Abstract:
During sound recording by a microphone, a recording device and method locate characteristics of noise by using a speaker as a second microphone to simultaneously receive sound, and then use the characteristics of noise for noise reduction of the received sound by the microphone, thereby improving the quality of sound recording. By using the speaker as a microphone to establish a microphone array with the existing microphone for noise reduction, no costs of additional microphones is required.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to a recording device and, more particularly, to circuit and method for noise reduction of a recording device. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wind noise and other ambient noise are usually troublesome for a sound recording device such as a digital video camera and a recording pen. Using a specific algorithm or filter to locate the characteristics of noise and remove it from original sound requires a lot of computing power and corresponding hardware cost. Using a microphone array (two or more microphones), for example, U.S. Pat. Nos. 7,174,023 and 7,895,036 and U.S. Pat. Publication No. 2009/0002498, can achieve better results but the cost of additional microphones is still an issue. 
         [0003]    In the field of communications, on the other hand, U.S. Pat. Publication No. 2011/0181452 teaches using a speaker at the far end that is communicating with the user of a communication device at the near end as a microphone for microphone array noise reduction of the sound transmitted to the far end. However, this art adopts a speaker as a microphone for a communication device to perform acoustic echo cancellation/suppression or level adjustment, but is not dedicated to enhance the quality of sound recording. 
         [0004]    Therefore, it is desired circuit and method for noise reduction of a recording device without using additional microphones. 
       SUMMARY OF THE INVENTION 
       [0005]    An objective of the present invention is to provide a low noise recording device and a noise reduction method thereof. 
         [0006]    Another objective of the present invention is to provide a recording device and method using a speaker as a microphone for noise reduction. 
         [0007]    According to the present invention, during sound recording by a microphone, a recording device and method locate characteristics of noise by using a speaker as a second microphone to simultaneously receive sound, and then use the characteristics of noise for noise reduction of the received sound by the microphone, thereby improving the quality of sound recording. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a circuit diagram of a first embodiment according to the present invention; and 
           [0010]      FIG. 2  is a circuit diagram of a second embodiment according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Since a recording device already has a microphone for receiving sound and a speaker for playing sound, using the speaker as a second microphone to characterize noise requires no additional microphones. Specifically, using the existing speaker of a recording device as a second microphone to establish a microphone array with the existing microphone of the recording device for noise reduction of the recording device needs no costs of additional microphones. 
         [0012]      FIG. 1  is a recording device according to the present invention, which includes a speaker  10 , a microphone  12 , analog processors  14  and  16 , analogy-to-digital converters (ADCs)  18  and  20 , a noise detector  22 , a noise processor  28 , a recording medium  34  and a play circuit  36 . During sound recording, the microphone  12  receives sound to generate an analog signal Sa 1 , meanwhile the speaker  10  acts as another microphone that also receives the sound to generate an analog signal Sa 2 . The analog processors  14  and  16  process the analog signals Sa 1  and Sa 2 , to generate analog signals Sa 3  and Sa 4 , respectively. The signal processing applied to the analog signals Sa 1  and Sa 2  by the analog processors  14  and  16  may include amplification, modulation or gain adjustment. The ADCs  18  and  20  convert the analog signals Sa 3  and Sa 4  into digital signals Sd 1  and Sd 2 , respectively. The noise detector  22  includes an equalizer  24  and a wind noise extractor  26 . The equalizer  24  adjusts the frequency response of the digital signal Sd 2  to generate a digital signal Sd 2 _e having a frequency response identical to that of the digital signal Sd 1 . The wind noise extractor  26  receives the digital signals Sd 1  and Sd 2 _e, and locates the characteristics of noise from the digital signal Sd 1  by using the digital signal Sd 2 _e to generate a digital signal Sn representative of the characteristics of noise. The noise processor  28  includes a subtractor  30  and a digital processor  32 . The subtractor  30  receives the digital signals Sd 1  and Sn, and subtracts Sn from Sd 1  to generate a digital signal Sd 1 _s. Then, the digital processor  32  processes the digital signal Sd 1 _s to generate a digital signal Sd 3 . The signal processing applied to the digital signal Sd 1 _s by the digital processor  32  may include sound optimization and background noise removal. The recording medium  34  stores the digital signal Sd 3 . When the recording device is to play the recorded sound, the play circuit  36  obtains the digital signal Sd 3  from the recording medium  34  to generate an analog signal Sa 5  for driving the speaker  10  to play sound. The play circuit  36  has a digital processor  38 , a digital-to-analogy converter (DAC)  40  and an analog processor  42 . The digital processor  38  processes the digital signal Sd 3  to generate a digital signal Sd 4 . The signal processing applied to the digital signal Sd 3  by the digital processor  38  may include volume adjustment. The DAC  40  converts the digital signal Sd 4  into an analog signal Sa 6 . The analog processor  42  processes the analog signal Sa 6  to generate an analog signal Sa 5 . The analog processor  42  functions like the analog processors  14  and  16 . 
         [0013]    Analog processors and digital processors for applying signal processing to sound in a recording device or a communication system are well known, for example, as mentioned in U.S. Pat. Publication No. 2011/0181452. Circuit and operation of a wind noise extractor may also refer to U.S. Pat. Nos. 7,174,023 and 7,895,036 and U.S. Pat. Publication No. 2009/0002498 for details. Thus, the circuits and operation of the analog processors  14 ,  16  and  42 , the digital processors  32  and  38 , and the wind noise extractor  26  are not described herein. 
         [0014]    Since major energy of wind noise is located as low frequency, a speaker as a microphone is enough to characterize it, and eliminates the requirement of additional microphones. Likewise, it may also use a speaker as a microphone to pick up other acoustic or mechanical noise and reduce those noise in other applications. For example, a speaker can be adopted to receive motor noise of lens module of a digital video camera or other collision noise occurred at recording device itself. 
         [0015]    In some applications, the equalizer  24  and the digital processors  32  and  38  shown in  FIG. 1  may be omitted in some applications, as shown in  FIG. 2 . In this embodiment, the noise detector  22  does not include an equalizer, and the wind noise extractor  26  directly uses the digital signals Sd 1  and Sd 2  to locate characteristics of noise to generate the digital signal Sn. The noise processor  28  does not further apply digital signal processing to the output of the subtractor  30  that subtracts Sn from Sd 1 , and directly sends it to the recording medium  34 . Likewise, the play circuit  36  does not further apply digital signal processing to the digital signal Sd 3  before converting it into the analog signal Sa 6 . 
         [0016]    While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.