background noise.

Referring to FIG. 8, automatic gain control device 10a
The overall operation will be described.

The operation of the automatic gain control device 10a shown in FIG. 8 is the same as the operation of the automatic gain control device 10 shown in FIG. 3 except that step S4 is performed between steps S3 and S5, and steps S15 and S17 are performed after step S13. , S19 are simply added, and the duplicate description is omitted here.

After step S3, the CPU 14
A threshold value Thl1 and a threshold value Thl2 are calculated based on the input signal level in_lev (step S4). Step S
4 corresponds to the processing in the threshold value calculation unit 32. Then, in step S5, the calculated threshold value Thl1 and threshold value T
Based on a comparison between hl2 and the input signal level in_lev, whether or not to perform gain control is determined.

After step S13, the CPU 14 determines whether the input signal is only background noise (step S15). Here, the input signal level is set based on a threshold value Thl1 set by the operation shown in FIG.
When in_lev is small, it is determined that the input signal is only background noise. Step S15 corresponds to a process in the operation determining unit 24 for controlling the switching of the switch 34.

If it is determined in step S15 that the input signal is only background noise, the input signal itself is output as an output signal (step S17), and the process returns to step S3. In this way, for example, amplification of unnecessary noise in a silent state of only background noise is suppressed. On the other hand, if it is determined that the input signal is not only background noise, an output signal obtained by amplifying the input signal is output (step S19), and the process returns to step S3. Step S
Steps S17 and S19 correspond to the switching of the output signal accompanying the switching of the switch.

Next, the operation in step S4 will be described in detail with reference to FIG.

First, the CPU 14 reads the input signal level in_lev stored in the RAM 18 (step S5).
1). Step S51 corresponds to the input of the input signal level in_lev from the input signal level calculation unit 22 to the operation determination unit 24.

It is determined whether the background noise reference value Thl_base <the input signal level in_lev (step S53). If background noise reference value Thl_base <input signal level in_lev, T_base = Thl_base + T_up is set, and background noise reference value Thl_base is increased (step S55).

Then, after the threshold value Thl1 is set according to Thl1 = Thl_base × a (step S57), Thl2 = Thl_base × a.
The threshold value Thl2 is set by Thl1 + b (step S59), and the process ends.

On the other hand, if it is determined in step S53 that the background noise reference value Thl_base <the input signal level in_lev,
It is determined whether or not background noise reference value Thl_base> input signal level in_lev (step S61). If background noise reference value Thl_base> input signal level in_lev, Thl_
base = Thl_base−T_down is set, and the background noise reference value Thl_base is lowered (step S63), and step S5 is performed.
Go to 7. On the other hand, if the background noise reference value Thl_base is not greater than the input signal level in_lev in step S61, the process directly proceeds to step S57.

Note that the initial value of the background noise reference value Thl_base stored in the ROM 16 is set to the automatic gain control device 10a.
Is set slightly larger than the value considered to be the background noise level of the system to which is applied. In addition, T_
By setting up≪T_down, the background noise reference value
The change in Thl_base is such that the rise time constant is large and the fall time constant is small. As a result, the background noise reference value Thl_base can be held at a value close to the background noise level.

Thus, the background noise reference value Thl_ba
se, the threshold value Thl1, and the threshold value Thl2 can be variably set.

Further, by the operation shown in FIG. 9, the optimum threshold value Thl1 for the system can be calculated by a simple calculation as shown in FIG. 10, following the fluctuation of the background noise level, and the threshold value Thl2 is set accordingly. it can. Therefore, more natural gain control for voice can be performed, and unnecessary amplification of background noise can be prevented.

As shown in FIG. 11, the automatic gain control device 10 of the present invention can be applied to a hands-free telephone 40.

The hands-free telephone 40 includes a microphone 42, and a microphone input signal from the microphone 42 is supplied to the A / D converter 12 of the automatic gain control device 10 and digitized. The output signal obtained by the automatic gain control by the automatic gain control device 10 is converted into an analog signal by the D / A converter 20 and supplied to the hybrid circuit 44 as a line output signal. In the hybrid circuit 44 to which the telephone line is connected, two-line four-line conversion is performed, the line input signal is amplified by the amplifier 46, the speaker output signal is given to the speaker 48, and the sound is output.

As described above, if the automatic gain control device 10 is used for the hands-free telephone 40 in which the input signal level in_lev greatly changes due to the movement of the near-end speaker, unnecessary gain control due to background noise is suppressed, and the gain is increased. Since the maximum gain is obtained, the fluctuation of the output signal level due to the movement of the near-end speaker can be suppressed smaller.

As shown in FIG. 12, the automatic gain control device 10 of the present invention can be applied to a voice memo device 50.

The voice memo device 50 includes a microphone 52,
The microphone input signal from the microphone 52 is transmitted to the automatic gain control device 1
0 is supplied to the A / D converter 12 and digitized. Then, the output signal obtained by automatic gain control by the automatic gain control device 10 is encoded by the audio encoding device 54 and recorded in the storage device 56. At the time of reproduction, the encoded audio data stored in the storage device 56 is decoded by the audio decoding device 58, and the D / A converter 2
The signal is converted into an analog signal by 0, a speaker output signal is supplied to the speaker 60, and sound is output.

In the voice memo device 50 for recording a conference or recording a voice memo, the input signal level in_lev greatly changes. In particular, when recording in a large conference room, it is necessary to record an input signal of a smaller level. When the automatic gain control device 10 is used for the voice memo device 50, a larger maximum gain can be obtained, so that even at a lower level input signal, it is possible to record at an optimum volume.

Further, as shown in FIG. 13, the automatic gain control device 10 of the present invention can be applied to a digital hearing aid 70.

The digital hearing aid 70 includes a microphone 72, and a microphone input signal from the microphone 72 is provided to the A / D converter 12 of the automatic gain control device 10 and digitized. The output signal obtained by the automatic gain control performed by the automatic gain control device 10 is converted into an analog signal by the D / A converter 20, supplied to the speaker 74 as a speaker output signal, and outputs a sound.

The input signal level in_lev from the target sound source may become very small due to the movement of the hearing aid user or the like. When the automatic gain control device 10 is used for the digital hearing aid 70, a larger maximum gain can be obtained. Therefore, even when the input signal level in_lev from the target sound source is very small, the hearing aid user can listen at the optimum level. Can be.

The application examples shown in FIGS. 11 to 13 include:
The automatic gain control device 10a may be used, and in this case, the same effect as when the automatic gain control device 10 is used can be obtained.

In each of the above embodiments, the CP
A DSP may be used instead of U14.

The input signal level in_lev may be calculated by the sum of the absolute values of the samples of the input signal.
The output signal level out_lev may be calculated by the sum of the absolute values of the samples of the output signal.

[0074]

According to the present invention, it is possible to suppress unnecessary gain control for the ending part of speech and background noise, and to obtain a larger maximum gain. Therefore, a more natural sound can be output.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a functional block diagram showing a main part of the embodiment of FIG.

FIG. 3 is a flowchart showing an example of the operation of the embodiment shown in FIGS. 1 and 2;

FIG. 4 is a flowchart showing details of the operation of step S5 in FIG. 3;

FIG. 5 is a flowchart showing details of the operation of step S9 in FIG. 3;

FIG. 6 is a graph showing a gain control section.

FIG. 7 is a functional block diagram showing a main part of another embodiment of the present invention.

FIG. 8 is a flowchart showing an example of the operation of the embodiment shown in FIGS. 1 and 7;

FIG. 9 is a flowchart showing details of the operation of step S4 in FIG. 8;

FIG. 10 shows a threshold value Th calculated by the operation shown in FIG.
It is a graph which shows l1.

FIG. 11 is a block diagram showing a case where the automatic gain control device is applied to a hands-free telephone.

FIG. 12 is a block diagram showing a case where the automatic gain control device is applied to a voice memo device.

FIG. 13 is a block diagram showing a case where the automatic gain control device is applied to a digital hearing aid.

FIG. 14 is a functional block diagram showing a main part of a conventional technique.

[Explanation of symbols]

 1, 2 flag 10, 10a automatic gain controller 12 A / D converter 14 CPU 16 ROM 18 RAM 20 D / A converter in_lev input signal level out_lev output signal level Thl1, Thl2 threshold value a multiplier b addition amount

Claims (4)

[Claims] 1. An automatic gain control device for multiplying and amplifying an input signal to obtain an output signal, wherein the input signal level is calculated based on the input signal. Output signal level calculation means for calculating an output signal level, gain control means for controlling the gain based on the output signal level and a preset target level, a first threshold value and a second threshold value larger than the first threshold value A storage unit that is stored respectively; a first unit that stops gain control by the gain control unit when the input signal level is smaller than the first threshold; a gain unit that stops gain control when the input signal level is larger than the second threshold value Second means for performing gain control, and when the input signal level is greater than or equal to the first threshold and less than or equal to the second threshold, When the input signal level increases from a value smaller than the first threshold, the gain control means executes gain control. On the other hand, when the input signal level decreases from a value larger than the second threshold, the gain control An automatic gain control device comprising: third means for stopping gain control by the means. 2. The apparatus according to claim 1, further comprising a flag indicating whether the input signal level has increased from a value smaller than the first threshold or decreased from a value larger than the second threshold, wherein the third unit refers to the flag. 2. The automatic gain control device according to claim 1, further comprising means for determining whether to execute or stop gain control by said gain control means. 3. The apparatus according to claim 2, further comprising: a threshold setting unit configured to variably set the first threshold and the second threshold. Means for setting a noise reference value, first threshold setting means for setting the first threshold based on the set background noise reference value, and based on the first threshold set by the first threshold setting means The automatic gain control device according to claim 2, further comprising a second threshold value setting unit that sets the second threshold value. 4. The first threshold value setting means multiplies the background noise reference value by a first predetermined value.
The automatic gain control according to claim 3, further comprising: means for setting a threshold, wherein the second threshold setting means includes means for setting the second threshold by adding a second predetermined value to the first threshold. apparatus.