FAST SYNTHESIS SUB-BAND FILTERING METHOD FOR DIGITAL SIGNAL DECODING

Abstract: In order to reproduce audio signals which have been compressed or encoded for storage or transmission using, for example, MPEG audio encoding, a synthesis sub-band filter is employed which performs an inverse modified discrete cosine transform (IMDCT). The computational cost of the IMDCT implementation is reduced by pre-calculating arrays of sum and difference data. The arrays of sum and difference data are then used in two separate transform calculations, the results of which can be used in the generation of pulse code modulation (PCM) audio data.

Description: FAST SYNTHES IS SUB-BAND FI LTER ING METHOD FOR D IGITAL SIGNAL DECODING This invention relates to digital signal decoding for the purposes primarily of audio reproduction. In particular, the invention relates to enhanced synthesis sub-band filtering during decoding of digital audio signals. In order to store or transmit data representing audio signals it is often desirable to first encode or compress the data so as to enable it to be stored or transmitted more efficiently. Decoding the data requires that the stored or transmitted data be reconstructed into audio signals by application of a decoding or decompression technique. The reconstruction process is typically quite computationally intensive, yet the process should be fast 'and reliable enough to enable the audio signals to be reconstructed in real time, on the fly, for example. In order for the decoding process to be carried out in relatively low-cost consumer products, the hardware utilised by the decoder should also preferably be relatively simple and inexpensive, or at least to the greatest extent reasonably possible. Efficient stereo and multichannel digital audio signal coding methods have been developed for storage or transmission applications such as Digital Audio Broadcasting (DAB), Integrated Service Digital Network (ISDN), High Definition Television (HDTV) and Set Top Box (STB) for video-on-demand. The formats used to encode and reciprocally decode digital audio and video information for storage and retrieval is subject to various standards, one of which has been established by the Moving Pictures Experts Group and is known as the MPEG standard. A standard on low bit rate coding for mono or stereo audio signals was established by MPEG- 1 Audio, published under ISO-EEC/JTC1 SC29 11172-3, entitled "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to About 1.5 Mbit/s", and the disclosure of that document is incorporated herein by reference. MPEG-2 Audio (ISO/IEC 13818-3) provides the extension to 3/2 multichannel audio and an optional low frequency enhancement channel (LFE). The audio part of the standard, ISO/IEC 11172-3, defines three algorithms. Layer 1 , 2 and 3 for coding PCM audio signals. MPEG-2 (Multichannel) also defines Layer 1 , 2, and 3 algorithms. The MPEG audio encoder processes a digital audio signal and produces a compressed bitstream for transmission or storage. The encoder algorithm is not standardised, and may use various means for encoding such as estimation of the auditory masking threshold, quantisation, and scaling. However, the encoder output must be such that a decoder conforming to the above-mentioned standards specification will produce audio suitable for the intended application. The decoder, subject to the application-dependent parameters, accepts the compressed audio bitstream in the defined syntax, decodes the data elements and uses the information to produce digital audio output, also according to the defined standard. The decoder first unpacks the received bitstream to recover the encoded audio information frame by frame. After the process of frame unpacking, the decoder performs an inverse quantisation (expansion process) and feeds a sub-band synthesis filter bank with a set of 32 scaled-up sub- band samples in order to reconstruct the output PCM audio signals. The sub-band filter banks used for Layer 1 and Layer 2 of MPEG 1 audio decoder and Layer 1 and Layer 2 of MPEG2 (Multichannel extension) audio decoder, are the same. The sub-band synthesis filter is one of the most computationally intensive blocks of the MPEG audio decoder. Sub-band filtering is performed for each sub-band in a frame and for ' every channel. Any reduction in its computational requirements thus enables less complexity and reduced cost of decoding. In accordance with the present invention there is provided a method of decoding digital audio data, comprising the steps of obtaining an input sequence of data elements representing encoded audio samples, calculating an array of sum data and an array of difference data using selected data elements from the input sequence, calculating a first sequence of output values using the array of sum data, calculating a second sequence of output values using the array of difference data, and forming decoded audio signals from the first and second sequences of output data. - J Preferably, the array of sum data is obtained by adding together respective first and second data elements from the input sequence, the first and second data elements being selected from mutually exclusive sub-sequences of the input sequence. Furthermore, the array of difference preferably obtained by subtracting respective first data elements from corresponding second data elements αf the input sequence, the first and second data elements being selected from mutually exclusive sub-sequences of the input sequence. In one form of the invention the step of calculating an array of sum data and an array of difference data comprises dividing the input data sequence into first and second equal sized sub-sequences, the first sub-sequence comprising the high order data elements of the input sequence and the second sub-sequence comprising the low order data elements of the input sequence, calculating the array of sum data by adding together each respective data element of the first sub-sequence with a respective corresponding data element of the second subsequence, and calculating the array of difference data by subtracting each respective data element of the first sub-sequence from a respective corresponding data element of the second sub-sequence. The invention also provides method of decoding a sequence of m, m an even positive integer, input digital audio data samples S[k], where k = 0, 1, ... (m-1), to produce a set of n, n an ' even positive integer, output audio data samples YTTj, where i = 0, 1, ...(n-1), comprising the steps of: a) calculating an array of sum data S ADD lk] according to S ADD M = S[k] + S[m-l-k] for k = 0, 1 , ...(m/2-1) b) calculating an array of difference data S sιs [k] according to SsαM = S[kj - S[m-l-k] for k = 0. 1 , ...(m/2-1) c) calculating a first output audio data sample by a multiply-accumulate operation according to for k = 0, 1 , ... (m/2-1) d) calculating a second output au io data sample a multiply-accumulate operation according to V[2i + 1] = V[2i+ 1] + N[2i + 1 , k]*S S0B [k] for k = 0, 1, ... (m/2-1) where v Mrr2ι- ■r ιl, n k] = cos f (32 + ( i2/-l) m)(2A + l ) —τ:l ι [ 64 J e) and repeating steps c) and d) for I = 0, 1, ... (n/2-1) to obtain a full set of output data. The invention further provides a synthesis sub-band filter for use in decoding digital audio data, comprising a means for receiving or retrieving an input sequence of data elements comprising encoded digital audio data, a pre-calculation means for calculating an array of sum data and an array of difference data using selected data elements from the input sequence, and a transform calculation means for calculating a first sequence of decoded output values using said array of sum data and a second sequence of decoded output values using said array of difference data. The invention is described in greater detail hereinbelow, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of major functional portions of an MPEG audio encoder; Figure 2 is a block diagram of major functional portions of an MPEG audio decoder; Figure 3 is a flow diagram of an MPEG decoding procedure; Figure 4 is a flow diagram showing a generalised form of a procedure according to the present invention; and Figure 5 is a flow diagram illustrating a preferred implementation of the invention. Figure 1 is a block diagram illustrating the major components of an MPEG audio encoder circuit 2 constructed in accordance with the aforementioned standards document. In the figure, an input signal 4, comprising a pulse code modulated (PCM) signal having a 48 kHz sampling frequency and a sample size of 16 bits per sampie, is provided as input to the single channel encoder 2. The input signal is first mapped from the time domain into the frequency domain by a sub-band filter bank 8. The resulting coefficients are normalized with scale factors which may be transmitted as side information. The coefficients thus obtained are then quantized and entropy encoded by a quantizer and encoding circuit 10. Masking thresholds of the quantization errors are calculated based on psychoacoustic values provided by a psychoacoustic model 14 to control the quantization step. The bit allocation is transmitted as side information. The coded signal is then multiplexed by a frame packing circuit 12 and an encoded bitstream 6 is produced at the output of the encoder 2. A block diagram illustrating the main components of an MPEG audio decoder circuit 20 is shown in Figure 2. In the figure, an encoded bitstream 22 is provided to the input of the decoder. A bitstream unpacking and decoding circuit 26 performs an error correction operation if such operation was applied in the encoder. The bitstream data are unpacked to recover the various pieces of encoded information, and a reconstruction circuit 28 reconstructs the quantized version of the set of mapped samples from the frames of input data. An inverse mapping circuit 30 transforms the mapped samples back into a uniform pulse code modulated (PCM) output signal 24 that reproduces the corresponding input signal which was provided to the encoder. The foregoing descriptions of the encoder and decoder are specific to the MPEG standard, and it is considered to be within the skill of those in the art to implement the various hardware functions described above. Accordingly, a more detailed hardware description of an MPEG coding system is not considered necessary for a full and complete understanding of the invention. It should be appreciated the invention described herein, although described in connection with the MPEG coding standard, is considered useful for other coding applications and standards. Referring to Figure 3, there is shown a flow diagram 40 of steps involved in signal processing in layers I and π in an MPEGl audio decoder. To begin with, the bit allocation of an input bitstream (42, 44) is decoded (46). Thereafter, various scale factors are also decoded (48) and the samples are requantized (50). The encoded signal is decoded in a synthesis sub-band filter (52) and the decoded pulse code modulated signals are output (54, 56) for further processing and/or real time reproduction. The present invention relates primarily to the synthesis sub-band filter portion of the decoding process, when implemented for MPEG decoding. The synthesis sub-band filter bank is composed of two main functions, an Inverse Modified Discrete Cosine Transform (TMDCT) and an Inverse Pseudo-Quadrature Mirror Filter (TPQMF). The IMDCT, which can be viewed as an overlap transform, performs a 32 x 64 cosine modulation transformation, which means a frequency shift of a filter bank into one single filter. Consider a system in which ouφut sub-band audio signal samples V; ( i=0....63) are decoded from sequences of 32 encoded input samples S k , k = 0....31. The inverse MDCT of the sequence S k , is defined as follows: Taking the cosine symmetric property wherein: cosθ =cos(2π-θ) (2) the IMDCT definition equation (1) may be modified as given below to implement a 32-point IMDCT. The remaining 32 output audio signal samples are obtained after post-processing from this IMDCT of S. This equation (3) may be computed according to the following algorithm: repeat i = 32 times repeat k = 16 times if I is even, Sum = S[k] + S[31-k] if I is odd, Sum = SfkJ- S[31-k] Vfi] = VfiJ + Nfi, k] * Sum end k end i where i is the index of output samples (i=0....31) k is the index of input samples (k=0....15) N(i,k) S[k] represents the input sample data sequence Vfi] represents the output of IMDCT The IMDCT equation, making use of the symmetrical property, is given in Equation (3) above, and the computational effort required for MPEG audio decoding is in large part dependant upon the efficiency with which the input samples can be processed through the IMDCT to obtain respective sub-band filter PCM samples. Embodiments of the present invention are able to reduce the number of arithmetic operations performed in implementing the IMDCT portion of the decoder, to thereby increase the computational efficiency of the decoding process. In particular, the number of addition operations required for the implementation of this equation can be reduced substantially by pre-computing the sum and difference of the sample data which is the input to the IMDCT. In addition, the pre- computation can take place outside the main IMDCT computational loop. Hence the main loop contains only the MAC operations, which can be executed very efficiently by any general purpose DSP in a minimum number of cycles. In the present invention, the dequantised sample data (e.g. 32 samples) from the encoded bitstream is pre-processed as per the symmetrical property of the cosine coefficients. The sample data is then split into two banks, each containing 16 samples. The sum and difference of respective data elements in the two banks is computed and stored in two arrays. These arrays are used as the input data for the subsequent MAC operations. Prior art implementations of equation (3) have required 32 x 16 Multiply-Accumulate operations and 32 x 16 Addition operations. By using the pre-computation operations described above, however, the number of Addition operations reduces to 2 x 16. This results in a saving of 30 x 16 Addition operations per Sub-band filter implementation, which in turn translates to a corresponding reduction in overall computational power. In the IMDCT equation (3), S k represents a sequence of m input data samples, where k = 0 ... (m-1). In a typical implementation for MPEG decoding 32 input data samples may be processed, such that m=32. For pre-computing the sum and difference of respective data elements, the input data sample sequence is first arranged into two equally sized data banks, one constituting the high order data elements and the other the low order data elements: Data bank (1) S k for k = 0 ... (m/2)-l Data bank (2) Sk for k = (m/2) ... (m-1) For example, in a preferred embodiment of the present invention where m=32, S k is split into two data banks comprising: (1) S k for k = 0 .. 15 (2) S k for k = 16 .. 31 The sum and difference data are calculated using respective data elements from the two data banks and is stored in two arrays of data, S^ D and S SUBι which are computed as follows: S ADD [k] =S[k] +S[m - l -k] for k = 0, 1 , (m/2) -l (4) S, ιn ,[k] =S[k] -S[m - l -k] for k = 0, 1 , .(m/2) - l (5) In the aforementioned example of 32 input data samples, equations (4) and (5) reduce to: S ADD M = S[k] + S[31-k] for k = 0, 1, ...15 S SUB M = S[k] - S[31-k] for k = 0, 1 , ...15 The IMDCT equation (3) may now be divided into two portions and rewritten as follows: for . =0,2,4,....30 15 V[(\ =∑ cos ^ κ+ l >" * SsmW *=0 64 (7) for 1 = 1 ,3,5 31 As shown in the above equations (6) and (7), the IMDCT may now be calculated in two passes, an 'even pass' where the sum of the sample data is used (equation (6)), and an 'odd pass' where the difference of the sample data is used (equation (7)). The computational algorithms of the above equations are shown below. Calculation of sum and difference of sample data (Addition operations) repeat k = 16 times SADDIAJ = "* ^ ^31-k. SSUBIA] = *S* " ^31-k end k Calculation of 'even' data of IMDCT {Multipl -Accumulate operations) repeat i — 16 times repeat k = 16 times Vfi] = Vfi] + Nf k^S^fk] end k end i Calculation of 'odd' data of IMDCT (Multiply-Accumulate operations) repeat i = 16 times repeat k = 16 times Vfi] = Vfi] + N[i,k]*S SUB [k] end k end i where i is the index of output samples (i=0...31) samples (k=0....15) represents t e input sample data sequence SΛDD represents the sum of data array SsUB represents the difference of data array Vfi] represents the output of the IMDCT Figures 4 and 5 illustrate the above procedure according to a preferred embodiment of the invention in the form of flow diagrams. The representation shown in Figure 4, illustrates the general steps involved, and the procedure illustrated in the flow diagram 80 of Figure 4 corresponds to the synthesis sub-band filter step 52 of the overall decoding procedure 40 of Figure 3. To begin with the input samples S k are received (82, 84) after having been isolated from the frames of encoded data received or retrieved. The input data samples are then utilised for pre-calculation of sum and difference data, as described above. This involves dividing the input data sample set into two equal sized sub-sets, which in the preferred embodiment consists of a first sub-set comprising the lower order data and a second sub-set comprising the higher order data. For example, in the case of 32 input samples S 0 to S 31 as described, the first sub-set of input sample data may comprise the lower order input data S 0 to S 15 and the second sub-set comprises the upper order data samples S 16 to S 31 . Respective ones of each sub-set of input sample data are then used to obtain a sets of sum and difference data, S ADD and § UB . As can be readily ascertained from the above description, in the preferred embodiment the calculation of the sum and difference data is performed using the lowest order samples from the first set with the corresponding highest samples from the second set. For example, in the case of 32 input samples, the sum and difference data elements may be calculated as follows: S ADD [0] = S[0] + S[31] S SUB [0] = S[0] - S[31] S ADD [1] = S[l] + S[30] S SUB [1] = S[l] - S[30] S ADD [2] = S[2] + S[29] S SUB [2] = S[2] - S[29] S ADD [15] = S[15] + S[16] S SUB [15] = S[15] - S[16] Once the arrays of sum and difference data have been calculated, the multiply-accumulate operations required to calculate the IMDCT can be performed iteratively in two steps. The first step (88) is used to obtain half of the ouφut samples (e.g. the "even" outputs) using the pre-calculated sum data comprising the data elements. The second step (90) is used to obtain the other half of the ouφut samples (e.g. the "odd" outputs) using the pre-calculated difference data comprising the S SUB data elements. Each of these steps (88, 90) is an iterative multiply-accumulate (MAC) operation involving each of the data elements from the respective S ADD or S SUB array. Furthermore, each of the MAC operations of steps 88, 90 are performed repeatedly (step 92) to obtain a full complement of ouφut samples. For example, where 32 ouφut samples V 0 to V 31 are required, each of the iterative' MAC steps 88, 90 would be performed 16 times. Once the data for each ouφut has been calculated, the data samples are ouφut for PCM processing (step 94). A more detailed preferred embodiment of the decoding procedure is illustrated in the flow diagram 100 shown in Figure 5. Beginning at step 102, a sequence of m input samples S k (k = 0 .... m-1) are received for decoding to n sub-band filter outputs V, (i = 0 .... n-1) at step 104. In the preferred embodiment for an MPEG implementation, both the number of input samples m and the number of ouφut samples n are the same, 32. Steps 106, 108 and 110 of procedure 100 form a loop for the pre-calculation process of determining and storing the sum and difference data arrays from the input data samples. The steps 112, 114, and 116 then form nested loops for the iterative multiple-accumulate calculation of the "even" ones of the ouφut data elements (e.g. Vj for i = 0, 2, 4, ... 30), using the pre-calculated sum data array S ADD - A calculation loop of steps 112 and 114 provides the iterative MAC operation, whilst the loop provided by step 116, enables calculation of each (even) alternate ouφut data element. The remaining (odd) alternate ouφut data elements are calculated in nested loop steps 118, 120, 122 using the difference data array S SUB . The resulting ouφut sub-band data is then provided at final step 124. The preferred form of the invention presented herein results in a reduction of 480 addition operations per 32 sub-band samples. For a stereo ouφut MPEGl Layer 2 audio decoder, this is a reduction of 480 *36*2 arithmetic operations per frame. -The overall reduction in arithmetic operations which is achieved is approximately 46.875% per IMDCT. It will be readily apparent to those of ordinary skill in the relevant art that the present invention may be implemented in numerous different ways, without departing from the spirit and scope of the invention as described herein, and it is to be understood that such modifications are considered to be within the scope of the invention. In any event, it is immediately recognisable that one way the invention can be carried out, relating as it does to the processing of data, is using general purpose computing apparatus operating under the instruction of software or the like which is produced separately and specially adapted to perform the methods of the invention. Alternatively, specialised computing apparatus such as a dedicated integrated circuit, chipset or the like may be constructed with the functions of the invention embedded therein. Many other variations to the particular implementation will of course be possible. It will also be recognised that in places in the description and appended claims where it is said that a data set is divided into sub-sets, for example, this division may be simply a notional one, and no physical separation need occur, as is known in the data processing art. The foregoing detailed description of the present invention has been presented by way of example only, and is not intended to be considered limiting to the invention which is defined in the claims appended hereto.

Claims: CLAIMS: 1. A method of decoding digital audio data, comprising the steps of obtaining an input sequence of data elements representing encoded audio samples, calculating an array of sum data and an array of difference data using selected data elements from the input sequence, calculating a first sequence of ouφut values using the array of sum data, calculating a second sequence of ouφut values using the array of difference data, and forming decoded audio signals from the first and second sequences of ouφut data. 2. A method as claimed in claim 1, wherein the array of sum data is obtained by adding together respective first and second data elements from the input sequence, the first and second data elements being selected from mutually exclusive sub-sequences of the input sequence. 3. A method as claimed in claim 1 or 2, wherein the array of difference data is obtained by subtracting respective first data elements from corresponding second data elements of the input sequence, the first and second data elements being selected from mutually exclusive subsequences of the input sequence. 4. A method as claimed in claim 1, wherein the step of calculating an array of sum data and an array of difference data comprises dividing the input data sequence into first and second equal sized sub-sequences, the first sub-sequence comprising the high order data elements of the input sequence and the second sub-sequence comprising the low order data elements of the input sequence, calculating the array of sum data by adding together each respective data element of the first sub-sequence with a respective corresponding data element of the second sub-sequence, and calculating the array of difference data by subtracting each respective data element of the first sub-sequence from a respective corresponding data element of the second sub-sequence. 5. A method as claimed in claim 1 , wherein the step of calculating a first sequence of output values comprises performing a multiply-accumulate operation utilising each of the sum data elements. 6. A method as claimed in claim 1 or 5, wherein the step of calculating a second sequence of ouφut values comprises performing a multiply-accumulate operation utilising each of the difference data elements. 7. A method as claimed in any preceding claim wherein the input sequence of data elements is derived from MPEG encoded audio data, and wherein the decoded audio signals comprise pulse code modulation samples. 8. A method of decoding a sequence of m, m an even positive integer, input digital audio data samples S[k], where k = 0, 1, ... (m-1), to produce a set of n, n an even positive integer, ouφut audio data samples Vfi], where i = 0, 1, ...(n-1), comprising the steps of: a) calculating an array of sum data S^uPe] according to S ADD M = Sfk] 4- S[m-l-k] for k = 0, 1, ...(m 2-1) b) calculating an array of difference data S SUB [k] according to S SUB M = Sfk] - S[m-l-k] for k = 0, 1, ...(m/2-1) c) calculating a first ouφut audio data sample by a multiply-accumulate operation according to for k = 0, 1, ... (m 2-1) d) calculating a second ouφut audio data sample a multiply-accumulate operation according to V[2i+ 1] = V[2i + 1] + Nfi, k]*S SUB [k] for k = 0, 1 , ... (m 2-1) where Nfi, k] = cos (32 +(2i + l ))(2* + l )π 64 e) and repeating steps c) and d for i = 0, 1 , ... (n/2-1) to obtain a full set of ouφut data. A method as claimed in claim 8, wherein the number m of input digital audio data samples is 32, and the number n of ouφut audio data samples is 32. 10. A method as claimed in claim 8 or 9, wherein the decoding steps are repeated for decoding a series of frames of encoded audio data in an MPEG format. 11. A synthesis sub-band filter for use in decoding digital audio data, comprising a means for receiving or retrieving an input sequence of data elements comprising encoded digital audio data, a pre-calculation means for calculating an array of sum data and an array of difference data using selected data elements from the input sequence, and a transform calculation means for calculating a first sequence of decoded ouφut values using said array of sum data and a second sequence of decoded ouφut values using said array of difference data. 12. A synthesis sub-band filter as claimed in claim 11 wherein the pre-calculation means and transform calculation means collectively perform an inverse modified discrete cosine transform of the encoded digital audio data. 13. An MPEG decoder including a synthesis sub-band filter as claimed in claim 11 or 12.

Inventors: SAPNA GEORGE, YANG HAIYUN, SAPNA, GEORGE, YANG, HAIYUN
Applicants: SAPNA GEORGE, SGS THOMSON MICROELECTRONICS A, YANG HAIYUN, SAPNA, GEORGE, SGS-THOMSON MICROELECTRONICS ASIA PACIFIC (PTE) LTD., YANG, HAIYUN
Application year: 1997
Main classification: H04H1
IPC codes: G10L19, H04H40