JPH0445006B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a level control circuit for performing gain adjustment such as volume adjustment in a circuit that processes an audio signal or the like into a digital signal. This prevents clipping. [Conventional technology] Conventionally, analog amplifiers and the like that process audio signals have been common, but with the advent of digital sources such as CDs (compact discs) in recent years, devices that process digital signals have appeared. I came to the conclusion. However, such digital signal processing circuits have had problems with level control such as volume adjustment because they are digital. FIG. 2 shows a level control method when analog signals are processed into digital signals. This involves inputting an analog input signal to a volume 10 whose gain is variable in the range of +G to -∞dB, converting the output into a digital signal with an A/D converter 12, and converting this into a digital signal with a digital signal processing section 14. Signal processing such as tone control was performed, and the processed signal was converted back to an analog signal by a D/A converter 16 and output. Here, gain +GdB gives the rated gain of the entire digital signal processing circuit, and in the case of an audio control amplifier, it is generally set to about 20 dB. The volume 10 is varied in conjunction with a volume adjustment operation means such as a master volume. In the level control method shown in Fig. 2, when the volume 10 is turned down to reduce the overall gain,
Digital processing system (A/D converter 12 to D/A
Since the signal is input at a low level to the system (up to the converter 16), the range (for example, 16 bits) of the digital processing system cannot be used effectively. Generally, the noise generated in a digital processing system (conversion error noise, quantization noise, etc.) is fixed at a value determined by the number of processing bits, and the smaller the signal level, the greater the proportion of noise included in the signal, and the S/ N gets worse. For this reason, the level control method shown in FIG. 2 has the disadvantage that when the volume 10 is turned down, the S/N becomes worse. Another conventional level control method is to place the volume 10 behind the D/A converter 16, as shown in FIG. According to this, even if the volume 10 is turned down, the digital signal does not become smaller, so there is no S/N deterioration due to the volume adjustment. However, when the input itself is at a small level, the digital signal remains small even if the gain of the volume 10 is increased, so that S/N deterioration occurs when the input level is small. Therefore, the level control method shown in FIG. 4 can be considered as a solution to the drawbacks of both FIGS. 2 and 3. In this, the volume 10 is divided into a front stage volume 10A whose gain is variable in the range of GdB to 0 dB, and a rear stage volume 10B whose gain is varied in the range of 0 dB to -∞dB. When setting the total gain of both volumes 10A and 10B to 0 dB or more, the rear volume 10B is set to 0 dB and the front volume 10A is controlled. Moreover, when setting the total gain to 0 dB or less, the front stage volume 10A is set to 0 dB and the rear stage volume 10B is controlled. According to this, even if the overall gain is reduced to 0 dB or less, the digital signal will not become smaller.
No deterioration of S/N occurs. Furthermore, for a small level input, if the overall gain is set to 0 dB or more, the digital signal becomes large, so no S/N deterioration occurs. [Problems to be Solved by the Invention] Generally, the digital filter, digital tone controller, digital equalizer, digital effector, digital reverberator, etc. that constitute the digital signal processing section 14 have a gain of + (that is, the digital digital signal processing that includes at least processing that amplifies the signal (that is, has +gain) is often performed. Therefore, in the level control method shown in FIG. 4, when the reference level is input, that is, when the gain of the volume 10A is 0 dB, the A/D converter 12 outputs a full bit (the value immediately before overflowing) at a level. When analog input is input,
Even if the gain of the volume 10A is narrowed down to 0dB,
Overflow occurs in the process of digital signal processing, resulting in clipping of the output waveform. Therefore, the allowable input level is reduced. For example, if the digital signal processing section 14 is configured with a digital graphic equalizer with a boost amount of +6 dB, if the graphic equalizer 14 is boosted during analog input of the reference level input, the graphic equalizer 14 will naturally overflow. , the output waveform will be clipped. Such inconveniences occur not only in the case of analog input but also in the case of digital input as shown in FIG. In other words, if the volume 10A' is configured with a digital volume so that the gain can be varied in the range of GdB to 0dB, the digital volume 1
When the gain of 0A' is narrowed down to 0dB, if there is a reference level digital input (full bit input), the digital signal processing section 14 will overflow and the output waveform will be clipped. Therefore, the allowable input level is reduced. The present invention solves these problems by preventing signal clipping due to overflow in the processing process when digital signal processing has positive gain (i.e. amplification), thereby reducing the allowable input level. This prevents [Means for Solving the Problems] The present invention provides a digital signal processing circuit having a digital signal processing section that performs digital signal processing including at least digital amplification processing on a controlled digital signal; A pre-stage volume having a gain of −gdB (a gain that does not overflow the digital signal processing unit when input is approximately at a reference level) and a variable gain disposed before the signal processing unit and a variable gain above the gain, and a pre-stage volume disposed after the digital processing unit. a rear-stage volume having a variable gain of approximately 0 dB or less, and a total gain that is the sum of the gain of the front-stage volume and the gain of the rear-stage volume between a rated gain greater than 0 dB and a maximum attenuation gain extremely smaller than 0 dB. When this gain adjustment operator is operated to instruct an overall gain of approximately -gdB or more, the gain of the latter stage volume is set to approximately 0 dB, and the gain of the latter stage volume is set to approximately 0 dB. Performs gain distribution to set the gain of the front volume so that the total gain of the gain of the front volume and the gain of the rear volume matches the total gain instructed by the gain adjustment controller, and controls the gain adjustment controller. is controlled to a total gain of approximately -gdB or less, the gain of the previous stage volume is set to approximately -gdB
At the same time, gain distribution is performed to set the gain of the latter stage volume so that the total gain of the gain of the first stage volume and the gain of the second stage volume matches the total gain instructed by the gain adjustment operator. The present invention is characterized by comprising a gain adjustment control means. [Operation] According to the present invention, if the total gain is set to -gdB when the input is approximately at the reference level, the pre-stage volume will have a gain of approximately -gdB, so even if there is a + gain in the digital signal processing section, it will be approximately equal to -gdB. The digital signal processing section no longer overflows, and a drop in the allowable input level is almost prevented. If the controlled digital input is A/D converted from the controlled analog input, the front volume is divided into the front side and the rear side of the A/D conversion means, and the front volume is set at approximately 0 dB and above. It has a variable gain of , and the rear volume of the front stage is almost 0 dB.
If the configuration is configured to have a variable gain of ~-gdB and the total gain is set to approximately 0 dB or more using both of these volumes and the rear volume, the gain of the rear volume of the front stage is set to approximately 0 dB, and the front volume of the front stage is controlled. When setting the gain from approximately 0 dB to -gdB, if you set the front volume of the front stage to approximately 0 dB or more and control the rear volume of the front stage, the entire range of -gdB or more can be achieved before A/D conversion, as described later. Compared to the control case, S/N deterioration in the A/D converter can be almost prevented. [Embodiment] FIG. 1 shows an embodiment in which the present invention is applied to an audio preamplifier. In the circuit shown in FIG. 1, the input/output can be either analog or digital. That is,
The following routes are respectively configured for analog input (controlled analog signal) and digital input (controlled digital signal). (1) Analog input route A This is the route in which an analog signal is input and an analog signal is output when digital signal processing is not performed, and consists of a volume 20 and a volume 24. Route B: When performing digital signal processing, this is a route in which an analog signal is input and an analog signal is output. →It consists of 24 volumes. Route C: A route in which an analog signal is input and a digital signal is output.Volume 20 → A/D converter 26 → Volume 30 → Digital signal processing section
→It consists of 36 volumes. (2) Digital input route D This route is input as a digital signal and output as an analog signal, from volume 38 to volume 30.
→Digital signal processing unit 32 →D/A converter 34
→It consists of 24 volumes. Route E: A route in which a digital signal is input and a digital signal is output.Volume 38 → Volume 30 → Digital signal processing section 32 → Volume 36
Consists of. The circuit shown in FIG. 1 constituting each of the routes A to E described above includes the following components. Volume 20 Analog input (controlled analog signal) to A/D
The gain is varied in the range of +G to 0 dB with an analog volume (pre-stage front volume that constitutes a pre-stage volume together with a volume 30 described later) that adjusts the gain before conversion. +G is analog input route A to C
These routes A to C with the maximum gain given by
gives a rated gain of The value of +G is set to about 20 dB in the case of an audio control amplifier. A/D converter 26 Converts the analog input into, for example, a 16-bit digital signal.The A/D converter 26 converts the analog input into, for example, a 16-bit digital signal.The A/D converter 26 converts the analog input into, for example, a 16-bit digital signal. /D conversion is performed. Volume 38 This digital volume adjusts the gain of a digital input (for example, a 16-bit controlled digital signal) (constituting a pre-stage volume together with volume 30, which will be described later), and the gain is variable in the range of +G to 0 dB. +G is the maximum gain given by the digital input routes D and E, and provides the rated gain of these routes D and E (for example, as mentioned above, in the case of an audio preamplifier, about +20 dB). Signal selection circuit 28 Selects and outputs either analog input or digital input, outputs analog input when only analog input is input, outputs digital input when only digital input is input, When both analog and digital inputs are input (for example, when both analog and digital signals are input from one CD player), the digital input, which is a high-quality input, is selected and output. Volume 30 When the digital signal processing section 32 has a gain, when the gain of the volumes 20 and 38 is 0 dB, the digital signal processing section 32 attenuates the input level to an attenuation amount -gdB that does not overflow with reference level input and performs the same processing. This is a digital volume for adjusting the head margin of the section 32, and the gain is variable in the range of 0 to -gdB. In the case of analog input, this volume 30 is
It becomes a front-stage rear volume that forms a front-stage volume together with the volume 20, and in the case of digital input, forms a front-stage volume together with the volume 38. Digital signal processing unit 32 A circuit that digitally processes input signals, and is composed of a digital filter, digital tone controller, digital graphic equalizer, digital effector, digital reverberator, etc. Note that the digital signal processing circuit in the claims includes the digital signal processing section 32,
It is composed of an A/D converter 26, a D/A converter 34, and the like. Patent application No. 61-230139 for digital signal processing section 32
A case in which the digital graphic equalizer described in the specification of the present invention is used will be explained. This digital graphic equalizer is composed of a three-band parametric equalizer with band-pass characteristics, and as shown in Figure 6, the center frequencies _L , _M , _H , level, and Q can be set. It is also possible to set the cutoff frequency _C and slope for low cut and high cut. The center frequencies _L , _M , and _H of each band can be set in 1/6 oct steps within the following ranges. _L : 20~500Hz _M : 100~5KHz _H : 1k~20kHz However, it is set under the condition of _L < _M < _H. The level is -6 to + in 0.1dB steps for each band.
Set in a range of 6dB. Q is set to 0.7, 1.0, 1.4, 2.0, or 3.0 for each band. The low-cut and high-cut frequencies _C are set in the ranges of 20 to 200 Hz and 5 k to 18 k, respectively, and the slope thereof is set to 12, 18, or 24 dB/oct. FIG. 7 shows an enlarged view of the parameter setting section 40 for setting parameters of this digital equalizer. This parameter setting section 40 has an operation section 41 at the bottom and a display section 42 at the top. The display section 42 displays the set values of each of the above parameters, and is composed of an LCD with a backlight. Of the parameters of the three divided bands, this display 42 displays the center frequencies _L , _M , and _H graphically on a scale, and displays the level and Q numerically. Since it is necessary to perform a huge amount of calculations to completely display the comprehensive frequency characteristics graphically, this is simply displayed, and the above configuration is adopted in consideration of the following () to). Since the center frequency is divided into three bands, it is preferable to display it graphically on a scale in order to know the mutual positional relationship. Levels have long been familiar even when expressed numerically, and are easy to recognize and understand. Q is generally difficult to understand, but if it can be recognized, it is sufficient for the purpose of reproducing characteristics. With this configuration of the display section 42, it is easy to
Easy-to-use and easy-to-understand frequency control characteristics can be displayed. In the display section 42, the upper frequency scale 4
4 displays the set values of the center frequencies _L , _M , and _H in each band, and the position of the set frequencies _L , _M , and _H among the ▼ marks 44a arranged in a row above the frequency scale 44. will be displayed in three places. The left part 45 of the numerical display shows the low-cut frequency in the upper row and its slope (12, 18, 18,
24dB/oct) are displayed respectively.
In the center section 46, the level is displayed in the upper row, and the Q is displayed in the lower row for the low range, middle range, and high range from the left. The right part 48 has a high cut frequency on the top and a slope on the bottom (either 12, 18, or 24 dB/octave).
are displayed respectively. The operation unit 41 includes the following various keys each consisting of a tact switch for setting parameters. (a) Equalizer on/off key 50 This is a key for turning on/off the function of the digital equalizer. When this key 50 is turned off, the characteristics of the digital equalizer become flat. At this time, if analog input is selected in the circuit of FIG. 1, route A is selected for analog input-analog output. Note that even if the switch is turned off, the parameter value before the switch is turned off is retained in the memory. When the equalizer on/off key 50 is turned on (it is also turned on by operating other equalizer related keys 52, 54, etc.), the state returns to the state before each parameter was turned off. (b) Frequency key 52, Q/slope key 54 These keys are used to select the setting mode, and are pressed to set the center frequency, Q, or slope, respectively. If neither is pressed, the mode will be set to level setting mode. (c) UP/DOWN key 56 This key is used to increase/decrease each set value.Press the right side to increase it, and press the left side to decrease it. (d) Low cut key 58 This key is used to set the low cut characteristics. After pressing this key 58, press the frequency key 52 to select
By operating the down key 56, the low cut frequency is set. Further, by pressing the low cut key 58, pressing the Q/slope key 54, and operating the up/down key 56, the slope of the low cut is set. The low cut key 58 is of a toggle type and turns on/off each time it is pressed. When it is off, the slope of the low cut is flat and the display on the display section 45 also disappears. Further, the operation of the up/down key 56 is no longer accepted. When turned on from off, the previous setting value is displayed. (e) Low key 60, Mid key 62, High key 6
4 Keys used to set the characteristics of the three divided bands, corresponding to the low range, mid range, and high range, respectively. By pressing the low key 60 and operating the up/down key 56, the low frequency level is set.
After pressing the low key 60, pressing the frequency key 52 and operating the up/down key 56 sets the center frequency _L of the low range. Further, by pressing the low key 60, pressing the Q/slope key 54, and operating the up/down key 56, the low-frequency Q is set. For midrange and high range, Midkey 62, 64
You can set each parameter by pressing and performing the same operation. (f) High-cut key 66 This is a key for setting high-cut characteristics, and the high-cut frequency and slope are set by the same operation as the low-cut key 58. Also, if you turn it off, the slope of the high cut will be flat, and the slope of the high cut will be flat.
The operation of the down key 56 is no longer accepted, and the display on the display section 48 also disappears. (The setting value before turning off is retained in memory.) When the digital graphic equalizer explained above in FIGS. 6 and 7 is used in the digital signal processing section 32 in FIG. 1, its maximum gain is +6 dB.
Therefore, by setting the minimum gain -g of the volume 30 to -6 dB, overflow can be prevented no matter how the equalizer level parameters are set. Note that although the above example shows the case where the value of -g is fixed, it can also be automatically varied according to the value of the parameter of the level to be set. In that case, cancel the gain where the peak is highest (i.e., if the gain is +5dB, -g=-
(If the gain is +4dB, -g = -4dB) is automatically set. (g) Volume 36 This is a digital volume (second stage volume) that adjusts the gain of the digital output, and the gain is variable in the range of 0 to -∞dB. (h) D/A converter 34 This converts the output of the digital signal processing section 32 into an analog signal, and is composed of, for example, 16 bits. Since the D/A converter 34 has an error at the lower end, if a floating (floating point) type is used, S/N deterioration here can be prevented. (i) Signal selection circuit 22 This circuit switches the analog output route to route A or route B or D. When digital signal processing is not performed between analog input and analog output (when digital signal processing section 32 is off), route A is switched. Otherwise, select routes B and D. (j) Volume 24 This is an analog volume (second stage volume) that adjusts the gain of analog output, and the gain is variable in the range of 0 to -∞dB. Incidentally, when the D/A converter 34 is of a floating type, a gain can be applied here to cancel out the amount of floating. (k) Microcomputer 39 This is gain adjustment control means for adjusting the gain of each volume 20, 38, 30, 24, and 36. For gain adjustment, in the case of an audio preamplifier, the total gain to be set is determined according to the operation of the volume adjustment operation means (gain adjustment control) such as the master volume 37, left/right volume balance, muting, etc., and this total gain is achieved. Each volume should be 20, 38, 30,
24 and 26 gain distributions are obtained and the gains are adjusted respectively. The amount of gain adjustment of each volume 20, 38, 30, 24, 26 by this microcomputer 39 is as follows, as shown in FIG. (1) Total gain +G to 0 dB The gains of the volumes 30, 24 and 36 are set to 0 dB, and the gains of the volumes 20 and 38 are variably controlled. When inputting a small level below the reference level,
Since the signal is amplified and input to the digital system, the digital system can effectively use the range, and S/N deterioration is prevented. (2) Overall gain 0 to -gdB The gains of volume 20, 38 and 24, 36 are
It is set to 0 dB, and the gain of the volume 30 is controlled. At this time, a head margin is formed in the digital signal processing section 32 according to the aperture amount of the volume 30, so that the level - gdB lower than the reference level ~
Clipping of the signal waveform due to overflow of the digital signal processing section 14 can be prevented even for inputs in the range of the reference level, and a decrease in the allowable input can be prevented. (3) Total gain -g ~ -∞dB (maximum attenuation gain) Gain of volume 20 and 38 is 0dB, volume 3
The gain of 0 is set to -gdB, and the gains of the volumes 24 and 36 are controlled. This narrowing down of the total gain is done later than in the digital system, so the range can be used effectively in the digital system.
S/N deterioration due to narrowing down is prevented. Also,
Since a head margin of -gdB is formed in the digital signal processing section 32, it is possible to input the signal up to the reference level without clipping, and a drop in the allowable input is prevented. When performing the control shown in FIG. 8 explained above, the first
The variable range of each volume 20, 38, 30, 24, 36 by operating the master volume 37 in the figure is 9th.
The result will be as shown in the figure. However, this is only when the gain of other volume adjustment control means (left/right balance volume, mute switch, etc.) is 0 dB, and if these are operated, the gain distribution handled by the master volume 37 will change, so each volume 20, 38 ,3
The variable gain range of 0, 24, and 36 also changes accordingly. As mentioned above, the value of -gdB is not fixed but varies according to the gain of the digital signal processing section 32 (the smaller the gain, the smaller the value of -g, and when the gain is 0 dB or less or when the circuit 32 is off). Time-
Let g=0. ) can also be done. In addition, when fixing the value of -g to a multiple of -6 dB, -12 dB, etc., 1 bit of the digital signal corresponds to 6 dB, so the volume 30 is configured to simply shift the input bit (when it is -6 dB, it is shifted down by 1 bit). , -12dB, 2-bit shift down). By the way, even if the volume 30 is not provided independently but is integrated with the volume 20, 38 (the variable gain range of the volume 20, 38 is +G to -gdB), the digital signal processing unit 3
It is possible to form a head margin of 2, but
If provided independently, it is possible to prevent S/N deterioration in the A/D converter 26 in the case of analog input. That is, when integrated with the volume 20, if the overall gain is set to -gdB, a -gdB attenuated signal is input to the A/D converter 26, so the range cannot be used effectively. That much S/N
deteriorates. On the other hand, the volume 30 is set to A/D
When installed independently after the converter 26, the A/D converter 26 can be used effectively up to the full range.
No S/N deterioration occurs. Furthermore, when the volume 30 is turned down, the digital signal is attenuated, so theoretically, some of the source information is lost (that is, the quantization error increases), the signal quality deteriorates, and the gain of the volume 30 decreases. The S/N will deteriorate compared to the case of 0 dB. However, in reality, there is a conversion error in the digital signal input to the volume 30, so the attenuation amount - gdB
is within a certain range, substantially no S/N deterioration occurs. In other words, for example, a 16-bit digital system should theoretically have a dynamic range of about 97 dB, but current consumer A/D converters cannot.
Only about 85dB of dynamic range was obtained. 84dB means 14 bits of accuracy.
This means that the lower two bits of the 16 bits of the A/D conversion output are included in the conversion error. The same is true for 16-bit signals on the supply source side, such as CDs, which have an accuracy of only about 15 bits even when the improvement in accuracy due to emphasis is taken into account. Therefore, even if the signal is attenuated within this error range, the S/N ratio does not deteriorate. In other words, the attenuation amount -gdB is -gdB≦D-D _AN , where D: The theoretical limit dynamic range corresponding to the number of digital bits (for example, in the case of 16 bits)
96dB) D _AN : Within the actual dynamic range that is reduced by noise generated in A/D conversion, the head margin of the digital signal processing section 32 can be increased without substantially losing any information transmission amount of the original signal. can be ensured. For example, 16
If the bit digital system actually has an accuracy of only 15 bits, the amount of attenuation -gdB will not lose any information transmission amount of the original signal up to 1 bit, that is, -6 dB. [Modifications] In the embodiments described above, the present invention was applied to an audio preamplifier, but the present invention can also be applied to digital signal processing other than audio signals. [Effects of the Invention] As explained above, according to the present invention, the signal can be attenuated to approximately -gdB in front of the digital signal processing section, so that a positive gain (i.e., amplification) is applied to the digital signal processing section. Even if there is a problem, the signal can be input without clipping to almost the reference level input level, and a drop in the allowable input level can be almost prevented. Moreover, even if the indicated value of the gain adjustment operator is narrowed down to -gdB or less, the gain in the front stage of the digital signal processing section is fixed at approximately -gdB, and variable attenuation is performed in the latter stage, so the digital signal processing section The input signal level does not decrease and S/N ratio does not deteriorate. Also, when inputting a small level, if the indicated value of the gain adjustment knob is set to -gdB or more, the gain of the downstream side of the digital signal processing section will be approximately
Since it is fixed at 0 dB and the gain on the front stage side is increased, the input signal level of the digital signal processing section is increased and the S/N ratio is improved. Also, when inputting an analog signal after A/D conversion, obtain a gain of approximately 0 dB or more before A/D conversion, and obtain approximately 0 dB to -gdB after A/D conversion. Since the input to the A/D conversion means is substantially not attenuated, the range of the A/D conversion means can be effectively utilized, and S/N deterioration due to A/D conversion can be almost prevented. Furthermore, if -gdB is approximately within the conversion error range of the A/D conversion means, S/N deterioration due to attenuation can be almost prevented.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment in which the present invention is applied to an audio preamplifier. FIGS. 2 and 3 are block diagrams showing conventional circuits, respectively. Figures 4 and 5 are
3A and 3B are block diagrams respectively showing level control methods that solve the drawbacks of FIGS. 2 and 3; FIG. 6th
The figure shows an example of a characteristic diagram when the digital signal processing section 12 of FIG. 1 is configured with a digital graphic equalizer. FIG. 7 is a diagram showing an example of the display section and operation section. Figure 8 shows
FIG. 2 is a diagram showing gain control by each volume in FIG. 1; Fig. 9 shows the first control when performing the control shown in Fig. 8.
3 is a diagram showing an example of a variable range of each volume by operating the master volume 37 shown in the figure. FIG. 20, 30, 38... Front stage volume (20...
Front stage front volume, 30... front stage rear volume),
24, 36...Late stage volume, 22, 28...Signal selection circuit, 32...Digital signal processing section, 2
6...A/D converter (A/D conversion means), 37...
...Master volume (gain adjustment control), 39...
Microcomputer (gain adjustment control means).

Claims (1)

[Scope of Claims] 1. A digital signal processing circuit having a digital signal processing section that performs digital signal processing including at least digital amplification processing on a controlled digital signal, and arranged before the digital signal processing section. -gdB (a gain that does not cause the digital signal processing section to overflow when the input is approximately at the reference level)
a pre-stage volume having a variable gain of approximately 0 dB or more, and a post-stage volume disposed after the digital processing section having a variable gain of approximately 0 dB or less; a gain adjustment operator that directs the overall gain to an arbitrary value between the rated gain that is greater than 0 dB and the maximum attenuation gain that is extremely smaller than 0 dB; If the gain of the latter stage volume is set to approximately 0 dB, the total gain that is the sum of the gain of the first stage volume and the gain of the second stage volume matches the total gain instructed by the gain adjustment operator. When the gain adjustment operator is operated to set the gain of the pre-stage volume to approximately -gdB or less, the gain of the pre-stage volume is set to approximately -gdB, and , gain adjustment control means that performs gain distribution to set the gain of the latter stage volume so that a total gain of the gain of the first stage volume and the gain of the second stage volume matches the total gain instructed by the gain adjustment operator; A level control circuit characterized by comprising: 2 The digital signal processing circuit has an A/
A/D conversion means is provided, and the controlled digital signal converts the controlled analog signal into an A/D conversion means.
The pre-stage volume has a variable gain of approximately 0 dB or more and is disposed before the A/D conversion means, and the A/D converter is a D-converted one.
The gain placed after the D conversion means is approximately 0 dB ~
a front-stage rear volume having a variable gain of −gdB; is set to 0 dB, and the gain of the front volume is set so that the total gain of the gain of the front volume and the gain of the rear volume matches the total gain instructed by the gain adjustment operator. allocation, and the gain adjustment operator is approximately 0~
-gdB, the gain of the front volume is set to approximately 0 dB, and the total gain, which is the sum of the gain of the front volume and the gain of the rear volume, is set by the gain adjustment operator. The level control circuit according to claim 1A, characterized in that the level control circuit performs gain distribution to set the gain of the front-stage rear volume so as to match the total gain instructed by .