Patent Publication Number: US-11641545-B2

Title: Conference terminal and feedback suppression method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 110132528, filed on Sep. 1, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Field of the Disclosure 
     The disclosure relates to a voice conference, and particularly relates to a conference terminal and a feedback suppression method. 
     Description of Related Art 
     Remote conferences allow people in different locations or spaces to have conversations, and conference-related equipment, protocols, and applications have also developed well. When a multi-person conference call is in progress, if there are more than two mobile devices operating together in the same space, the speakers may generate a howling that causes interference and discomfort. 
     For example,  FIG.  1    is a schematic diagram of a conventional sound processing framework. Please refer to  FIG.  1   , the sound signal S 1  of the mobile device D 1  is played through the speaker S. The sound signal S 2  received by the microphone R can use the echo cancellation mechanism C to cancel the echo component that passes through the echo path EP and belongs to the sound signal S 1 , thereby generating the sound signal S 3 . The sound signal S 3  is transmitted to the mobile device D 2  through the network and can be played through its speaker S. However, if the sound played by the mobile device D 2  is received by the microphone R of the mobile device D 1 , a closed loop system (for example, the howling path HP) is formed, and a howling that causes interference and discomfort is more likely to be generated. 
       FIG.  2    is an example describing a time-frequency diagram of a howling. Please refer to  FIG.  2   , the howling is about 0.8 kilohertz (kHz) and becomes louder along with increase of time length. It should be noted that the current suppression technologies for howling are performed to eliminate sound signals of specific frequencies. However, the signal of a specific frequency in the main sound signal that was originally intended to be retained is also eliminated, resulting in distortion of the sound and auditory perception. 
     SUMMARY OF THE DISCLOSURE 
     In view of the above problem, the embodiments of the disclosure provide a conference terminal and a feedback suppression method, which can eliminate howling and retain signals of all frequency bands in the main sound signal. 
     The feedback suppression method of the embodiment of the disclosure is suitable for conference terminals. This conference terminal includes a microphone array and a speaker. The feedback suppression method includes (but is not limited to) the following steps: dividing the transmitted sound signal into sub sound signals of multiple frequency bands; the transmitted sound signal is transmitted through the network; different sub sound signals correspond to different frequency bands; detecting the interfered frequency band corresponding to howling interference according to the sub sound signals of the multiple frequency bands; the power of the sub sound signal of the interfered frequency band among the multiple frequency bands increases along with time; the interfered frequency band is affected by the howling interference; determining an interference direction according to multiple input sound signals received by the microphone array and merely pass through the interfered frequency band; a sound from the interference direction leads to the howling interference; determining a beam pattern of the microphone array according to the interference direction; and the gain of the beam pattern in the interference direction is reduced. 
     The conference terminal of the embodiment of the disclosure includes (but is not limited to) a microphone array, a speaker, a communication transceiver, and a processor. The microphone array is configured for sound collection. The speaker is configured to play sound. The communication transceiver is configured to transmit or receive data. The processor is coupled to the microphone array, the speaker, and the communication transceiver. The processor is configured to divide the transmitted sound signal into sub sound signals of multiple frequency bands, detect the interfered frequency band corresponding to howling interference according to the sub sound signals of the multiple frequency bands, determine an interference direction according to multiple input sound signals received by the microphone array and merely pass through the interfered frequency band, and determine a beam pattern of the microphone array according to the interference direction. The transmitted sound signal is transmitted through the network through the communication transceiver. Different sub sound signals correspond to different frequency bands. The power of the sub sound signal of the interfered frequency band among these frequency bands increases along with time. The interfered frequency band is affected by howling interference. A sound from the interference direction leads to the howling interference. The gain of the beam pattern in the interference direction is reduced. 
     Based on the above, according to the conference terminal and feedback suppression method of the embodiment of the disclosure, the sub sound signal whose power increases along with time and its corresponding interfered frequency band are detected among multiple frequency bands, the interference direction leading to the howling interference is determined through the microphone array, and the gain of the interference direction is suppressed through beamforming. In this way, all frequency bands in the main voice signal can be retained, and howling can also be suppressed. 
     In order to make the above-mentioned features and advantages of the disclosure more comprehensible, the following specific embodiments are described in detail in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a conventional sound processing framework. 
         FIG.  2    is an example describing a time-frequency diagram of a howling. 
         FIG.  3    is a schematic diagram of a conference call system according to an embodiment of the disclosure. 
         FIG.  4    is a flowchart of a feedback suppression method according to an embodiment of the disclosure. 
         FIG.  5    is a schematic diagram showing feedback suppression according to an embodiment of the disclosure. 
         FIG.  6 A  is a schematic diagram showing a beam pattern according to an embodiment of the disclosure. 
         FIG.  6 B  is a schematic diagram showing a beam pattern according to an embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG.  3    is a schematic diagram of a conference call system  1  according to an embodiment of the disclosure. The conference call system  1  includes (but is not limited to) conference terminals  10  and  20  and a cloud server  50 . 
     The conference terminals  10  and  20  may be wired phones, mobile phones, Internet phones, tablet computers, desktop computers, notebook computers, or smart speakers. 
     The conference terminal  10  includes (but is not limited to) a microphone array  11 , a speaker  13 , a communication transceiver  15 , a memory  17 , and a processor  19 . 
     The microphone array  11  includes multiple microphones of dynamic type, condenser type, or electret condenser type. The microphone array  11  may also include a combination of other electronic components, analog-to-digital converters, filters, and audio processors that can receive sound waves (for example, human voice, ambient sound, machine operation sound, etc.) and convert them into sound signals. In an embodiment, the microphone array  11  is configured to collect/record the voice of the speaker to obtain the input sound signal. In some embodiments, the input sound signal may include the voice of the speaker, the sound generated by the speakers  13  and  23 , and/or other ambient sounds. 
     The speaker  13  may be a speaker or a loudspeaker. In an embodiment, the speaker  13  is configured to play sound. 
     The communication transceiver  15  is, for example, a transceiver (which may include, but not limited to, connection interface, signal converters, communication protocol processing chips and other components) that supports wired networks such as Ethernet, optical fiber networks, or cables. The transceiver may also be a transceiver (which may include, but are not limited to, antennas, digital-to-analog/analog-to-digital converters, communication protocol processing chips and other components) that supports Wi-Fi, fourth-generation (4G), fifth-generation (5G) or later generation mobile networks and other wireless networks. In an embodiment, the communication transceiver  15  is configured to transmit or receive data through a network  50  (for example, the Internet, a local area network, or other types of networks). 
     The memory  17  can be any type of fixed or removable random access memory (RAM), read only memory (ROM), flash memory, hard disk drive (HDD), solid-state drive (SSD) or similar components. In an embodiment, the memory  17  is configured to store program codes, software modules, configuration, data (for example, sound signal, interfered frequency band, interference direction, or beam pattern) or files. 
     The processor  19  is coupled to the microphone array  11 , the speaker  13 , the communication transceiver  15  and the memory  17 . The processor  19  may be a central processing unit (CPU), a graphic processing unit (GPU), or other programmable general-purpose or special-purpose microprocessors, digital signal processor (DSP), programmable controller, field programmable gate array (FPGA), application-specific integrated circuit (ASIC) or other similar components or a combination of the above components. In an embodiment, the processor  19  is configured to perform all or part of the operations of the conference terminal  10  to which it belongs, and can load and execute various software modules, files, and data stored in the memory  17 . 
     The conference terminal  20  includes (but is not limited to) a microphone array  21 , a speaker  23 , a communication transceiver  25 , a memory  27 , and a processor  29 . The implementation and functions of the microphone array  21 , the speaker  23 , the communication transceiver  25 , the memory  27 , and the processor  29  can be derived from the description of the microphone array  11 , the speaker  13 , the communication transceiver  15 , the memory  17  and the processor  19 , and no further description is incorporated herein. The processor  29  is configured to perform all or part of the operations of the conference terminal  20  to which it belongs, and can load and execute various software modules, files, and data stored in the memory  27 . 
     Hereinafter, various devices, components, and modules in the conference communication system  1  will be incorporated to describe the method in the embodiment of the disclosure. Each process of the method can be adjusted according to the actual implementation, and is not limited thereto. 
     In addition, it should be noted that, for the convenience of description, the same component can achieve the same or similar operations, and no further description will be incorporated herein. For example, the processor  19  of the conference terminal  10  and the processor  19  of the conference terminal  20  can both implement the same or similar method in the embodiment of the disclosure. 
       FIG.  4    is a flowchart of a feedback suppression method according to an embodiment of the disclosure. Referring to  FIG.  4   , the processor  19  of the conference terminal  10  may divide the transmitted sound signal S TX  into sub sound signals S B1 , S B2 , . . . , S BL  (L is a positive integer) of multiple frequency bands B 1 , B 2 , . . . , BL (step S 410 ). Specifically, transmitting the sound signal S TX  is performed by transmitting the sound signal Six to the conference terminal  20  through the communication transceiver  15  via the network  50 . Generally speaking, the processor  19  can perform sound processing such as filtering, echo cancellation, and gain adjustment on the input sound signals S M1  to S MN  recorded by the microphone array  11 , and generate the transmitted sound signal Six accordingly. In an embodiment, the processor  19  may divide the transmitted sound signal Six into sub sound signals S B1 ˜S BL  of L (for example, 64, 128, or 512) frequency bands B 1 ˜BL through Fourier transform, wavelet transform, or impulse response. Different sub sound signals S B1 ˜S BL  correspond to different frequency bands B 1 ˜BL. For example, the sub sound signal S B1  only covers the frequency band B 1  ranging from 600 Hz to 700 Hz, and the sub sound signal S B2  only covers the frequency band B 2  ranging from 700 Hz to 800 Hz. 
     The processor  19  can detect the interfered frequency band B H  corresponding to the howling interference according to the sub sound signals S B1  to S BL  of the frequency bands B 1  to BL (step S 430 ). Specifically, the howling interference is characterized in that the sound signal of a certain frequency becomes louder and louder along with the increase of time length. It can be obtained that if one or more of the interfered frequency bands B H  among the frequency bands is affected by howling interference, the power of the sub sound signal of the interfered frequency band B H  increases along with time. 
     In an embodiment, the processor  19  may determine the interfered frequency band according to the power change and the single frequency ratio. The power change is related to the difference or the amount of change in the power of the sub sound signal of the interfered frequency band at different time points, such as the highest, average or other statistically measured power difference of the interfered frequency band between time point t−1 and time point t. The greater the power change, the greater the probability that frequency band is affected by howling interference. On the other hand, if the power change is smaller, the probability that frequency band is affected by howling interference is smaller. In addition, the single frequency ratio is the proportion that the power of the sub sound signal of the interfered frequency band accounts for in all or part of the sub sound signals S B1 ˜S BL . If the single frequency ratio is greater, the probability that frequency band is affected by howling interference is greater. On the other hand, if the ratio of single frequency is smaller, the probability that frequency band is affected by howling interference is smaller. 
     In an embodiment, the processor  19  may determine that the product of the power change of the interfered frequency band and the single frequency ratio is greater than a threshold value. Assuming that the power corresponding to the sub sound signals S B1 ˜S BL  of each frequency band B 1 ˜BL at time t is P t   3 , P t   2 , . . . , P t   L , respectively, then the possibility that the frequency band Bb (b is a value from 1 to L) is an interfered frequency band B H  is: 
     
       
         
           
             
               
                 
                   
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     The left half 
             (         P   t   b     -     P     t   -   1     b           P   t   b     +     P     t   -   1     b         )         
reflects the influence of power change over time (i.e., the aforementioned power change). If the power of frequency band Bb is increased over time, this value (which is between −1 and 1) will get closer and closer to 1. On the other hand, the right half
 
             (       P   t   b         ∑     b   =   1       N   B       ⁢           ⁢     P   t   b         )         
reflects the proportion (which is between 0 and 1) that the power of the frequency band Bb accounts for the power of the overall sub sound signals S B1 ˜S BL  (i.e., the aforementioned single frequency ratio). Therefore, if the sub sound signal S Bb  of the frequency band Bb becomes louder and louder, the value of the possibility F t   b  (that is, the product of the power change and the single frequency ratio) will become larger and larger. In addition, if the possibility F t   b  exceeds the defined threshold value T H  (for example, 0.5, 0.4 or 0.45), it is very likely that howling interference will occur at this frequency band Bb. On the other hand, if the possibility F t   b  does not exceed the threshold value T H , the processor  19  may determine that no howling interference occurs at this frequency band Bb. The formula (2) for determining the detection of howling interference at time point t can be defined as:
 
     
       
         
           
             
               
                 
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     In light of the above, it can be obtained that when howling interference occurs, the frequency band with the highest possibility, the second highest possibility or the possibility that is higher than the threshold value is the interfered frequency band B H . 
     It should be noted that the number of interfered frequency bands B H  at the same time point is not limited to one. As long as the detection conditions are met (for example, the possibility or power change exceeds the corresponding threshold value), the frequency band can be regarded as the interfered frequency band B H . 
     The processor  19  may determine the interference direction OB according to the multiple input sound signals S M1 ˜S MN  received by the microphone array  11  and merely pass through the interfered frequency band B H  (step S 450 ). Specifically,  FIG.  5    is a schematic diagram showing feedback suppression according to an embodiment of the disclosure. Referring to  FIG.  5   , it is assumed that the microphone array  11  includes N microphones M 1 ˜MN (N is a positive integer greater than 1). The sound signals obtained by recording sounds through the microphones M 1 ˜MN are the input sound signals S M1 ˜S MN , respectively. Assume that the conference terminals  10  and  20  establish a conference call. For example, the conference is established through video software, voice call software, or making a phone call, the caller can start talking. After recording/receiving sounds through the microphones M 1 ˜MN, the processor  19  can obtain the input sound signals S M1 ˜S MN . 
     The processor  19  may perform filtering processing on the input sound signals S M1 ˜S MN  respectively (step S 510 ). Specifically, this filtering process, for example, allows only signals of the interfered frequency band B H  to pass through, and blocks signals of other frequency bands that are not the interfered frequency band B H  from passing through. In an embodiment, the processor  19  can divide the input sound signals S M1 ˜S MN  into L frequency bands B 1 ˜BL respectively through Fourier transform, wavelet transform, or impulse response, and extract only the input sound signals S H1 ˜S HN  that belong to the interfered frequency band B H . In another embodiment, a bandpass filter can be set according to the interfered frequency band B H , and the processor  19  filters the input sound signals S M1 ˜S MN  respectively through the bandpass filter to obtain the input sound signals S H1 ˜S HN . 
     In an embodiment, the processor  19  determines the time difference Δt between the input sound signals S H1 ˜S HN  according to the correlation between the input sound signals S H1 ˜S HN . Here, the correlation referred to corresponds to the phase/time delay between two of the input sound signals S H1 ˜S HN . Taking two microphones M 1  and M 2  as an example, the processor  19  uses cross-correlation or other correlation algorithms on the input sound signals S H1  and S H2  to obtain the correlation. Each correlation corresponds to a phase/time delay, and the processor  19  can obtain a time difference Δt based on the phase/time delay. 
     It should be noted that the sound from the interference direction θ B  causes howling interference. For example, the conference terminal  20  is located at the interference direction θ B  of the conference terminal  10 . The processor  19  can determine the interference direction θ B  according to the distance between the time difference Δt and the multiple microphones M 1 ˜MN in the microphone array  11 . That is, the distance traveled by the sound over the time difference Δt is the adjacent side of the interference direction θ B  in the right triangle, and the distance between the two microphones is the hypotenuse of the interference direction θ B . Taking two microphones M 1  and M 2  as an example, the distance between the two microphones is d, and the interference direction θ B  is: 
                     θ   B     ≅       cos     -   1       ⁡     (       Δ   ⁢           ⁢     t   ·     v   S         d     )               (   3   )               
ν s  is the speed of sound propagation.
 
     In other embodiments, the processor  19  may also obtain the interference direction θ B  through other sound source positioning algorithms. 
     Referring to  FIG.  4   , the processor  19  may determine the beam pattern of the microphone array  11  according to the interference direction θ B  (step S 470 ). Specifically, the beamforming technology adjusts the parameters (for example, phase and amplitude) of the basic unit of the phased array, so that signals at certain angles obtain constructive interference, and signals at other angles obtain destructive interference. Therefore, different parameters will form different beam patterns. 
     Referring to  FIG.  5   , if howling interference is detected, the processor  19  performs howling cancellation according to the interference direction θ B  corresponding to the howling interference (step S 530 ). The processor  19  may determine the beamforming parameters according to the interference direction θ B  (step S 550 ), and reduce the gain of the beam pattern at the interference direction θ B . 
     In an embodiment, the processor  19  aligns the null steering of the beam pattern with the interference direction θ B . In another embodiment, the processor  19  aligns the position between the main lobe and sidelobe in the beam pattern with the interference direction θ B . 
     In an embodiment, the memory  17  records the corresponding relationship between beamforming parameters (for example, the amplitude and phase corresponding to different microphones M 1  to MN) and various interference directions θ B , and the corresponding relationship is recorded for the processor  19  to use. For example, when the interference direction θ B  is 30 degrees, the null steering in this beam pattern is towards 30 degrees. In another embodiment, the processor  19  may use a model based on a machine learning algorithm to infer an appropriate beam pattern of the interference direction θ B , and generate corresponding parameters accordingly. 
     The processor  19  may generate an input sound signal SB according to the beamforming parameters. Under the circumstances, the input sound signal SB still retains signals of all frequency bands. 
     In an embodiment, even if no howling interference is detected, the processor  19  can still aim the main beam or main lobe at the speaker of the conference terminal  10  based on the beamforming technology. 
     For example,  FIG.  6 A  is a schematic diagram showing a beam pattern PB 1  according to an embodiment of the disclosure. Please refer to  FIG.  6 A . Assuming that no howling interference is detected, the beam pattern PB 1  faces forward, and receives the sound signal at the front accordingly. 
       FIG.  6 B  is a schematic diagram showing a beam pattern PB 2  according to an embodiment of the disclosure. Please refer to  FIG.  6 B , the beam pattern PB 2  includes a main lobe MS and a sidelobe SS. The conference terminal  20  is located at the interference direction θ B  of the conference terminal  10 . The interference direction θ B  is substantially located at null steering between the main lobe MS and the sidelobe SS. 
     On the other hand, referring to  FIG.  5   , the processor  19  can still perform echo cancellation on the input sound signal SB subjected to beamforming process according to the received sound signal S Rx  (the sound signal to be played through the speaker  13  thereof) in the call (step S 570 ). 
     In summary, in the conference terminal and feedback suppression method in the embodiment of the disclosure, the microphone array technology is adopted to determine the interference direction corresponding to the howling interference, and cancel the howling interference based on beamforming technology. In this way, the howling of a specific frequency can be canceled, and all frequency bands of the input sound signals can be retained. 
     Although the present disclosure has been disclosed in the above embodiments, it is not intended to limit the present disclosure, and those skilled in the art can make some modifications and refinements without departing from the spirit and scope of the disclosure. Therefore, the scope of the present disclosure is subject to the definition of the scope of the appended claims.