JP3519935B2 - Automatic gain control device - Google Patents

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 device, and more particularly to an automatic gain control device for voice used for adjusting a voice level in a telephone system, voice memo, etc.

[0002]

2. Description of the Related Art Conventionally, in a telephone system, a system for handling a voice signal such as a voice memo, an automatic gain control device for a voice which automatically amplifies a voice having a small input level to an optimum level is used. FIG. 14 is a functional block diagram showing an example of a main part of the conventional automatic gain control device 1. In the automatic gain control device 1, the input signal level calculation unit 2
The input signal level is calculated by the operation determination unit 3, the operation determination unit 3 compares the input signal level with a predetermined threshold value, and the gain control unit 4 is turned on / off according to the comparison result. When the input signal level is higher than the threshold value, the gain control unit 4 outputs O
The gain control section 4 controls the gain of the amplification section 6 so that the output signal level calculated by the output signal level calculation section 5 becomes constant. On the other hand, if the input signal level is less than or equal to the threshold value, the gain control unit 4 is turned off and the gain control is stopped to prevent unnecessary amplification of background noise in the silent section.

[0003]

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

On the other hand, in order to prevent malfunction due to noise and unnecessary gain increase at the end of the word, 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 value is made variable so that the threshold value follows the background noise level, it is possible to prevent a malfunction due to the background noise, but it is not possible to prevent an unnecessary increase in the gain for the ending portion.

Therefore, a main object of the present invention is to provide an automatic gain control device capable of suppressing unnecessary gain control for the end portion of voice and background noise and obtaining a larger maximum gain.

[0007]

In order to achieve the above object, an automatic gain control apparatus according to claim 1 is an automatic gain control apparatus which applies an gain to an input signal to amplify the input signal to obtain an output signal. Input signal level calculating means for calculating the input signal level based on the input signal, output signal level calculating means for calculating the output signal level based on the output signal, and gain based on the output signal level and a preset target level. Gain control means for controlling, storage means for 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 if the input signal level is smaller than the first threshold value, input Second means for performing gain control by the gain control means if the signal level is higher than the second threshold, and if the input signal level is 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, the gain control means executes the gain control, while when the input signal level decreases from a value larger than the second threshold value, the gain control means performs it. A third means for stopping the gain control is provided.

The automatic gain control apparatus according to a second aspect is the automatic gain control apparatus according to the first aspect, in which the input signal level has increased from a value smaller than the first threshold value or decreased from a value greater than the second threshold value. The third means includes means for determining whether to execute or stop the gain control by the gain control means by referring to the flag.

An automatic gain control apparatus according to a third aspect is the automatic gain control apparatus according to the second aspect, further comprising threshold setting means for variably setting the first threshold value and the second threshold value, and the threshold setting means. Means for setting a background noise reference value in accordance with the result of comparison between the input signal level and the background noise reference value, first threshold value setting means for setting a first threshold value based on the set background noise reference value, and first threshold value The second threshold value setting means for setting the second threshold value based on the first threshold value set by the setting means is included.

An automatic gain control device according to a fourth aspect is the automatic gain control device according to the third aspect, wherein the first threshold value setting means multiplies 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 the preset target level. If the input signal level calculated by the input signal level calculating means is smaller than the first threshold value, the gain control is stopped and the input signal level is set to the second value.
If it is larger than the threshold value, the gain control is executed. When the input signal level increases from a value smaller than the first threshold value and becomes equal to or higher than the first threshold value and equal to or lower than the second threshold value, the gain control is executed, while the input signal level decreases from the value higher than the second threshold value. When it is equal to or more than the first threshold and equal to or less than the second threshold, the gain control is stopped. During gain control, the gain determined by the gain control means is applied to the input signal and amplified to obtain the output signal.

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

In this way, two kinds of thresholds are set for the input signal level, and hysteresis is provided between these thresholds. That is, the first threshold value set to a lower value and the second threshold value set to a higher value are prepared. By using the first threshold value 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 with respect to the ending portion and background noise can be suppressed.

The first threshold and the second threshold may be variable.

In the automatic gain control device according to the third aspect,
In the threshold setting means, first, the background noise reference value is set according to the result of comparison 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 background noise reference value that has been set.
For example, the first threshold value may be set by multiplying the background noise reference value by the first predetermined value (claim 4). The second threshold value setting unit sets the second threshold value based on the first threshold value set by the first threshold value setting unit.
For example, the second threshold value may be set by adding the second predetermined value to the first threshold value (claim 4).

By making the first threshold value and the second threshold value variable in this way, the first threshold value and the second threshold value that follow the fluctuation of the background noise level are set.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION 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. CPU14
The flag 1 and the flag 2 are provided, and the flag 1 indicates whether or not the gain control is executed. 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 Thl1,
It is indicated whether the value has decreased from a value larger than the threshold Thl2. Further, the CPU 14 has a ROM 16 and a RAM 1
8 are connected.

The ROM 16 stores a program for controlling the operation of the automatic gain control device 10 and preset data. The data stored in the ROM 16 is output signal level out_ by the automatic gain control device 10.
Target level that is the target value for adjusting lev, initial value of gain AGC_gain applied to the input signal, gain increase width G_up per unit time when gain is increased, gain per unit time when gain is decreased Reduction amount G_down, threshold value Thl1 and threshold value Thl1 for determining whether to perform gain control
Larger threshold values Thl2 and the like are included. The RAM 18 temporarily stores data such as a calculated value obtained by the calculation by the CPU 14. The data stored in the RAM 18 includes the gain AGC_gain applied to the input signal, the input signal level in_lev, the 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 configured as described above
Automatic gain control of 0, CPU14, ROM16 and R
2 is a functional block diagram executed by the AM18.
Shown in.

The operation of the entire automatic gain control device 10 will be described with reference to FIGS. 2 and 3.

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, and the input signal level in_lev and the output signal level
out_lev is cleared to zero, 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 thereof 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, for example, by the 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, the CPU 14 will be described later with reference to FIG. 4 based on the input signal level in_lev stored in the RAM 18.
It is determined whether or not to perform the gain control by the operation shown in (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 decided to perform the gain control, the flag 1 is set to "1". In step S5, the action determining unit 24
Corresponds to the ON / OFF determination process of the gain control unit 26.

Whether or not the gain control is performed is determined by the CPU 14 with reference to the flag 1 (step S7). When the gain control is performed, the gain is controlled by the CPU 14 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 gain control is performed, the adjusted gain is adjusted.
The input signal is amplified by AGC_gain. On the other hand, when it is determined that the gain control is not performed in step S7, the process proceeds to step S11, and the gain AGC_gain determined immediately before the gain control is stopped is held as it is.
It is amplified by C_gain. As a result, it is possible to prevent unnecessary gain change due to ON / OFF switching of gain control. Step S11 corresponds to the amplification processing of the input signal by the amplification unit 28.

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

Then, the process returns to step S3, and the above-described operation is repeated.

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.

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

In step S23, the input signal level
in_lev <threshold value Thl1, unless the input signal level in_l
It is determined whether or not ev> threshold 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 gain control, the flag 1 is set to "1" (step S33), and the process is ended.

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

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

As described above, by comparing the input signal level in_lev with the threshold values Thl1 and Thl2 and referring to the flag 2, it is determined whether or not the gain control is performed.

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 calculation unit 30 to the gain control unit 26.

Then, it is judged whether or not the output signal level out_lev <target level (step S43). If the output signal level out_lev <target level, the output signal level o
Since it is necessary to increase ut_lev, AGC_gain ＝
AGC_gain + G_up, which is the gain applied to the input signal AGC_
The gain is increased (step S45). Step S4
In 3, if the output signal level out_lev is not the target level, it is determined whether or not the output signal level out_lev> the target level (step S47). Output signal level out_le
If v> the target level, it is necessary to reduce the output signal level out_lev, 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, if the output signal level out_lev> target level is not satisfied, the output signal level out_lev = target level, so the gain AGC_gain applied to the input signal is not changed.

The input signal is amplified by the gain AGC_gain thus set.

By the above operation, the gain control section is set as shown in FIG. 6, for example. In the graph of FIG. 6, the input signal level in_lev is shown 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 threshold value Thl1 and the threshold value Thl2.

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

Therefore, the sections a and b are gain control sections.

As described above, in the automatic gain control device 10,
Two kinds of threshold values Thl1 for the input signal level in_lev,
The threshold value Thl2 is set so that the threshold value Thl1 and the threshold value Thl2 have hysteresis. That is, the threshold value Thl1 set lower and the threshold value Thl2 set higher.
By preparing and and 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 setting 2 as a reference, it is possible to suppress unnecessary gain control for the ending portion of voice and background noise. The automatic gain control device 10 is particularly effective when performing automatic gain control on a voice containing 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 structure 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 upward correction width T_up of the background noise reference value Thl_base, the downward correction width T_down of the background noise reference value Thl_base, and the background noise reference value Thl_base. Further, a predetermined multiplier a for the above and a predetermined addition amount b for calculating the threshold Thl2 from the threshold Thl1 are further stored. The multiplier a and the addition amount b are set according to the system. Further, the threshold value Thl1 and the threshold value 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 section 32 and a switch 34. The threshold calculation unit 32 uses the threshold Thl.
1 and the threshold value Thl2 are not fixed values but the input signal level
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, an automatic gain control device 10a.
The overall operation will be described.

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

After step S3, the CPU 14
The threshold value Thl1 and the 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 calculation unit 32. Then, in step S5, the calculated threshold value Thl1 and threshold value T1 are calculated.
Whether or not to perform gain control is determined based on the comparison between hl2 and the input signal level in_lev.

After step S13, the CPU 14 determines whether the input signal is only background noise (step S15). Here, the input signal level from the threshold 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 the 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 the output signal (step S17), and the process returns to step S3. In this way, for example, amplification of unnecessary noise in the 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, the output signal obtained by amplifying the input signal is output (step S19), and the process returns to step S3. Step S
17 and S19 correspond to the switching of the output signal accompanying the switching of the switch 34.

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 or not the background noise reference value Thl_base <input signal level in_lev (step S53). If the background noise reference value Thl_base <input signal level in_lev, T_base = Thl_base + T_up is set, and the background noise reference value Thl_base is increased (step S55).

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

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

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.
Set a little larger than the value that is considered to be the background noise level of the system to which is applied. Furthermore, T_
By setting up << T_down, the background noise reference value
The change of Thl_base has a characteristic that the rising time constant is large and the falling 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.

In this way, 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 threshold value Thl1 most suitable for the system can be calculated by a simple calculation as shown in FIG. 10 by following the fluctuation of the background noise level, and the threshold value Thl2 corresponding thereto is set. it can. Therefore, it is possible to perform more natural gain control for voice and prevent unnecessary background noise from being amplified.

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

The hands-free telephone 40 includes a microphone 42, and a microphone input signal from the microphone 42 is given to the A / D converter 12 of the automatic gain control device 10 and digitized. Then, the output signal obtained by the automatic gain control by the automatic gain control device 10 is analogized by the D / A converter 20 and is given to the hybrid circuit 44 as a line output signal. In the hybrid circuit 44 to which the telephone line is connected, 2-line to 4-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 voice 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 changes greatly due to the movement of the near-end speaker or the like, unnecessary gain control due to background noise can be suppressed and a larger value can be obtained. Since the maximum gain is obtained, the fluctuation of the output signal level due to the movement of the near-end speaker can be further suppressed.

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

The voice memo device 50 includes a microphone 52,
The microphone input signal from the microphone 52 is the automatic gain control device 1.
0 is supplied to the A / D converter 12 and digitized. Then, the output signal obtained by the automatic gain control by the automatic gain control device 10 is encoded by the voice 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
It is analogized by 0, the speaker output signal is given to the speaker 60, and the voice is output.

In the voice memo device 50 for performing conference recording and voice memo recording, the input signal level in_lev changes greatly. Especially, 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 an input signal of a smaller level can be recorded with an optimum volume.

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

The digital hearing aid 70 includes a microphone 72, and a microphone input signal from the microphone 72 is given to the A / D converter 12 of the automatic gain control device 10 and digitized. Then, 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 is given to the speaker 74 as a speaker output signal to output a sound.

The input signal level in_lev from the target sound source may become extremely small due to the movement of the hearing aid user. When the automatic gain control device 10 is used for the digital hearing aid 70, a larger maximum gain is obtained, so that even if the input signal level in_lev from the target sound source is very small, the hearing aid user should listen at the optimum level. You can

The application examples shown in FIGS. 11 to 13 are as follows.
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, and similarly,
The output signal level out_lev may be calculated by the sum of absolute values of samples of the output signal.

[0074]

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

[Brief description of 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 shown in 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 of FIG.

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

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.

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

10 is a threshold 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 and 2 flags 10, 10a Automatic gain control device 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 a multiplier b Addition amount

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Claims (4)

(57) [Claims] 1. An automatic gain control device for obtaining an output signal by applying a gain to an input signal to amplify the input signal, the input signal level calculating means calculating an input signal level based on the input signal, and based on the output 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. Storage means respectively stored therein; first means for stopping the gain control by the gain control means if the input signal level is smaller than the first threshold value; and by the gain control means if the input signal level is larger than the second threshold value. A second means for performing gain control, and a case where the input signal level is equal to or higher than the first threshold and equal to or lower than the second threshold, When the input signal level increases from a value smaller than the first threshold value, the gain control means executes the gain control, while when the input signal level decreases from a value larger than the second threshold value, the gain control. An automatic gain control device comprising third means for stopping the gain control by the means. 2. A flag indicating whether the input signal level has increased from a value smaller than the first threshold value or decreased from a value larger than the second threshold value, and the third means refers to the flag. 2. The automatic gain control device according to claim 1, further comprising means for determining whether to perform or stop the gain control by the gain control means. 3. Further comprising threshold setting means for variably setting said first threshold and said second threshold, said threshold setting means said said background according to the result of comparison between said input signal level and background noise reference value. Based on the means for setting a noise reference value, the first threshold value setting means for setting the first threshold value based on the set background noise reference value, and the first threshold value set by the first threshold value 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 is configured to multiply 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 value, wherein the second threshold value setting means includes means for setting the second threshold value by adding a second predetermined value to the first threshold value. apparatus.