Abstract:
A time-division method for playing multi-channel voice signals is provided to improve voice qualities of the multi-channel voice signals. The first feature of the present invention is that after time-division sampling, the voice signals can be directly sent to a subsequent voice generator without being demodulated beforehand. The second feature is that the ratio of a specific voice signal to a synthesized voice can be easily adjusted by changing the numbers of channels occupied by the specific voice. Consequently, the adjusting circuitry can be significantly simplified.

Description:
BACKGROUND OF THE INVENTION 
   A. Field of the Invention 
   The invention relates to a synthesizing method of multi-channel voice signals. In particular, the invention relates to a synthesizing method, which utilizes time-division theory, capable of playing multi-channel voice signals with good voice qualities and easily adjusting the ratio of voice signals. 
   B. Description of the Prior Art 
   Conventionally, voice signals can be played via digital/analog conversion (DAC) or pulse width modulation (PWM) in consumer ICs. Synthesis of the multi-channel voice signals can be implemented by current summing or by using adder. 
     FIG. 1  is a block diagram showing a circuitry for playing four-channel voice signals by using DAC method and current summing. With reference to  FIG. 1 , the four channels  16  each has an associated voice data generator  11  and an associated D/A converter  12 . When the four-channel voice is to be played, each voice data generator  11  generates a voice data that will be converted into an analog signal by the associated D/A converter  12 . Then, the output signals of the D/A converters  12  are wire-anded and thus current summing is performed. The summed signal is amplified by a power amplifier  14  and then sent to a speaker  15  to generate the multi-channel voice. 
   Good voice qualities can be obtained when using the above-mentioned method to play multi-channel voice. However, the disadvantage is that similar circuit unit (DAC circuit) needs to be provided for each channel. Consequently, the entire circuitry will occupy a large area. 
     FIG. 2  is a block diagram showing a circuitry for playing four-channel voice signals by using an adder. When four-channel voice signals are to be played, the voice data generator  21  of each channel generates a voice data. Then, the two least significant bits of the voice signal in each channel are removed (that is, divide by four), and the voice signal is sent to an adder  22  to obtain a summed data. The summed data are then sent to a D/A converter  23  to be converted into an analog signal. The analog signal is amplified by a power amplifier  24  and then sent to a speaker  25  to generate the multi-channel voice. 
   The associated disadvantages of the method are that it requires an extra adder circuit and the resolutions of voices are lowered. 
     FIG. 3  is a block diagram showing a conventional circuitry for playing four-channel voice signals by using a PWM method and current summing. When multi-channel voice is to be played, the voice data generator  31  of each channel generates a respective voice signal. Then, each voice signal is sent to a PWM voice generator  32 . Conventionally, the PWM voice generator  32  includes a high-speed counter and a switch. According to the positive phase or negative phase of sound wave, each PWM voice generator  32  converts the associated voice signal into two binary bits, and represents the voice volume by pulse width. Subsequently, the first bit BIT 1  and the second bit BIT 2  of the output of each channel are wire-anded such that current summing are implemented. The summed signals are sent to a speaker  34  to generate voices. 
   A disadvantage for playing multi-channel voices using a PWM method and current summing is that similar sound generator circuit unit needs to be provided for each channel. Consequently, the entire circuitry occupies a large area. Moreover, since each output is connected to each other, the output current of each channel flows to other channels, which results in waste of power. 
     FIG. 4  is a block diagram showing a conventional circuitry for playing four-channel voice signals by using a PWM method and an adder. When the four-channel voice signals are to be played, each voice data generator  41  generates a voice signal. Next, two least significant bits of the voice signal in each channel are removed (that is, divided by four) and then sent to an adder  42  to obtain a summed data. Subsequently, the summed data are sent to a PWM voice generator  43 , which includes a high-speed counter and a switch. The output of the PWM voice generators  43  is sent to a speaker  44  for generating voices. 
   The associated disadvantages of this method is that it requires an extra adder circuit and the resolutions of voices are lowered because synthesis of the four-channel voice signals is based on average pulse width of each channel. 
   The approaches as described above cannot provide excellent synthesis of multi-channel voice signals. In view of this, a time-division multiplexing (TDM) method, which is widely used in communication systems, is utilized to play multi-channel voice signals, thereby solve the above-mentioned problems. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a time-division method for playing multi-channel voice signals, thereby improve voice qualities of the multi-channel voice signals. 
   In accordance with the present invention, the time-division method for playing multi-channel voice signals comprises the following steps: 
   (1) Each voice data generator generates a voice signal of an associated channel. 
   (2) Under the control of a set of periodical channel selecting signals, a channel selector is utilized to successively sample the voice signals of the channels such that a time-division signal comprising periodically alternative voice signal can be generated. The sampling rate is that each channel is sampled once per cycle. 
   (3) The channel selector sends the time-division signal without being demodulated to a voice generator including a power amplifier, for driving a speaker to generate voices. 
   (4) Voice volume can be adjusted by adjusting numbers of channels being occupied by that voice, thereby achieves the effect of mixing voices. 
   The first feature of the present invention is that after several voice signals are subject to time-division sampling, the voice signals are directly inputted to a subsequent voice generator without being demodulated, thereby achieves the effect of mixing voices. The second feature is that the ratio of a specific voice signal in the synthesized voice can be adjusted by changing the numbers of channels occupied by that voice signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become apparent by reference to the following description and accompanying drawings, wherein: 
       FIG. 1  is a block diagram showing a circuitry for playing four-channel voice signals using a DAC method and current summing; 
       FIG. 2  is a block diagram showing a circuitry for playing four-channel voice signals using a DAC method and an adder; 
       FIG. 3  is a block diagram showing a circuitry for playing four-channel voice signals using a PWM method and current summing; 
       FIG. 4  is a block diagram showing a circuitry for playing four-channel voice signals using a PWM method and an adder; 
       FIG. 5  is a schematic diagram showing a conventional system of a time-division multiplexing method; 
       FIG. 6  is a block diagram showing a circuitry for playing four-channel voice signals using a time-division multiplexing method according to an embodiment of the present invention; 
       FIG. 7  is a diagram showing the waveform of a channel selecting signal SC which comprises two periodical signals S f1 , and S f2 ; 
       FIG. 8  is a block diagram showing a conventional PWM voice generatorry. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   To better understand the spirit of the present invention, a conventional time-division method, which is widely applied to various communication systems, is explained hereinafter. 
     FIG. 5  shows a conventional communication system which utilizes a time-division multiplexing method. As shown in  FIG. 5 , four voice signals X 1 ˜X 4  are respectively pre-filtered by low pass filters LPF 1 ˜LPF 4 , and then sent to a transmitter  52 . Subsequently, in the transmitter  52 , a multiplexer (not shown), which receives the four output signals of the low pass filters LPF 1 ˜LPF 4  as its input signals, samples each input signal once per switching cycle. Therefore, the output signal of the transmitter  52  has a waveform of pulse amplitude modulation (PAM), wherein the periodically alternative sampled signals are included. These signals are sent to a receiver  54  via a transmission channel  53 . In the receiver  54 , a demultiplexer (not shown), which receives the signal from the transmission channel  53  as its input signal, divides the received signal into four signals and sends them to four low pass filters LPF 1 ′˜LPF 4 ′ respectively. Finally, four output signals X 1 ′˜X 4 ′, which represent the four original voice signals can be derived. In theory, if each voice signal has the same bandwidth, then a sampling frequency larger than or equal to two times of the bandwidth can be utilized. 
   The time-division method for playing multi-channel voice signals according to the present invention is based on the above-mentioned time-division multiplexing method. 
     FIG. 6  is a block diagram showing a circuitry for playing four-channel voice signals with eight-bits resolutions by using a time-division method according to the present invention. Referring to  FIG. 6 , each one of the voice data generators  61  generates an eight-bits voice signal Si (i=1,2,3,4), respectively. Since the most significant bit (MSB) of each of the voice signals Si (i=1,2,3,4) can be “0” or “1”, each of these voice signals S 1 ˜S 4  has a positive half cycle and a negative half cycle. The remaining seven bits represent the envelope of the associated voice signal (or the amplitude of the associated voice signal). 
   On the other hand, a two-bits channel selecting signal S c  is sent to the channel selector  62 . In this embodiment, the channel selecting signal S c  consists of two synchronous periodical signals S f1  and S f2 , which respectively have frequencies of f 1  kHz and f 2  kHz (f 2 =2f 1 ), as illustrated in  FIG. 7 . Accordingly, four different states of the channel selecting signal S c  can be obtained, as illustrated in table 1. 
   
     
       
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               State of signal S C   
               Signal S f1   
               Signal S f2   
             
             
                 
             
           
           
             
               State 1 
               “1” 
               “1” 
             
             
               State 2 
               “1” 
               “0” 
             
             
               State 3 
               “0” 
               “1” 
             
             
               State 4 
               “0” 
               “0” 
             
             
                 
             
           
        
       
     
   
   If the states 1˜4 of the signal S c  corresponds to each of the voice data generators 1˜4, the channel selector  62  can sample the output signals S 1 ˜S 4  of the voice data generators  61  once per sampling cycle under the control of the channel selecting signal S c . Subsequently, the channel selector  62  directly outputs sampled signals S d  (MSB) which is periodically alternative and envelop signals S 0 ˜S 6  to a voice generator  63  (by using a DAC method or a PWM method). 
   Taking the PWM voice generatorry as an example, the method for driving voice generator using the sampling signals (S d (MSB) and S 0 ˜S 6 ) are illustrated as follows.  FIG. 8  is a block diagram showing a conventional PWM voice generatorry. With reference to  FIG. 8 , the one-bit voice signal S d (MSB) and seven-bits voice signal S 0 ˜S 6  outputted from the channel selector  62  are sent to a high speed counter  9  and a switch  8  of the PWM voice generator, respectively. The envelope signal S 0 ˜S 6  is connected to the input terminal of the high speed counter  9 , wherein a control signal C r  is used to control the counting of the high speed counter  9 . The overflow signal of the high-speed counter  9  is used for generating a high frequency pulse S se , whose width represents amplitude of the voice signals. Subsequently, the high frequency pulse S se  is sent to a switch  8 . Under the control of S d (MSB), the switch  8  separates the high frequency pulse S se  into a positive half cycle signal SS sp1  and a negative half cycle signal SS sp2  that represent the voice in that period. Finally, the signals SS sp1  and SS sp2  are sent to a speaker  7  to directly drive the speaker  7  to generate voices. 
   If several input channels are connected to one voice source, or several channels are distributed to a particular voice source, then amplitudes of the voice signals (speech or music) can be adjusted. Consequently, volume can be easily controlled and numbers of channels occupied by a specific voice signal can be easily adjusted. 
   As illustrated in the above embodiment, by using the time-division multiplexing method to play multi-channel voice signals in accordance with the present invention, the problem that several sets of voice generators are required and thus occupies a large area when current summing is utilized can be avoided. Besides, the problem of a deteriorated resolution caused by incorporating an adder can be avoided as well. Moreover, no extra circuit is needed since the ratio of a specific voice in the multi-channel voices can be adjusted by controlling numbers of channels occupied by a specific voice signal. 
   While the present invention has been described with reference to the specific embodiments, the description is only illustrative and is not to be construed as limiting the invention. For example, the numbers of channels are not limited to four. Moreover, the time-division method can be applied to a voice generator, which uses only DAC method or any other types of voice generators.