Patent Publication Number: US-8111099-B2

Title: Multi-channel audio playback apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to audio playback apparatuses and methods, and in particular relates to a multi-channel audio playback apparatus and method. 
     2. Description of the Related Art 
     Switching amplifiers, also named as class D amplifiers, are used as audio playback power amplifiers and have become more and more popular in portable devices due to their power efficiency. Moreover, switching amplifiers do not require heat sink devices to dissipate heat, thus, taking up less volume when used in portable devices. 
       FIG. 1  shows a schematic diagram of a conventional multi-channel audio playback apparatus. The multi-channel audio playback apparatus  100  comprises a serial-to-parallel data formatter  102 , a switching amplifier  104 , and loudspeakers  191  and  192 . 
     The serial-to-parallel data formatter  102  receives multi-channel digital data  120  from a source (not shown) and separates the multi-channel digital data  120  in serial format into first channel digital data  121  and second channel digital data  122  in parallel format. As is well known in the art, the first channel digital data  121  and second channel digital data  122  can be left channel data and right channel data in a stereo audio system. Moreover, the serial-to-parallel data formatter  102  can separate the multi-channel digital data  120  into five channels which are left, right, center, left-back, right-back and subwoofer channels in a Dolby 5.1 system. 
     Taking a stereo audio system for example, the switching amplifier  104  further comprises a first digital-to-analog converter (DAC)  141 , a second DAC  142 , a reference signal generator  110 , a first comparator  151 , a second comparator  152 , a first driver  161  and a second driver  162 . The first DAC  141  and the second DAC  142  respectively convert the first channel digital data  121  and the second channel digital data  122  into first channel analog data  131  and second channel analog data  132 . The reference signal generator  110  generates a reference signal  111  with a specific frequency and outputs the reference signal  111  to the first comparator  151  and the second comparator  152 . 
       FIG. 2A  illustrates the relationship between the first channel analog data  131  and the reference signal  111  of  FIG. 1 . The first comparator  151  receives the first channel analog data  131  from the first DAC  141  and the reference signal  111  from the reference signal generator  110  and compares the first channel analog data  131  with the reference signal  111  in order to generate the first pulse width modulation (PWM) signal  181 .  FIG. 2B  illustrates the first PWM signal of  FIG. 1 . To explain in detail, when the first channel analog signal  131  is higher than the reference signal  111 , the first PWM signal  181  is high (labeled as “1” in  FIG. 2B ). When the first channel analog signal  131  is lower than the reference signal  111 , the first PWM signal  181  is low (labeled as “0” in  FIG. 2B ).  FIG. 2C  illustrates the relationship between the second channel analog data  132  and the reference signal  111  of  Fig. 1  and  FIG. 2D  illustrates the second PWM signal  182  of  FIG. 1 . Accordingly, the second comparator  152  compares the second channel analog data  132  with the reference signal  111  in order to generate the second PWM signal  182 . Then, the first driver  161  and the second driver  162  respectively use the first PWM signal  181  and the second PWM signal  182  to drive the first loudspeaker  191  and the second loudspeaker  192 . 
     However, while the multi-channel audio playback apparatus  100  is playing sounds through the loudspeaker  191  and  192 , severe radio frequency (RF) interference occurs.  FIGS. 3A ,  3 B and  3 C respectively shows the frequency spectrum of the first PWM signal  181 , the second PWM signal  182  and combinations thereof of  FIG. 1 . The first PWM signal  181  in the frequency spectrum comprises a first channel audio frequency  312  corresponding to the first channel analog data  131  and a first carrier frequency  314  corresponding to the reference signal  111 . Accordingly, the second PWM signal  182  in the frequency spectrum comprises a second channel audio frequency  322  corresponding to the second channel analog data  132  and a second carrier frequency  324  corresponding to the same reference signal  111 , wherein the first carrier frequency  314  is the same as the second carrier frequency  324 . However, while channel analog data  312  and  322  are being played as sounds from the loudspeakers, the carrier frequencies  314  or  324 , in the range of 100 kHz˜400 kHz in most cases, contain non-ideal components in the PWM signals. Since most loudspeakers are made of magnetic materials, non-ideal components in the PWM signals radiate easily within the loudspeakers, thus affecting radio signals. In addition, with the same frequency (as shown in  FIG. 3C ), radio signals are further deteriorated when the amplitude of the second carrier frequency  324  is superposed onto the amplitude of the first carrier frequency  314 . For example, the intensity of EMI caused by a 5.1 Dolby audio system is about  6  times higher than that caused by a mono-channel audio system. 
     As such, reducing RF interference of multi-channel audio playback apparatuses is desired. 
     BRIEF SUMMARY OF INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     In a first aspect of the present invention, a multi-channel audio playback apparatus comprising a channel interface, a first switching amplifier and a second switching amplifier is provided. The channel interface is used to receive multi-channel digital data and generate first channel digital data and second channel digital data. The first switching amplifier is used to convert the first channel digital data into a first pulse width modulation (PWM) signal according to a first reference signal with a first frequency, and the second switching amplifier is used to convert the second channel digital data into a second PWM signal according to a second reference signal with a second frequency, wherein the second frequency is different from the first frequency. 
     In a first aspect of the present invention, a multi-channel audio playback method comprises the step of receiving multi-channel digital data and generating first channel digital data and second channel digital data. Next, a first reference signal with a first frequency and a second reference signal with a second frequency are generated, wherein the second frequency is different from the first frequency. Following, the first channel digital data is converted into a first pulse width modulation (PWM) signal according to the first reference signal with the first frequency, and the second channel digital data is converted into a second PWM signal according to the second reference signal with the second frequency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a schematic diagram of a conventional multi-channel audio playback apparatus; 
         FIG. 2A  illustrates the relationship between the first channel analog data  131  and the reference signal of  FIG. 1 ; 
         FIG. 2B  illustrates the first PWM signal of  FIG. 1 ; 
         FIG. 2C  illustrates the relationship between the second channel analog data  132  and the reference signal of  FIG. 1 ; 
         FIG. 2D  illustrates the second PWM signal of  FIG. 1 ; 
         FIGS. 3A ,  3 B and  3 C respectively shows the frequency spectrum of the first PWM signal, the second PWM signal and combinations thereof of  FIG. 1 ; 
         FIG. 4  shows a schematic diagram of a multi-channel audio playback apparatus according to the present invention; 
         FIG. 5A  illustrates the relationship between the first channel analog data  431  and the first reference signal of  FIG. 4 ; 
         FIG. 5B  illustrates the first PWM signal of  FIG. 4 ; 
         FIG. 5C  illustrates the relationship between the second channel analog data  432  and the second reference signal of  FIG. 4 ; 
         FIG. 5D  illustrates the second PWM signal of  FIG. 4 ; 
         FIGS. 6A ,  6 B and  6 C respectively shows the frequency spectrum of the first PWM signal, the second PWM signal and combinations thereof of  FIG. 4 ; 
         FIG. 7A  is a flow chart of the multi-channel audio playback method according to the present invention; 
         FIG. 7B  is a detailed flow chart of the step S 704  of  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 4  shows a schematic diagram of a multi-channel audio playback apparatus according to the present invention. For convenience, the multi-channel audio playback apparatus  400  is described as a two-channel audio playback apparatus (stereo audio system) hereinafter, however, those skilled in the art will appreciate that the invention is not limited in this regard. The multi-channel audio playback apparatus  400  comprises a channel interface  402 , a first switching amplifier  404 , a second switching amplifier  405 , a first loudspeaker  491  and a second loudspeaker  492 . 
     The channel interface  402  can be a serial-to-parallel data formatter, which receives multi-channel digital data  420  from a source (not shown) and separates the multi-channel digital data  420  in serial format into first channel digital data  421  and second digital data  422  in parallel format. The first switching amplifier  404  further comprises a first digital-to-analog converter (DAC)  441 , a first reference signal generator  411 , a first comparator  451  and a first driver  461 . The second switching amplifier  405  further comprises a second DAC  442 , a second reference signal generator  412 , a second comparator  452 , and a second driver  462 . In this embodiment, the DACs, comparators, reference signal generators and drivers herein are disposed in pairs to be applied on two channels. In other embodiments, the number of DACs, comparators, and reference signal generators increase along with the number of channels that a multi-channel audio playback apparatus has. The first DAC  441  and the second DAC  442  respectively convert the first channel digital data  431  and the second channel digital data  432  into first channel analog data  441  and second channel analog data  442 . The first reference signal generator  411  generates a first reference signal  471  with a first frequency and outputs the first reference signal  471  to the first comparator  451 , and the second reference signal generator  412  generates a second reference signal  472  with a second frequency and outputs the second reference signal  472  to the second comparator  452 . The first reference signal  471  and the second reference signal  472  are provided to the first comparator  451  and the second comparator  452  respectively and independently. Specifically, the first frequency of the first reference signal  471  is different from the second reference signal  472 , which will be described as follows. 
       FIG. 5A  illustrates the relationship between the first channel analog data  431  and the first reference signal  471  of  FIG. 4 . In this embodiment, the first channel analog data  431  is a sine wave with a frequency, for example, of 7 kHz, while the first reference signal  471  is a saw-toothed wave with a first frequency, for example, of 100 kHz. The first comparator  451  receives the first channel analog data  431  from the first DAC  441  and the first reference signal  471  from the first reference signal generator  410  and compares the first channel analog data  431  with first reference signal  471  in order to generate a first pulse width modulation (PWM) signal  481 .  FIG. 5B  illustrates the first PWM signal  481  of  FIG. 4 . Like the prior art described above, when the first channel analog signal  431  is higher than the first reference signal  471 , the first PWM signal  481  is high (labeled as “1” in  FIG. 5B ). When the first channel analog signal  431  is lower than the first reference signal  471 , the first PWM signal  481  is low (labeled as “0” in  FIG. 5B ).  FIG. 5C  illustrates the relationship between the second channel analog data  432  and the second reference signal  472  of  FIG. 4  and  FIG. 5D  illustrates the second PWM signal  482  of  FIG. 4 . In this embodiment, for example, the second channel analog data  432  is a sine wave with a frequency, for example, 13.3 kHz, while the second reference signal  471  is a saw-toothed wave with a second frequency, for example, 131 kHz. Accordingly, the second comparator  452  compares the second channel analog data  432  with the second reference signal  470  in order to generate the second PWM signal  482 . Then, the first driver  461  and the second driver  462  respectively use the first PWM signal  481  and the second PWM signal  482  to drive the first loudspeaker  491  and the second loudspeaker  492 . Then, the first driver  461  and the second driver  462  respectively use the first PWM signal  481  and the second PWM signal  482  to drive the first loudspeaker  491  and the second loudspeaker  492 . 
     To summarize, the first switching amplifier  404  converts the first channel digital data  421  into the first PWM signal  481  according the first reference signal  471  with a first frequency, while the second switching amplifier  405  converts the second channel digital data  422  into the second PWM signal  482  according the second reference signal  472 , wherein the second frequency is different from the first frequency.  FIG. 6A and 6B  respectively show the frequency spectrum of the first PWM signal  481  and the second PWM signal  482 . The first PWM signal  481  in the frequency spectrum comprises a first channel audio frequency  612  corresponding to the first channel analog data  431  and a first carrier frequency  614  corresponding to the first reference signal  471 . Accordingly, the second PWM signal  482  comprises a second channel audio frequency  622  corresponding to the second channel analog data  432  and a second carrier frequency  624  corresponding to the second reference signal  472 . In this embodiment, because the first frequency of the first PWM signal  481 , 100 kHz, is different from the second frequency of the second PWM signal  482 , which is 133 kHz, the first carrier frequency, which is 100 kHz, is different from the second carrier frequency, which is 133 kHz. Specifically, since the two carrier frequencies  614  and  624  are different, the amplitude thereof will not be superimposed together like that in the prior art. Therefore, the RF interference caused by the multi-channel audio playback apparatus  400  according to the present invention is significantly reduced. Moreover, in another embodiment, the first frequency of the first reference signal  471  provided by the first reference signal generator  411  and the second frequency of the second reference signal  472  provided by the second reference signal generator  412  are not only different but also relatively prime frequencies. In this case, the harmonics of the first frequency and the second frequency will exceed the frequency band which causes the RF interference. 
     The following describes a multi-channel audio playback method for reducing the RF interference.  FIG. 7A  is a flow chart of the multi-channel audio playback method according to the present invention. Please refer to  FIGS. 7A and 7B  and  FIG. 4  together. In step S 702 , the serial-to-parallel formatter interface  402  receives multi-channel digital data  420  and generates first channel digital data  421  and second channel digital data  422 . In step S 704 , the first switching amplifier  404  generates a first reference signal  471  with a first frequency, and the second switching amplifier  405  generates a second reference signal  472  with a second frequency, wherein the second frequency is different from the first frequency. In step S 706 , the first switching amplifier  404  converts the first channel digital data  421  into a first PWM signal  481  according to the first reference signal  471  with the first frequency, and the second switching amplifier  405  converts the second channel digital data  422  into a second PWM signal  482  according to the second reference signal  472  with the second frequency. 
       FIG. 7B  is a detailed flow chart of the step S 704  of  FIG. 7A . The method further comprises the steps S 712 , S 714  and S 716 . In step S 712 , the first DAC  441  converts the first channel digital data  421  into first channel analog data  441 , and the second DAC  442  converts the second channel digital data  422  into second channel analog data  432 . In step S 714 , the first comparator  451  compares the first channel analog data  431  with the first reference signal  471  to generate the first PWM signal  481 , and the second comparator  452  compares the second channel analog data  432  with the second reference signal  472  to generate the second PWM signal  482 . In step S 716 , the first driver  461  uses the first PWM signal  481  to drive a first external loudspeaker  491 , and the second driver  462  uses the second PWM signal  482  to drive a second external loudspeaker  492 . 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.