Abstract:
An apparatus and method for conversion of direct stream digital (DSD) signal samples to pulse code modulated (PCM) signal samples using a look-up table. The apparatus includes a first-in-first-out (FIFO) buffer that contains a plurality of bits from a DSD signal, the plurality of bits further divided into a plurality of words of the same size. The apparatus comprises a look-up table coupled to the FIFO buffer, the look-up table generating a result for each of the plurality of words. In one embodiment, the apparatus includes an accumulator coupled to the look-up table, the accumulator holding the results added together. After adding the result for the last word in the plurality of bits, the accumulator generates at an output a multiple bit PCM signal sample. The apparatus includes an address generator connected to the FIFO buffer and look-up table.

Description:
[0001]    This application claims priority under 35 USC § 119(e)(1) of Provisional Application No. 60/452,397, filed Mar. 6, 2003. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to conversion of information signals. More particularly, the invention relates to the efficient conversion of audio signals using a look-up table. Still more particularly, the invention relates to the use of a look-up table to efficiently and quickly convert direct stream digital (DSD) signals to pulse code modulated (PCM) signals.  
         BACKGROUND OF THE INVENTION  
         [0003]    Audio systems capable of reproducing digital format signals allow high fidelity sound and theater like effects compared to audio systems that can reproduce analog format signals. New digital audio disk formats such as super audio compact disk (SACD) allow reproduction of an extended range of frequencies compared to that of the more conventional digital compact disk (CD). The SACD contains a DSD signal while CDs contain PCM signals. Each sample of an analog audio signal is translated into either of the two binary digits 0 or 1 in a DSD signal encoder. The number of 0 and 1 binary digits over a given period of time determines the value of the analog audio signal.  
           [0004]    PCM audio signal samples on a CD are translated into 16 bits representing the value of the analog audio signal. PCM signals have a uniform time period between samples, with the rate of sampling varying from 4 KiloHertz (KHz) to 192 KHz. DSD signals are sampled at much higher rates such as 2.8224 MegaHertz (MHz). Thus, an audio signal sampled using a PCM system at 44.1 KHz sampling rate and a DSD system at 2.8224 MHz sampling rate would result in 64 DSD samples occurring between each PCM sample.  
           [0005]    Sampling an analog audio signal into a DSD signal may be performed by a sigma-delta modulator. A sigma-delta modulator includes analog circuitry that captures the analog audio signal and converts it into a single bit stream. Because of its single bit format, DSD signals can be converted back into the analog audio signal using minimal hardware. However, manipulation of the single bit DSD stream representing the analog audio signal can be very difficult. For example, tasks such as increasing the volume, adjusting treble or bass, etc. is very difficult because the DSD signal cannot be easily processed using existing digital filters and digital signal processing (DSP) techniques. One solution is to convert the DSD signal into a PCM signal using a Finite Impulse Response (FIR) digital filter. PCM signals can be processed using digital filters and DSP techniques to allow manipulation of the PCM signal to accomplish tasks such as increasing the volume or adjusting bass and for more complex tasks such as surround sound effects. After manipulation of the PCM signal, the signal may be converted back to a DSD signal and/or into analog audio signal format and transmit to speakers.  
           [0006]    Conversion of the DSD signal to a PCM signal may require a high quality and expensive FIR digital filter containing large quantities of complex hardware. The DSD to PCM converter may have an odd sized binary multiplier for multiplying 1 bit by the number of bits needed to encode the PCM signal (i.e. 1 by 16 bit multiplier, 1 by 24 bit multiplier, 1 by 32 bit multiplier, and so on . . . ). The DSD to PCM converter may also include a sign controller and an N-coefficient buffer to implement the FIR filter.  
           [0007]    Digital signal processors that do not contain the dedicated hardware described above for DSD to PCM conversion are not capable of efficiently performing this conversion. Thus, there has been a longfelt need for an improved and low-cost method implemented in software or firmware and apparatus for efficient conversion of DSD signals to PCM signals in a digital signal processor (DSP).  
         SUMMARY OF THE INVENTION  
         [0008]    The problems noted above are solved in large part by a method for conversion of signals that includes receiving a first plurality of bits from a first signal. The first signal may be a direct stream digital (DSD) signal. The method also includes performing a look-up in a table with a first subset of bits in the first plurality of bits to generate a result and adding the result to a sum. The subset of bits is a word. The method further includes performing another look-up in the table with the next subset of bits in the first plurality of bits and adding the result to the sum until a look-up with a last subset of bits in the first plurality of bits is performed and the result added to sum. Next, the method comprises providing the sum as a first multiple bit value of a second signal. The second signal may be a pulse code modulated (PCM) signal. The method also includes receiving a second plurality of bits from the first signal and converting to a second multiple bit value of the second signal using the steps described above until all bits in the first signal have been converted.  
           [0009]    In the preferred embodiment of the invention, the table is a two dimensional array containing a plurality of elements. Preferably, the size of the first dimension equals the number of bits in the plurality of bits divided by the number of bits in the subset of the plurality of bits and the size of the second dimension is equal to 2 (number of bits in subset) . Each element in the table contains one multiple bit result. Preferably, performing the look-up in the table comprises accessing the element in the array that corresponds to the number of the subset in the plurality of bits and the value of the subset of bits.  
           [0010]    An apparatus for conversion of signals is described that includes a first-in-first-out (FIFO) buffer that contains a plurality of bits from a first signal, the plurality of bits further divided into a plurality of subset of bits of the same size. Each subset of bits is a word. The first signal may be a DSD signal. The apparatus comprises a look-up table coupled to the FIFO buffer, the look-up table generating a result for each of the plurality of subset of bits. In the preferred embodiment of the invention, the apparatus includes an accumulator coupled to the look-up table, the accumulator holding the results added together. After adding the result for the last subset of bits in the plurality of bits, the accumulator generates at an output a multiple bit second signal. The second signal may be a PCM signal.  
           [0011]    The apparatus for conversion of signals also includes an address generator connected to the FIFO buffer and look-up table. Preferably, the address generator provides to the look-up table the address of a section in the look-up table corresponding to each of the plurality of subset of bits. Each section includes a plurality of results for each subset of bits, with one of the plurality of results selected by the value of the subset of bits. The address of each section in the look-up table corresponding to each of the plurality of subset of bits is sequential. In some embodiments of the present invention, the look-up table is contained in a memory located on a DSP. In alternative embodiments of the invention, the look-up table is contained in an external memory coupled to the DSP.  
           [0012]    An object of the present invention is to provide a method and apparatus for efficiently converting DSD signals to PCM signals using a look-up table.  
           [0013]    The present invention provides significant advantages over the prior art. One advantage is the simplified circuitry and elimination of special hardware (e.g. 1 bit by 32 bit multiplier) for conversion of DSD signals to PCM signals. Another advantage is the reduced processor resources and bandwidth needed to convert signals using the present invention. The apparatus and method of the present invention can process multiple DSD sample bits (e.g. 16 bits, 32 bits, 64 bits, and so on) to a PCM signal sample in one clock cycle resulting in much faster conversion. Finally, because the present invention may be implemented in firmware or software, another advantage is that modifications to the firmware or software code can be easily and quickly performed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 is a block diagram of a DSD to PCM conversion device containing an N-coefficient buffer;  
         [0016]    [0016]FIG. 2 is a block diagram of a DSD to PCM conversion device that uses a look-up table in accordance with the preferred embodiment of the invention;  
         [0017]    [0017]FIG. 3 shows a flow chart for generating the look-up table shown in FIG. 2 in accordance with the preferred embodiment of the invention; and  
         [0018]    [0018]FIG. 4 shows a flow chart for conversion of DSD signals to PCM signals using the look-up table generated in FIG. 3 in accordance with the preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    [0019]FIG. 1 is a block diagram of a DSD to PCM signal conversion device  100 . The device includes an N-bit delay line  110  that may be implemented as a bit addressable register. N sequential DSD bit samples are stored into the N-bit delay line  110  for conversion to a PCM signal sample  180 . DSD signal bit  105  is the most recently sampled DSD bit. The N-bit delay line  110  receives a plurality of DSD signal bits that may have been sampled from the analog audio signal at a rate of 2.8224 MHz. N-bit delay line  110  may be capable of holding N=32 bits. Alternatively, the N-bit delay line  110  may hold N=16 bits, N=64 bits, N=128 bits, and so on.  
         [0020]    DSD to PCM signal conversion device  100  also includes an N-coefficient buffer  120 . The N-coefficient buffer includes coefficients c1  125   a , c2  125   b , c3  125   c , . . . , and cN  125 N that are each 32 bits in length  124 . Coefficients c1 to cN are general FIR low pass filter coefficients derived using well known general methods. The resulting coefficients c1 to cN in the coefficient buffer are used to multiply each coefficient by its corresponding DSD signal bit value over a period of time, as described below, to generate the correct values to reproduce the analog audio signal.  
         [0021]    Each bit in the N-bit delay line has a corresponding coefficient in the N-coefficient buffer. Thus, the most significant bit  105  in the N-bit delay line  110  corresponds to coefficient c1  125   a  in the N-coefficient buffer  120 . An address generator  110  moves from left to right and sequentially accesses each bit in the N-bit delay line  110 . For each bit in the N-bit delay line  110 , the address generator  110  determines the corresponding coefficient in the N-coefficient buffer  120 . Sign controller  130  receives the bit value of the N-bit delay line  110  currently accessed by the address generator  110  and sets output line  133  high or low depending on whether the bit value is one or zero.  
         [0022]    Multiplier MPY  135  receives at a first input  133  a high or low signal from sign controller  130  indicating the current bit value from the N-bit delay line  110 . As shown in FIG. 1, multiplier MPY  135  receives at a second input  138 , a 32 bit coefficient  125  from N-coefficient buffer  120 . If the multiplier MPY receives a high signal from sign controller  130 , the multiplier MPY generates a 32 bit binary value representing decimal +1.0 and multiplies this 32 bit binary value with the 32 bit coefficient  125  received at its second input  138 . If the multiplier MPY receives a low signal from sign controller  130 , the multiplier MPY generates a 32 bit binary value representing decimal −1.0 in two&#39;s complement form and multiplies this 32 bit binary value with the 32 bit coefficient  125  received at its second input  138 .  
         [0023]    Alternatively, the sign controller  130  may connect to the multiplier MPY  135  through a 32 bit bus (not shown). The sign controller  130  may provide to the multiplier MPY  135  a binary 32 bit value of decimal +1.0 or −1.0 depending on whether the bit value from N-bit delay line  110  is one or zero, respectively. In this system, multiplier MPY  135  performs a 32 bit by 32 bit multiply with the output from sign controller  130  and coefficient  125  from N-coefficient buffer  120 . The multiplier MPY  135  sends the result of the multiplication via 32 bit output bus  140  to adder  150 .  
         [0024]    Accumulator  170  couples to adder  150  through 32 bit bus  160  and initially contains a zero value. The current 32 bit value contained in the accumulator  170  is fed back to the adder  150  to be summed with the next output from multiplier MPY  135 . The result of this addition is then loaded into accumulator  170  and then summed with the next output from multiplier MPY  135 . Once all bits have been processed in the N-bit delay line  110  (i.e. the address generator has reached the least significant bit  185  in N-bit delay line  110 ) the accumulator  170  contains a 32 bit PCM signal sample that it transmits through output bus  175 . The N-bit delay line  110  may then be loaded with the next Nbit samples from the DSD signal for conversion to a PCM signal sample.  
         [0025]    described above, the DSD to PCM conversion device  100  requires N multiplies to convert N DSD bit samples to a multiple bit PCM signal sample. Multiplier MPY  135  may be an odd sized multiplier with one 32 bit input bus and a single bit input line. Because a single bit is processed from the N-bit delay line  110  through the multiplier MPY  135  and accumulator  170  at a time, multiple cycles may be required to generate the multiple bit PCM signal sample  180 .  
         [0026]    [0026]FIG. 2 shows another implementation, in accordance with the preferred embodiment of the invention, of a DSD to PCM conversion device contained in a digital signal processor (DSP)  200 .. This device uses a look-up table  260  that, preferably, may be stored on a memory  210 . The memory may be a Read-Only-Memory (ROM) that stores a permanent copy of the look-up table or a Random Access Memory (RAM) in which the look-up table may be built, as described below, each time power is applied to the DSP. In some preferred embodiments of the invention, the memory is located in the DSP  200 . In alternative embodiments of the invention, the memory  210  is an external memory that couples to the DSP  200  through a multiple bit bus.  
         [0027]    The DSD to PCM conversion device shown in FIG. 2 includes a First-In-First-Out (FIFO) buffer  205  that contains multiple lines each of size N bits. Line  202  at the top of the FIFO buffer contains N DSD signal sample bits that are subdivided into a number of words Word( 0 )  212 , Word( 1 ), Word( 2 ), . . . , Word(N/n−1)  214 . Each word contains n bits  218  such that line  202  of the FIFO buffer  205  is subdivided into N/n words. Thus, for example, if line  202  of the FIFO buffer  205  contains N=32 bits, and as shown in FIG. 2 each word contains n=8 bits, there would be 4 words (N/n=32/8=4) in line  202 .  
         [0028]    Look-up table  210  may be organized as a two dimensional array data structure containing section  0   262 , section  1 , section  2 , . . . , section(N/n−1)  265 . Each section may include a 2 n  entry array  270  containing sum[0]  275 , sum[1], sum[2], . . . , sum[2 n −1]  280 . As shown in FIG. 2, each entry of the array sum  270  may contain a 32 bit binary value  279 . As described in greater detail below and shown in FIG. 3, each 32 bit binary value in the array  270  is a precomputed partial sum for one of the bit patterns index=00 . . . 00b  276   a , 00 . . . 01b  276   b , 00 . . . 10b  276   c , . . . , 11 . . . 11b  276   n  using the corresponding coefficient for each bit in the bit pattern to calculate the sum. The look-up of a word Word( 0 ) . . . Word(N/n−1) in the table  260  includes determining the section corresponding to the word in the FIFO buffer  205  and matching the bit pattern of the word to a bit pattern in the array  270  to determine the precomputed partial sum. Thus, for the ekample described above where N=32 bits and n=8 bits, the look-up table  210  would contain four sections section  0 , section  1 , section  2 , and section  3 . Each section would include 256 entries (2 n =2 8 =256 entries) in array  270  starting at sum[0], sum[1], sum[2], . . . , sum[255]. Each entry of the array sum would contain a 32 bit precomputed sum for one of the bit patterns index=00 . . . 00b  276   a,  00 . . . 01b  276   b,  00 . . . 10b  276   c , . . . , 11 . . . 11b  276   n . Thus, if the n-bit word from the FIFO buffer  205  was Word( 0 )=11001101b  212 , section  0  corresponding to Word( 0 ) would be accessed (see below for description) and a look-up of the appropriate entry in array  270  would be performed. Look-up of the appropriate entry in array  270  includes matching Word( 0 )=11001101b  212  to one of the bit patterns 00000000b , 00000001b, 00000010b, . . . , 11111111b to determine the 32 bit precomputed sum. The index of each entry in the array sum  270  sum[index=0], sum[index=1], . . . , sum[index=2 n −1 9  corresponds to the bit pattern for each entry and so Word( 0 )=11001101b=205 dec would retrieve the 32 bit precomputed sum at sum[205].  
         [0029]    As mentioned above, each word in the FIFO buffer has a corresponding section in the look-up table  260 . Thus, the most significant Word( 0 )  212  in the FIFO buffer  205  corresponds to section  0   262  in the look-up table  260 . An address generator  220  moves from left to right and sequentially accesses each word in the FIFO buffer  205 . For each word in the FIFO buffer  205 , the address generator  220  determines the corresponding section in the look-up table  260 . The bit pattern of the n-bit word accessed by the address generator is also passed to the corresponding section through bus  230  so that a look-up in array  270  for the precomputed sum can be performed.  
         [0030]    The resulting 32 bit precomputed sum from the look-up table  260  for the corresponding word from FIFO buffer  205  is provided to output bus  235  and added in adder  240  to the current value of accumulator  255 . Preferably, accumulator  255  is initially set to a zero value. The current 32 bit value contained in the accumulator  255  is fed back to the adder  240  through bus  245  to be summed with the next 32 bit precomputed sum from look-up table  260 . The result of this addition is then loaded into accumulator  255  through bus  250  and then summed with the next output from look-up table  260 . Once all words have been processed in the current line  202  of the FIFO buffer  205  (i.e. the address generator has reached the least significant word  214  in FIFO buffer  205 ) the accumulator  255  contains a 32 bit PCM signal sample that it transmits through output bus  285 . The FIFO buffer  205  then flushes line  202  from the FIFO buffer and moves the next line below line  202  to the top of the FIFO buffer for conversion from DSD signal bits to a PCM signal sample.  
         [0031]    Turning now to FIG. 3, a flow chart  300  for generating the look-up table shown in FIG. 2 in accordance with the preferred embodiment of the invention is shown. Generating the look-up table  260  includes determining the precomputed sums for each of the 2 n  bit patterns index=00 . . . 00b . . . 11 . . . 11b in each section of the look-up table  260 . As mentioned above, for each word in the FIFO buffer  205 , the address generator determine the appropriate section and the bit pattern of the n-bit word is matched with one of the 2 n  bit patterns in array  270  of the section.  
         [0032]    Each bit position in the n-bit words of the FIFO buffer has a corresponding coefficient in a two dimensional coefficient array of size coeff[N/n][n]. Thus, a total of N coefficients exist for each of the N bits in the FIFO buffer  205 . The coefficients in the coefficient array are accessed based on section for N/n sections and bit position i within the n bits of a word such that coeff[section][i] corresponds to a bit position i of word(section). Thus, the most significant bit i=0 in the most significant Word(section= 0 )  212  in the FIFO buffer  205  corresponds to coeff[section=0][i=0] in the coefficient array. As mentioned above, each coefficient coeff[section][i] in the coefficient array is a constant that is a general FIR low pass filter coefficient derived using well known general methods.  
         [0033]    Referring to FIG. 3, generating the look-up table is performed using the following technique. A variable section corresponding to the sections shown in look-up table  260  of FIG. 2 is initialized to zero at block  315 . Similarly, variable index is initialized to zero in block  320 . Variable index corresponds to the n-bit index of the two dimensional array table for look-up table  260  table[section=0][index=0], table[section=0][index=1], table[section=0][index=2], . . . table[section=0][index=2 n −1]. Variable index is also shown in FIG. 2 as binary bits index=00 . . . 00b  276   a , index=00 . . . 01b  276   b , index=00 . . . 10b  276   c  . . . index=11 . . . 11b  276   n . In block  330 , the variable i that goes from i=0 to i=n and keeps track of the current bit in the n-bit index is also initialized to zero and sum containing the value of the precomputed sum is initialized to zero.  
         [0034]    Starting at index=0 and for each of the 2 n  values of index, then for each index starting with the most significant i=O bit in the index and going to the least significant i=n bit in the index, the 2 n  precomputed sums are determined for each section using the two dimensional coefficient array coeff[section][i]. Thus, in branch condition  335  if the i&#39;th bit of index is 1 then go to block  345 . In block  345 , the coeff[section][i] from the coefficient array is added to the current value in sum and i incremented to the next bit position in index. If the i&#39;th bit of index is zero, then branch condition  335  goes to block  340 . In block  340 , the coeff[section][i] from the coefficient array is subtracted from the current value in sum and i incremented to the next bit position in index. After all bits in index have been evaluated and the condition i=n in block  350 , the precomputed sum for that index value is present in sum. In block  360 , the precomputed sum stored in the sum variable is placed into look-up table entry corresponding to table[section][index] and index incremented to the next bit pattern  276 . If the precomputed sums for all values of index from index=0 to index=2 n  for the section have been determined and index=2 n , the condition in block  370  is not true and the variable section is incremented in block  375 . Thus, the precomputed sums for each index value going from index=0 to index=2 n  for the next section are determined as given above until the precomputed sums for all index values in all sections of the look-up table have been determined and section=N/n in block  380 . Generation of the look-up table  360  is complete and the technique stops in block  390 .  
         [0035]    DSP  200  is now ready to receive DSD signal samples for conversion to multiple bit PCM signal samples using the generated look-up table as described with reference to flow chart  400  in FIG. 4. Referring also to FIG. 2, the FIFO buffer  205  receives N bits from the DSD signal, and flushes out the oldest N bits that have been converted to a multiple bit PCM signal sample as shown in block  420 . The N bits are subdivided into words corresponding to sections as shown in FIG. 2. The address generator  220  starting at the most significant word  212  and traversing sequentially from left to right to access each word, determines the correct precomputed sum by performing a look-up in table  260  to match the bit pattern in word to the correct bit pattern 00 . . . 00b  276   a . . .  11 . . . 11b  276   n . Thus, in block  440 , a look-up of the two dimensional array table[section][word[section]] containing the precomputed sums is performed and added to the sum. In the two dimensional table array, word[section] corresponds to the bit pattern in the word. Thus, as shown in FIG. 2 for section= 0 , word[section=0]=11001101b=205 dec and element table[section=0][word[section=0]=205] contains the precomputed sum corresponding to word[0].  
         [0036]    After adding the precomputed sum for the word to sum, the variable section is incremented to determine the precomputed sum for the next word and this value is added to the sum variable. Thus, in block  450  after all words have been evaluated and the address generator has reached the section(N/n−1) in the look up table  260  and corresponding word(N/n−1)  214 , the condition is not true since section=N/n. Block  460  is evaluated and the multiple bit PCM signal sample in variable sum is generated. Finally, in block  470  if input continues from the DSD signal, then N DSD samples are loaded into the FIFO buffer as given in block  420  and the oldest N bits are flushed out. If no more bits from the DSD signal are in the FIFO buffer for conversion to PCM signal samples, the conversion technique stops in block  480 .  
         [0037]    The technique described above for conversion of DSD signals to PCM signals reduces the sampling rate for the PCM signal samples. Thus, if the DSD signal is sampled at a rate of 2.8224 MHz and N DSD samples are converted to one PCM signal sample, the PCM signal sampling rate is decimated to 2.8224/N MHz. Common values of N=16, 32, and 64 would given respective sampling rates of 176.4 KHz, 88.2 KHz, and 44.1 KHz.  
         [0038]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. Thus, in an alternative embodiment, the two dimensional array table may be a partial linked list data structure with each section containing a one dimensional array of precomputed sums and pointing to the address of the next sequential section in memory. Use of the linked list data structure allows use of non-contiguous blocks of memory in the DSP. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.