Abstract:
A coder-decoder simultaneously processing a plurality of first analog signals with only one analog-to-digital converter is provided. In an exemplary embodiment, the coder-decoder comprises a multiple access modulator, the analog-to-digital converter, and a multiple access demodulator. The multiple access modulator combines the first analog signals according to a multiple access algorithm to obtain a first multiple access signal comprising the first analog signals. The analog-to-digital converter then converts the first multiple access signal from analog to digital to obtain a second multiple access signal. The multiple access demodulator then separates the second multiple access signal according to the multiple access algorithm to obtain a plurality of first digital signals respectively corresponding to the first analog signals.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to signal processing, and more particularly to coder-decoders (CODEC). 
   2. Description of the Related Art 
   Coder-decoders are circuits converting signals between analog and digital forms. For example, after analog signals are converted to digital signals by coder-decoders, the digital signals can then be processed by digital processors of computers or other devices to generate audio or video samples. Conventional audio and video samples can be further compressed to reduce the transmission bandwidth thereof. 
   Multiple access is a modulation technique used in satellite and long-distance communication. A communication satellite or switching center can simultaneously service multiple satellite terminals or mobile connections via a single communication connection link with multiple access modulation. Because bandwidth of mobile communication systems is limited, the system only assigns bandwidth to clients when clients initiate a communication session and revokes assigned bandwidth for use of other clients when the communication session is over. Thus, the mobile communication system must dynamically allocate bandwidth, and multiple access modulation is suitable for mobile communication systems to reduce bandwidth resource requested by a single client. Current multiple access modulation methods include a Time Division Multiple Access (TDMA) method, a Frequency Division Multiple Access (FDMA) method, and a Code Division Multiple Access (CDMA) method. 
   FDMA slices a frequency band into multiple channels of equal bandwidth, each FDMA channel carrying data of a specific client. TDMA slices a transmission period into multiple time slots of equal length, each transmitting data of a specific client. CDMA modulates data of multiple clients with spreading codes to obtain a CDMA signal, and a CDMA receiver retrieves data of a specific client from the CDMA signal with the specific spreading code corresponding to the specific client. Thus, the bandwidth of a mobile communication system can be dynamically allocated to multiple clients simultaneously accessing the system. 
   A conventional codec requires an analog-to-digital converter for conversion of audio or video signals. If a codec has multiple audio inputs or video inputs, the codec requires multiple analog-to-digital converters accordingly. The multiple analog-to-digital converters occupy considerable chip area of a codec chip, increasing production costs. For example, two analog-to-digital converters can substantially occupy 40% area of a codec chip, complicating chip design and increasing production cost. If a codec handles multiple analog inputs with a single analog-to-digital converter, cost of the codec is effectively decreased. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides a coder-decoder simultaneously processing a plurality of first analog signals with only one analog-to-digital converter. In an exemplary embodiment, the coder-decoder comprises a multiple access modulator, the analog-to-digital converter, and a multiple access demodulator. The multiple access modulator modulates the first analog signals according to a multiple access method to obtain a first multiple access signal comprising the first analog signals. The analog-to-digital converter then converts the first multiple access signal from analog to digital to obtain a second multiple access signal. The multiple access demodulator then demodulates the second multiple access signal according to the multiple method to obtain a plurality of first digital signals respectively corresponding to the first analog signals. 
   the invention also provides a method for simultaneously processing a plurality of analog signals with only one analog-to-digital converter. First, a plurality of first analog signals are modulated according to a multiple access method to obtain a first multiple access signal comprising the first analog signals. The first multiple access signal is then converted from analog to digital with the analog-to-digital converter to obtain a second multiple access signal. Finally, the second multiple access signal is demodulated according to the multiple access method to obtain a plurality of first digital signals respectively corresponding to the first analog signals. 
   A detailed description is given in the following embodiments with reference to The accompanying drawings. 

   
     BRIEF DESCRIPITON OF THE DRAWINGS 
     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of a portion of a codec according to the invention, wherein the codec simultaneously converts multiple input signals from analog to digital with an analog-to-digital converter according to a TDMA method; 
       FIG. 2  is a schematic diagram of the TDMA signal shown in  FIG. 1 ; 
       FIG. 3  is a block diagram of a portion of a codec according to the invention, wherein the codec simultaneously converts multiple input signals from analog to digital with an analog-to-digital converter according to a FDMA method; and 
       FIG. 4  is a block diagram of a portion of a codec according to he invention, wherein the codec simultaneously converts multiple input signals from analog to digital with an analog-to-digital converter according to a CDMA method 
   

   DETAILED DESCRIPTION OF THE 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. 
   The invention provides a multiple access method modulating multiple analog audio or video signals to obtain a single multiple access signal, and then converts the multiple access signal from analog to digital with a single analog-to-digital converter. The multiple access method can be a TDMA method, a FDMA method, or a CDMA method. Thus, the codec requires only one analog-to-digital converter. Because the number of analog-to-digital converters is decreased, the chip area occupied by the analog-to-digital converter in the codec chip is reduced, as is manufacture cost of the codec chip. 
     FIG. 1  is a block diagram of a portion of a codec  100  according to the invention, wherein the codec  100  simultaneously converts multiple input signals from analog to digital with an analog-to-digital converter  106  according to a TDMA method. The codec  100  receives multiple analog input signals A 1 ˜A N , and the codec  100  must convert analog input signals A 1 ˜A N  from analog to digital before the input signals are further decoded. The input signals A 1 ˜A N  may be analog audio signals, such as multi-channel audio signals, or analog video signals, such as three video inputs of RGB format. 
   Input low pass filters  102 A˜ 102 N first respectively filter off high frequency noise from the analog input signals A 1 ˜A N  to obtain analog input signals A 1 ′˜A N ′, fitting the frequency band of the analog input signals A 1 ′˜A N ′ to the range derived from the sampling rate of the analog-to-digital converter  106  according to Shannon-Nyquist sampling theorem and avoiding signal distortion due to inter-symbol interference. Thus, the input low pass filters  102 A˜ 102 N are also referred to as “anti-alias filters”. The filtered analog input signals A 1 ′˜A N ′ are then delivered to a TDMA modulator  104 . The TDMA modulator  104  modulates the analog input signals A 1 ′˜A N ′ according to a TDMA method to an analog TDMA signal T 1 . 
     FIG. 2  is a schematic diagram of the TDMA signal T 1  shown in  FIG. 1 . If a TDMA system must simultaneously transmit N input signals, the TDMA system slices a transmission period into N time slots with equal length, and each time slot is responsible for transmitting one of the N input signals. Thus, a communication link can simultaneously transmit N input signals. For example, a TDMA frame  200  is sliced into N time slots respectively corresponding to the analog input signals A 1 ′˜A N ′, wherein time slots  202 ,  204 ,  206 ,  208 , and  210  respectively transmit a portion of the analog input signals A 1 ′, A 2 ′, A N-2 ′, A N-1 ′, and A N ′. 
   The TDMA modulator  104  includes a commutator  114  and a pulse width modulator  124 . The pulse width modulator  124  generates pulse waves with pulse widths respectively proportional to the amplitudes of the analog input signals A 1 ′˜A N ′. The commutator  114  respectively inserts one of the pulse waves generated by the pulse width modulator  124  into corresponding time slots of the TDMA signal T 1 . The analog-to-digital converter  106  then converts the TDMA signal T 1  from analog to digital to obtain a digital TDMA signal T 2 . The sampling rate of the analog-to-digital converter  106  is at least N times the sampling frequency of the pulse width modulator  124 . The digital TDMA signal T 2  is then delivered to a TDMA demodulator  108 . The TDMA demodulator  108  then demodulates the digital TDMA signal T 2  according to the TDMA method to obtain N digital signals D 1 ′˜D N ′ respectively corresponding to the analog input signals A 1 ′˜A N ′. 
   The TDMA demodulator  108  includes a de-commutator  118  and a pulse width demodulator  128 . The de-commutator  118  respectively retrieves the digital samples corresponding to the analog input signals A 1 ′˜A N ′ from the time slots of the digital TDMA signal T 2  to obtain the N sample series respectively corresponding to the analog input signals A 1 ′˜A N ′. The pulse width demodulator  128  then implements pulse width demodulation to demodulate the N sample series from high to low frequency to obtain digital signals D 1 ′˜D N ′ respectively corresponding to the analog input signals A 1 ′˜A N ′. 
   Output low pass filters  110 A˜ 110 N then filter off high frequency noise generated in the demodulation process of the TDMA demodulator  108  from the digital signals D 1 ′˜D N ′ to obtain the digital signals D 1 ˜D N , wherein the digital signals D 1 ˜D N  respectively correspond to one of the analog input signals A 1 ˜A N . The output low pass filters  110 A˜ 110 N are also referred to as “decimation filters” or “reconstruction filters”. 
   In addition, the commutator  114  and the de-commutator  118  must be synchronized. Thus, the de-commutator  118  can correctly identify the samples inserted in the times slots of the TDMA signal T 2  by the commutator  114  and correctly retrieve the samples corresponding to the analog signals A 1 ′˜A N ′ to precisely generate the sample series corresponding to the analog signals A 1 ′˜A N ′. Otherwise, if the commutator  114  and the de-commutator  118  are not synchronized, the de-commutator  118  may erroneously put samples of a time slot corresponding to an ananlog signal A X ′ into sample series corresponding to an analog signal A Y ′. The synchronization can be achieved by a synchronization module independent of the TDMA modulator  104  and the TDMA demodulator  108 . Moreover, modulation implemented by the pulse width modulator  124  and demodulation implemented by the pulse width demodulator  128  must also be synchronized. 
     FIG. 3  is a block diagram of a portion of a codec  300  according to the invention, wherein the codec  300  simultaneously converts multiple input signals from analog to digital with an analog-to-digital converter  306  according to a FDMA method. The codec  300  receives multiple analog input signals A 1 ˜A N , and the codec  300  must convert the analog input signals A 1 ˜A N  from analog to digital before the input signals are further processed. The input signals A 1 ˜A N  may be analog audio signals, such as multi-channel audio signals, or analog video signals, such as three video inputs of RGB format. 
   Input low pass filters  302 A˜ 302 N first respectively filter off high frequency noise from the analog input signals A 1 ˜A N  to obtain analog input signals A 1 ′˜A N ′, fitting the frequency band of the analog input signals A 1 ′˜A N ′ to the range derived from the sampling rate of the analog-to-digital converter  306  according to Shannon-Nyquist sampling theorem and avoiding signal distortion due to inter-symbol interference. Thus, the input low pass filters  302 A˜ 302 N are also referred to as “anti-alias filters”. The filtered analog input signals A 1 ′˜A N ′ are then delivered to a FDMA modulator  304 . The FDMA modulator  304  modulates the analog input signals A 1 ′˜A N ′ according to a FDMA method to an analog FDMA signal F 1 . 
   If a FDMA system must simultaneously transmit N input signals, the FDMA system modulates the N input signals with N carriers of different frequencies to obtain N modulated signals and then mixes the N modulated signals to obtain a single FDMA signal. Thus, N input signals can be transmitted with the same communication link. A frequency synthesizer  312  of  FIG. 3  first generates N carriers of different frequencies. The FDMA modulator  304  then modulates the input analog signals A 1 ′˜A N ′ with the N carriers generated by the frequency synthesizer  312  to obtain N modulated signals. The FDMA modulator  304  further comprises a mixer and a band pass filter. The mixer mixes the N modulated signals to obtain a single mixed signal, and the band pass filter filters the mixed signal to eliminate noise. Finally, an analog FDMA signal F 1  is generated by the FDMA modulator  304 . 
   The analog-to-digital converter  306  then converts the FDMA signal F 1  from analog to digital to obtain a digital FDMA signal F 2 . The digital FDMA signal F 2  is then delivered to a FDMA demodulator  308 . The FDMA demodulator  308  then demodulates the digital FDMA signal F 2  with the carriers generated by the frequency synthesizer  312  to obtain N digital signals D 1 ′˜D N ′ respectively corresponding to the analog input signals A 1 ′˜A N ′. 
   Output low pass filters  310 A˜ 310 N then filter off high frequency noise generated in the demodulation process of the FDMA demodulator  308  from the digital signals D 1 ′˜D N ′ to obtain the digital signals D 1 ˜D N , wherein the digital signals D 1 ˜D N  respectively correspond to one of the analog input signals A 1 ˜A N . The output low pass filters  310 A˜ 310 N are also referred to as “decimation filters” or “reconstruction filters”. 
   In addition, the FDMA modulator  304  and the FDMA demodulator  308  must be synchronized, such that demodulator  308  can correctly demodulate the FDMA signal F 2  with adequate carriers to precisely generate the digital signals D 1 ′˜D N ′ corresponding to the analog signals A 1 ′˜A N ′. The synchronization can be achieved by a synchronization module independent of the FDMA modulator  304  and the FDMA demodulator  308 . 
     FIG. 4  is a block diagram of a portion of a codec  400  according to the invention, wherein the codec  400  simultaneously converts multiple input signals from analog to digital with an analog-to-digital converter  406  according to a CDMA method. The codec  400  receives multiple analog input signals A 1 ˜A N , and the codec  400  must convert the analog input signals A 1 ˜A N  from analog to digital before the input signals are further processed. The input signals A 1 ˜A N  may be analog audio signals, such as multi-channel audio signals, or analog video signals, such as three video inputs of RGB format. 
   Input low pass filters  402 A˜ 402 N first respectively filter off high frequency noise from the analog input signals A 1 ˜A N  to obtain analog input signals A 1 ′˜A N ′, fitting the frequency band of the analog input signals A 1 ′˜A N ′ to the range derived from the sampling rate of the analog-to-digital converter  406  according to Shannon-Nyquist sampling theorem and avoiding signal distortion due to inter-symbol interference. Thus, the input low pass filters  402 A˜ 402 N are also referred to as “anti-alias filters”. The filtered analog input signals A 1 ′˜A N ′ are then delivered to a CDMA modulator  404 . The CDMA modulator  404  modulates the analog input signals A 1 ′˜A N ′ according to a CDMA method to an analog CDMA signal C 1 . 
   If a CDMA system must simultaneously transmit N input signals, a CDMA modulator modulates the N input signals with different CDMA codes to obtain a single CDMA signal carrying the N input signals. Thus, N input signals can be transmitted with the same communication link. After a receiver receives the CDMA signal through the communication link, a CDMA demodulator demodulates the CDMA signal with the CDMA codes identical to those used by the CDMA modulator to recover the N input signals. The code generator  412  of  FIG. 4  first generates different CDMA codes. The CDMA modulator  404  then modulates the input analog signals A 1 ′˜A N ′ with the CDMA codes generated by the code generator  412  to obtain a single CDMA signal C 1 . 
   The analog-to-digital converter  406  then converts the CDMA signal C 1  from analog to digital to obtain a digital CDMA signal C 2 . The digital CDMA signal C 2  is then delivered to a CDMA demodulator  408 . The CDMA demodulator  408  then demodulates the digital CDMA signal C 2  with the codes generated by the code generator  412  to obtain N digital signals D 1 ′˜D N ′ respectively corresponding to the analog input signals A 1 ′˜A N ′. 
   Output low pass filters  410 A˜ 410 N then filter off high frequency noise generated in the demodulation process of the CDMA demodulator  408  from the digital signals D 1 ′˜D N ′ to obtain the digital signals D 1 ˜D N , respectively corresponding to one of the analog input signals A 1 ˜A N . The output low pass filters  410 A˜ 410 N are also referred to as “decimation filters” or “reconstruction filters”. 
   In addition, the CDMA modulator  404  and the CDMA demodulator  408  must be synchronized. Thus, the CDMA demodulator  408  can correctly demodulate the CDMA signal C 2  with the same CDMA codes as the CDMA modulator  404  to precisely generate the digital signals D 1 ′˜D N ′ corresponding to the analog signals A 1 ′˜A N ′. The synchronization can be achieved by a synchronization module independent of the CDMA modulator  404  and the CDMA demodulator  408 . 
   The invention adopts a multiple access modulation method to decrease the number of analog-to-digital converters required by a codec which must simultaneously process multiple analog video or audio signals. Three embodiments respectively corresponding to a TDMA method, a FDMA method, and a CDMA method are provided. Reducing the number of required analog-to-digital converters further reduces the chip area occupied by the analog-to-digital converter, reducing cost of the codec chip. In addition, because multiple access modulation of analog input signals and multiple access demodulation of a multiple access modulated signal is synchronized in the codec, the digital signals generated in the multiple access demodulation are synchronized to reduce signal distortion due to asynchronicity between the digital signals, such as ghost shadows or ripples of video signals. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. 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.