Patent Publication Number: US-2022215825-A1

Title: Method and device for eliminating unstable noise

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This non-provisional application claims priority under 35 U.S.C. § 119(a) to Patent Application No. 110100185 filed in Taiwan, R.O.C. on Jan. 4, 2021, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The invention relates to an audio processing technology, and in particular, to a method and device for eliminating unstable noise. 
     Related Art 
     For a general digital microphone in recording, when the microphone is just activated or when switching into a mode, the microphone often produces unstable noise, such as pop noise, which leads to poor sound quality and hurts user experience. Conventional techniques employ software for processing an audio, where the software compensates a high pass filter, so that a DC value converges faster, and a volume mask is used in coordination to make initial pop noise unperceivable due to a lower volume. However, when the microphone generates a large DC value, such technique cannot guarantee that the applied volume mask is able to render the noise inaudible, and a high pass filter that maintains a high cutoff frequency (−3 dB corner) also filters out part of audio bandwidth (audio BW), resulting in audio distortion. 
     SUMMARY 
     In view of this, the invention provides a method for eliminating unstable noise, which is applicable to a sound recording device and implemented by a codec. The method for eliminating unstable noise includes: activating the sound recording device to start recording; setting a suppression duration and a cutoff frequency switching duration according to unstable noise and a DC offset value of the sound recording device; processing a front-end audio of a recorded sound by a filter having a first cutoff frequency to make the unstable noise in the front-end audio quickly converge, and outputting a filtered audio signal; suppressing the filtered audio signal according to the suppression duration to eliminate the unstable noise; and adjusting the first cutoff frequency of the filter to a second cutoff frequency according to the cutoff frequency switching duration, where the first cutoff frequency is greater than the second cutoff frequency. 
     The invention also provides a device, which includes a sound recording device and a codec, where the codec is electrically coupled to the sound recording device. The codec includes a filter, an audio signal path, and a suppression circuit. The filter is electrically coupled to the sound recording device and the audio signal path. When the sound recording device is activated to start recording, the filter processes a front-end audio of a recorded sound by using a first cutoff frequency and outputs a filtered audio signal, and then the filter adjusts the first cutoff frequency to a second cutoff frequency according to a cutoff frequency switching duration, where the first cutoff frequency is greater than the second cutoff frequency. The audio signal path is electrically coupled to the suppression circuit for the transmission of the filtered audio signal to the suppression circuit, and the suppression circuit suppresses the filtered audio signal according to a suppression duration to eliminate unstable noise in the front-end audio. 
     According to some embodiments, the step of activating the sound recording device is performed when the sound recording device is powered on or when the sound recording device performs mode switching. 
     According to some embodiments, before the step of setting the suppression duration and the cutoff frequency switching duration according to the unstable noise and the DC offset value of the sound recording device, the method further includes: detecting the unstable noise and the DC offset value of the sound recording device by using a detection circuit. 
     According to some embodiments, the detection circuit further transmits the unstable noise and the DC offset value to a decision circuit, so that the decision circuit sets the suppression duration and the cutoff frequency switching duration according to the unstable noise and the DC offset value. 
     According to some embodiments, in the step of setting the suppression duration and the cutoff frequency switching duration according to the unstable noise and the DC offset value of the sound recording device, the decision circuit also sets the first cutoff frequency according to the DC offset value. 
     According to some embodiments, the second cutoff frequency is a cutoff frequency of the sound recording device in normal operation. 
     According to some embodiments, the sound recording device is a digital microphone. 
     In summary, by using a hardware control method, the invention enables unstable noise generated by a sound recording device during recording to converge faster and be suppressed, so as to eliminate the unstable noise (transient change) in a front-end audio of the sound recording device. In addition, in the invention, a suppression duration, a cutoff frequency switching duration, and a first cutoff frequency can be dynamically adjusted and set according to the unstable noise and a DC offset value of the sound recording device to adapt to the transient change of the sound recording device during recording and reduce the duration of recording distortion, so as to achieve the effect of small distortion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a device for eliminating unstable noise according to an embodiment of the invention; 
         FIG. 2  is a schematic flowchart of a method for eliminating unstable noise according to an embodiment of the invention; 
         FIG. 3  is a block diagram of a device for eliminating unstable noise according to another embodiment of the invention; 
         FIG. 4  is a schematic flowchart of a method for eliminating unstable noise according to another embodiment of the invention; 
         FIG. 5  is a schematic diagram of an audio signal on which the method for eliminating unstable noise of the invention is not performed; and 
         FIG. 6  is a schematic diagram of an audio signal on which the method for eliminating unstable noise of the invention has been performed. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a device for eliminating unstable noise according to an embodiment of the invention. Referring to  FIG. 1 , the device  1  for eliminating unstable noise includes a codec  10  and a sound recording device  12 , and the recording device  12  can also be called a transducer. The codec  10  is electrically coupled to the sound recording device  12 , and the codec  10  includes a register control circuit  14 , a filter  16 , an audio signal path  18 , and a suppression circuit  20 . The filter  16  is electrically coupled to the sound recording device  12 , the register control circuit  14 , and the audio signal path  18 , and the suppression circuit  20  is electrically coupled to the register control circuit  14  and the audio signal path  18 , so that the filter  16  can be used to filter an audio signal, and the suppression circuit  20  can be used to suppress the audio signal in a specific duration. 
     In an embodiment, the sound recording device  12  is any electroacoustic conversion device, such as a digital microphone. In another embodiment, the sound recording device  12  may alternatively be composed of an analog microphone and an analog-to-digital converter. 
     In an embodiment, the filter  16  is a high pass filter. 
     Referring to both  FIG. 1  and  FIG. 2 , the method for eliminating unstable noise is applicable to the sound recording device  12  and is implemented by the codec  10 . The method for eliminating unstable noise includes: as shown in step S 10 , activating the sound recording device  12  to start recording, where the activation of the sound recording device  12  can be performed when the sound recording device  12  is powered on or when the sound recording device  12  performs mode switching, in response to different transient changes generated by the sound recording device  12 . Because features of the sound recording device  12  are known, unstable noise and a DC offset value corresponding to the sound recording device  12  can be known. As shown in step S 12 , software or firmware may be applied for setting a suppression duration Tm and a cutoff frequency switching duration Tc according to the unstable noise and the DC offset value of the sound recording device  12 . The suppression duration Tm and the cutoff frequency switching duration Tc can be stored in the register control circuit  14 , and the register control circuit  14  transmits the suppression duration Tm to the suppression circuit  20  and transmits the cutoff frequency switching duration Tc to the filter  16 . As shown in step S 14 , when the sound recording device  12  transmits recorded audio data to the filter  16 , the filter  16  filters a front-end audio of the recorded sound by using a relatively high first cutoff frequency, so that unstable noise in the front-end audio can quickly converge, and the filter  16  outputs a filtered audio signal. The filtered audio signal processed by the filter  16  is transmitted to the suppression circuit  20  through the audio signal path  18 . As shown in step S 16 , the suppression circuit  20  suppresses the filtered audio signal according to the suppression duration Tm and holds for the suppression duration Tm to eliminate the unstable noise. Finally, as shown in step S 18 , the register control circuit  14  adjusts the cutoff frequency of the filter  16  from the first cutoff frequency to a second cutoff frequency according to the cutoff frequency switching duration Tc, where the first cutoff frequency is greater than the second cutoff frequency, and the second cutoff frequency is a cutoff frequency of the sound recording device  12  in normal operation. Therefore, the relatively high first cutoff frequency can be used to make the unstable noise in the front-end audio quickly converge. In other words, at the beginning of recording, the filter  16  works at a relatively high first cutoff frequency, and after the cutoff frequency switching duration Tc, the filter  16  is adjusted from the initially relatively high first cutoff frequency back to the normal second cutoff frequency, and filters the subsequent audio data by using the normal second cutoff frequency. The processed audio signal passes sequentially through the audio signal path  18  and the suppression circuit  20 , so that the codec  10  outputs an audio stream. When the audio signal passes through the suppression circuit  20 , because the suppression duration Tm has passed, the suppression circuit  20  stops the suppression. Therefore, the audio signal directly passes through the suppression circuit  20  without being suppressed, and the audio stream is outputted normally. 
     In an embodiment, the unstable noise may be, but is not limited to, DC pop noise. 
     In an embodiment, when the suppression duration Tm and the cutoff frequency switching duration Tc are set in step S 12 , the initial first cutoff frequency of the filter  16  can also be set according to the DC offset value of the sound recording device  12 , so that the relatively high first cutoff frequency can be used to make the unstable noise (transient change) of the front-end audio quickly converge. 
     In an embodiment, in step S 16 , a muting method can alternatively be used in the suppression step to eliminate the unstable noise. That is, the suppression circuit  20  can mute the filtered audio signal according to the suppression duration Tm and continue for the suppression duration Tm to eliminate the unstable noise. 
       FIG. 3  is a block diagram of a device for eliminating unstable noise according to another embodiment of the invention. Referring to  FIG. 3 , a device  1  for eliminating unstable noise includes a codec  10  and a sound recording device  12 . The codec  10  is electrically coupled to the sound recording device  12 , and the codec  10  includes a detection circuit  22 , a decision circuit  24 , a filter  16 , an audio signal path  18 , and a suppression circuit  20 . The detection circuit  22  is electrically coupled to the sound recording device  12  and the decision circuit  24 . The filter  16  is electrically coupled to the sound recording device  12 , the decision circuit  24 , and the audio signal path  18 . The suppression circuit  20  is electrically coupled to the decision circuit  24  and the audio signal path  18 . In the codec  10 , the detection circuit  22  detects unstable noise and a DC offset value corresponding to the sound recording device  12 , and transmits the unstable noise and the DC offset value to the decision circuit  24 , so that the decision circuit  24  determines a suppression duration Tm and a cutoff frequency switching duration Tc according to the unstable noise and the DC offset value, and transmits Tm and Tc to the suppression circuit  20  and the filter  16  respectively. Then the suppression circuit  20  performs suppression according to the suppression duration Tm, and the filter  16  switches a cutoff frequency according to the cutoff frequency switching duration Tc. 
     Referring to both  FIG. 3  and  FIG. 4 , the method for eliminating unstable noise is applicable to the sound recording device  12  and is implemented by the codec  10 . This method includes: as shown in step S 20 , activating the sound recording device  12  to start recording when the sound recording device  12  is powered on or performs mode switching. As shown in step S 22 , the detection circuit  22  detects unstable noise and a DC offset value of the sound recording device  12 , and transmits the detected unstable noise and DC offset value to the decision circuit  24 . As shown in step S 24 , the decision circuit  24  sets the suppression duration Tm and the cutoff frequency switching duration Tc according to the unstable noise and the DC offset value, and transmits the suppression duration Tm to the suppression circuit  20  and transmits the cutoff frequency switching duration Tc to the filter  16 . As shown in step S 26 , when the sound recording device  12  transmits recorded audio data to the filter  16 , the filter  16  filters a front-end audio of the recorded sound by using a relatively high first cutoff frequency, so that the unstable noise in the front-end audio can quickly converge, and the filter  16  outputs a filtered audio signal. The filtered audio signal processed by the filter  16  is transmitted to the suppression circuit  20  through the audio signal path  18 . As shown in step S 28 , the suppression circuit  20  suppresses the filtered audio signal according to the received suppression duration Tm and continues for the suppression duration Tm to eliminate the unstable noise. Finally, as shown in step S 30 , the decision circuit  24  adjusts the cutoff frequency of the filter  16  from a relatively high first cutoff frequency to a normal second cutoff frequency according to the cutoff frequency switching duration Tc. After the cutoff frequency switching duration Tc, the filter  16  is adjusted from the initially relatively high first cutoff frequency back to the normal second cutoff frequency, and filters the subsequent audio data by using the normal second cutoff frequency. The processed audio signal passes sequentially through the audio signal path  18  and the suppression circuit  20 , so as to output an audio stream. When the audio signal passes through the suppression circuit  20 , because the suppression duration Tm has passed, the suppression circuit  20  stops the suppression. Therefore, the audio signal directly passes through the suppression circuit  20  without being suppressed, and the audio stream is outputted normally. 
     In an embodiment, when the decision circuit  24  sets the suppression duration Tm and the cutoff frequency switching duration Tc in step S 24 , the decision circuit  24  can also set the initial first cutoff frequency of the filter  16  according to the DC offset value of the sound recording device  12 , so that the relatively high first cutoff frequency can be used to make the unstable noise of the front-end audio quickly converge. 
     In an embodiment, in step S 28 , a muting method can alternatively be used in the suppression step to eliminate the unstable noise. That is, the suppression circuit  20  can mute the filtered audio signal according to the suppression duration Tm and continue for the suppression duration Tm to eliminate the unstable noise. 
     In summary, in the operating modes shown in  FIG. 3  and  FIG. 4 , the detection circuit  22  can be used to automatically detect the unstable noise and the DC offset value contributed by the sound recording device  12  and dynamically adjust the suppression duration Tm and the cutoff frequency switching duration Tc. In this case, the system is more adaptable to different sound recording devices  12 , so as to eliminate unstable noise (transient change) generated by a sound recording device  12  during recording. 
     In an embodiment, the codec  10  can be built in a host (not shown), and the sound recording device  12  can be plugged into the host to form an electrical connection with the codec  10  to eliminate unstable noise by using the codec  10 . The host can be, but is not limited to, a notebook computer, a tablet computer, or a personal computer. 
     Referring to both  FIG. 5  and  FIG. 6 , before the method for eliminating unstable noise of the invention is used, as shown in  FIG. 5 , after the sound recording device is activated, the audio signal starts from 0 seconds. At this duration, there is unstable noise in a front-end audio of an audio signal, and the DC signal (audio signal) is not stabilized until 400 milliseconds. In contrast, after the method for eliminating unstable noise of the invention is applied, as shown in  FIG. 6 , after the sound recording device is activated, the audio signal also starts from 0 seconds, but unstable noise in a front-end audio of an audio signal has already been suppressed, and the DC signal (audio signal) is already stable at 150 milliseconds. Therefore, the total duration required for stabilization is greatly reduced. 
     Therefore, by using a hardware control method, the invention enables unstable noise generated by a sound recording device during recording to converge faster and be suppressed, so as to eliminate the unstable noise (transient change) in a front-end audio of the sound recording device. In addition, in the invention, a suppression duration, a cutoff frequency switching duration, and a first cutoff frequency can be dynamically adjusted and set according to the unstable noise and a DC offset value of the sound recording device to adapt to the transient change of the sound recording device during recording and reduce the duration of recording distortion, so as to achieve the effect of small distortion. 
     Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, the disclosure is not for limiting the scope of the invention. Persons having ordinary skill in the art may make various modifications and changes without departing from the scope and spirit of the invention. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments described above.