Patent Publication Number: US-2022225025-A1

Title: Signal processing apparatus and signal processing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2021-004199 filed on Jan. 14, 2021, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a signal processing apparatus and a signal processing method suitable for an amplifier, a mixer, and the like. 
     BACKGROUND ART 
     In a signal processing apparatus such as an amplifier capable of performing gain switching, stepwise noise is generated in an output signal at the time of gain switching. In a technique disclosed in WO2019/069439, in order to reduce such stepwise noise, at the time of switching a gain of an amplifier, a total gain value is gradually changed over a predetermined time by correcting the gain. 
     Among amplifiers, there is, for example, a combination of an amplifier using an analog circuit and an amplifier using a digital circuit. Here, the amplifier using the analog circuit has a configuration in which gain switching is performed by a switch, and in this type of amplifier, impulse-like noise is generated in an output signal at the time of gain switching. The technique disclosed in WO2019/069439 has a problem in that such impulse-like noise cannot be removed and the impulse-like noise is generated in a final output signal. 
     SUMMARY OF INVENTION 
     The present disclosure is made in view of the above circumstances, and an object of the present disclosure is to provide a signal processing device and a signal processing method capable of reducing an influence of noise generated in association with gain switching in an output signal. 
     Aspect of non-limiting embodiments of the present disclosure relates to provide a signal processing apparatus including: a first signal processing circuit configured to generate a first sound signal based on an input signal, and a second signal processing circuit configured to generate a second sound signal based on the first sound signal. In a case that a gain of the first signal processing circuit is switched, the second signal processing circuit generates, for a predetermined time after the gain of the first signal processing circuit is switched, a value of the second sound signal based on a value of the first sound signal at or before a timing of the gain of the first signal processing circuit being switched. 
     Another aspect of non-limiting embodiments of the present disclosure relates to provide a signal processing method including: generating a first sound signal based on an input signal, generating a second sound signal based on the first sound signal, and in a case that a gain is switched in the generating the first sound signal and (ii) for a predetermined time after the gain is switched, a value of the second sound signal based on a value of the first sound signal at or a before a timing of the gain being switched. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a signal processing apparatus according to an embodiment of the present disclosure. 
         FIG. 2  is a circuit diagram showing a configuration example of an analog amplification portion in the signal processing apparatus. 
         FIG. 3  is a time chart showing operations in the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. 
       FIG. 1  is a block diagram showing a configuration of a signal processing apparatus  1  according to the embodiment of the present disclosure. As shown in  FIG. 1 , the signal processing apparatus  1  includes a first signal processing circuit  100 , a second signal processing circuit  200 , and a control portion  300 . 
     The first signal processing circuit  100  generates a first sound signal S 1  which is a digital sound signal from an input signal Sin. The input signal Sin is an analog sound signal or the like picked up by a microphone. The first signal processing circuit  100  includes an analog amplification portion  110  and a digital amplification portion  160 . Under control of the control portion  300 , the analog amplification portion  110  performs analog amplification on the input signal Sin, performs A/D conversion on the input signal Sin, and outputs the input signal Sin. Under the control of the control portion  300 , the digital amplification portion  160  performs digital amplification on the output signal of the analog amplification portion  110  and outputs the amplified signal as the first sound signal S 1 . 
     The signal processing apparatus  1  is a circuit that amplifies the input signal Sin as a whole. A total gain of the signal processing apparatus  1  as a whole is a gain obtained by adding an analog gain (dB) of the analog amplification portion  110  and a digital gain (dB) of the digital amplification portion  160  in the first signal processing circuit  100 . 
     An instruction of the total gain of the signal processing apparatus  1  is given to the control portion  300  by an operation of an operation device (not shown) or the like. When the total gain of the signal processing apparatus  1  is switched from, for example, a total gain TG 1  to a total gain TG 2 , the control portion  300  instructs the analog amplification portion  110  and the digital amplification portion  160  to provide the analog gain and the digital gain for implementing the total gain TG 2  after the switching. 
     A reason why the analog amplification performed by the analog amplification portion  110  and the digital amplification performed by the digital amplification portion  160  are performed on the input signal Sin is as follows. 
       FIG. 2  is a circuit diagram showing a configuration example of an analog amplification unit  111  which is a part of the analog amplification portion  110 . The analog amplification portion  110  has a configuration in which the analog amplification units  111  are connected in multiple stages. 
     The analog amplification unit  111  includes an amplification unit  120  and a negative feedback circuit  130  which is connected to the amplification unit  120 . the amplification unit  120  is configured by an operational amplifier. Here, the negative feedback circuit  130  includes resistors  141 ,  142 ,  143  having resistance values different from one another, and switches  151 ,  152 ,  153  that switch whether these resistors are respectively connected between input and output terminals of the amplification unit  120 . 
     In the analog amplification unit  111 , based on the analog gain instructed from the control portion  300 , one or a plurality of switches among the switches  151 ,  152 ,  153  are turned on, a resistance value between the input and output terminals of the amplification unit  120  is switched, and a gain of the analog amplification unit  111  is determined. The same operation is performed in other analog amplification units  111  connected in multiple stages, and the analog gain of the analog amplification portion  110  becomes the analog gain instructed by the control portion  300 . 
     In the analog amplification portion  110 , in order to improve accuracy of the gain switching (step width of gain switching), it is necessary to make the analog amplification portion  110  large in scale. However, this is difficult. Therefore, in the present embodiment, the digital amplification portion  160 , which is an amplification circuit using a digital circuit, is provided at a subsequent stage of the analog amplification portion  110 , so that the digital amplification portion  160  performs fine adjustment of the gain that cannot be implemented by the analog amplification portion  110 . 
     In addition, the digital amplification portion  160  has a function of reducing stepwise noise appearing in the first sound signal S 1  to be output by the first signal processing circuit  100  under the control of the control portion  300  at the time of switching the total gain. 
     More specifically, at the time of switching the total gain, the control portion  300  obtains the analog gain and the digital gain for implementing the total gain TG 2  after the switching, and instructs the analog amplification portion  110  to provide the analog gain. In addition, at the time of switching the total gain, the control portion  300  obtains a digital gain DG 1  for implementing the total gain TG 1  before switching, a digital gain DG 2  for implementing the total gain TG 2  after switching, and a gain step ADG obtained by dividing a difference DG 2 −DG 1  between the digital gains DG 1  and DG 2  by a predetermined number of steps N. Then, the control portion  300  repeats a process of changing the digital gain provided by instructing to the digital amplification portion  160  from the initial value DG 1  to the final value DG 2  by the gain step ADG at a predetermined time interval. 
     In this manner, at the time of gain switching, the control portion  300  gradually changes the total gain of the first signal processing circuit  100  from the total gain TG 1  before switching to the total gain TG 2  after switching over time, and reduces the stepwise noise appearing in the first sound signal S 1  to be output by the first signal processing circuit  100 . 
     As shown in  FIG. 1 , the second signal processing circuit  200  includes a sample hold portion  210 , a first cross-fade portion  220 , a low-pass filter  230 , and a second cross-fade portion  240 . The second signal processing circuit  200  is a circuit that removes noise from the first sound signal S 1  output by the first signal processing circuit  100 . 
     A portion including the sample hold portion  210  and the first cross-fade portion  220  constitutes a circuit for generating a second sound signal S 2  from the first sound signal S 1 . The portion including the sample hold portion  210  and the first cross-fade portion  220  has a function of preventing passage of impulse-like noise appearing in the first sound signal S 1  output by the first signal processing circuit  100  at the time of gain switching. 
     Here, the impulse-like noise appearing in the first sound signal S 1  will be described. In the analog amplification unit  111  ( FIG. 2 ) in the analog amplification portion  110 , for example, only the switch  151  is turned on in order to implement the total gain TG 1  before switching, and only the switch  152  is turned on in order to implement the total gain TG 2  after switching. In this case, at the time of gain switching, when a switching timing of the switch  151  from being turned on to being turned off and a switching timing of the switch  152  from being turned off to being turned on do not coincide with each other, a state in which both of the switches  151  and  152  are turned on or a state in which both of the switches  151  and  152  are turned off occurs instantaneously. Due to such an instantaneous occurrence state, the total gain of the first signal processing circuit  100  instantaneously changes, so that the impulse-like noise is generated in the first sound signal S 1 . 
     In order to prevent the passage of the impulse-like noise, when the switching of the total gain of the first signal processing circuit  100  (specifically, a first switching of a stepwise gain switching) is detected, the portion including the sample hold portion  210  and the first cross-fade portion  220  generates a value of the second sound signal S 2  based on a value of the first sound signal S 1  at a gain switching timing for a predetermined time after the switching is detected. 
     More specifically, when it is detected that the gain of the first signal processing circuit  100  is switched, the sample hold portion  210  holds the value of the first sound signal S 1  at the gain switching timing or holds the value of the first sound signal S 1  at the timing before the gain is switched. 
     The first cross-fade portion  220  generates the value of the second sound signal S 2  by performing first mixing (cross-fade in this example) of the value of the first sound signal S 1  at the gain switching timing, that is, the value of the first sound signal S 1  held by the sample hold portion  210  and the value of the first sound signal S 1  generated by the first signal processing circuit  100  for the predetermined time after the timing at which the gain switching is detected. 
     Specifically, the first cross-fade portion  220  generates a coefficient r that monotonously change from 0 to 1 over the predetermined time and a coefficient  1 - r , and outputs a value obtained by adding a value obtained by multiplying the first sound signal S 1  held by the sample hold portion  210  by the coefficient  1 - r  and a value obtained by multiplying the first sound signal S 1  output from the first signal processing circuit  100  by the coefficient r over the predetermined time. In this manner, the first cross-fade portion  220  increases a ratio of the value of the first sound signal S 1  generated by the first signal processing circuit  100  in the second sound signal S 2  obtained by the first mixing with the lapse of time. Accordingly, it is possible to obtain the second sound signal in which the impulse-like noise generated in association with the gain switching is prevented. 
     In the second signal processing circuit  200 , a portion including the low-pass filter  230  and the second cross-fade portion  240  constitutes a circuit that generates a third sound signal S 3  and a fourth sound signal S 4  based on second sound signal S 2 . The portion including the low-pass filter  230  and the second cross-fade portion  240  has a function of preventing noise generated in the second sound signal S 2  when the total gain of the first signal processing circuit  100  changes linearly (more specifically, in a fine stepwise manner) at the time of gain switching. 
     The low-pass filter  230  selects a low-pass component in the second sound signal S 2  and generates the third sound signal S 3 . The second cross-fade portion  240  performs second mixing of the second sound signal S 2  and the third sound signal S 3  to generate the fourth sound signal S 4 . More specifically, the second cross-fade portion  240  generates a coefficient s that monotonously change from 0 to 1 over a predetermined time when the gain switching is detected and a coefficient  1 - s , and outputs a value obtained by adding a value obtained by multiplying the third sound signal S 3  output from the low-pass filter  230  by the coefficient  1 - s  and a value obtained by multiplying the second sound signal S 2  by the coefficient s over the predetermined time. In this manner, the second cross-fade portion  240  increases a ratio of the value of the second sound signal S 2  in the fourth sound signal S 4  obtained by the second mixing with the lapse of time. Then, the fourth sound signal S 4  becomes a final output signal of the signal processing apparatus  1 . 
       FIG. 3  is a time chart showing operations of each device of the signal processing apparatus  1 . When the total gain TG of the first signal processing circuit  100  is switched from the total gain TG 1  to the total gain TG 2 , the control portion  300  instructs the analog amplification portion  110  to switch an analog gain AG. Specifically, the control portion  300  instructs the analog amplification portion  110  to switch from an analog gain AG 1  for implementing the total gain TG 1  before switching to an analog gain AG 2  for implementing the total gain TG 2  after switching. Further, at the time of switching the total gain, the control portion  300  starts control of gradually changing a digital gain DG of the digital amplification portion  160  from the digital gain DG 1  for implementing the total gain TG 1  before switching to the digital gain DG 2  for implementing the total gain TG 2  after switching. 
     In the example shown in  FIG. 3 , when the analog gain AG of the analog amplification portion  110  is switched from the analog gain AG 1  to the analog gain AG 2 , the impulse-like noise is generated in the first sound signal S 1 . In order to prevent the passage of the impulse-like noise, in the present embodiment, the sample hold portion  210  holds the value of the first sound signal S 1  at the timing at which the gain switching of the first signal processing circuit  100  is detected. For a predetermined time thereafter, the first cross-fade portion  220  generates the value of the second sound signal S 2  by performing the first mixing of the value of the first sound signal S 1  output by the first signal processing circuit  100  and the value of the first sound signal S 1  held by the sample hold portion  210 . The ratio of the value of the first sound signal S 1  output by the first signal processing circuit  100  in the second sound signal S 2  is increased with the lapse of time. By such processing, the passage of the impulse-like noise generated in the first sound signal S 1  is prevented. 
     When the analog gain AG of the analog amplification portion  110  is switched from the analog gain AG 1  to the analog gain AG 2 , the second cross-fade portion  240  starts the process of generating the fourth sound signal S 4  by performing the second mixing of the second sound signal S 2  and the third sound signal S 3  output by the low-pass filter  230 . The ratio of the second sound signal S 2  in the fourth sound signal S 4  is increased with the lapse of time. By such processing, the noise in the fourth sound signal S 4  that is caused by the gain switching is reduced. 
     As described above, according to the present embodiment, it is possible to reduce an influence of the noise generated in association with the gain switching on the output signal. 
     Although an embodiment of the present disclosure is described above, the present disclosure may include other embodiments. For example, the present disclosure includes the following embodiments.
         (1) In the above embodiment, the first signal processing circuit  100  includes the analog amplification portion  110  and the digital amplification portion  160 , but the first signal processing circuit  100  may have any configuration, and the entire first signal processing circuit  100  may be configured by analog circuits.       

     (2) In the above embodiment, the second signal processing circuit  200  may be implemented by a digital circuit or an analog circuit. The second signal processing circuit  200  may be implemented by a processor that executes a program. 
     (3) The present disclosure can also be implemented as a program that causes a processor to function as the signal processing apparatus  1  of the above-described embodiment.