USOO5884269A United States Patent (19) 11 Patent Number: 5,884,269 Cellier et al. (45) Date of Patent: Mar 16, 1999 54) LOSSLESS COMPRESSION/ 4,841,299 6/1989 Weaver ..................................... 341/36 DECOMPRESSION OF DIGITALAUDIO 4.882,754 11/1989 Weaver et al. ............................ 381/35 DATA OTHER PUBLICATIONS 75 Inventors: Claude Cellier, CheXbres, Switzerland; C. Cellier et al., “Lossless Audio Data Compression for Real Pierre Chenes, Ferreyres, Switzerland Time Applications,” 95th AES Convention, Oct. 1993. 73 ASSignee: Merging Technologies, Puidoux, Primary Examiner-Chi H. Pham Switzerland Assistant Examiner Ricky Q. Ngu Attorney, Agent, or Firm-Kenyon & Kenyon 21 Appl. No.: 422,457 57 ABSTRACT 22 Filed: Apr. 17, 1995 6 An audio signal compression and decompression method 51 Int. Cl. ................................. G10L 7/00; G10L 3/02 and apparatus that provide lossleSS, realtime performance. 52 U.S. Cl. ......................... 704/501; 704/504; 371/37.8; The compression/decompression method and apparatus are 341/64 based on an entropy encoding technique using multiple 58 Field of Search .................................. 395/2.38, 2.39, Huffman code tables. Uncompressed audio data Samples C 395/2.3–2.32; 375/242, 243, 254; 371/30, first processed by a prediction filter which generates predic 37.1, 37.2, 37.7, 37.8; 382/56; 358/246, tion error Samples. An optimum coding table is then Selected 261.1, 261.2, 427; 704/200, 500, 501, 503, from a number of different preselected tables which have 504; 341/50, 51, 64, 65 been tailored to different probability density functions of the prediction error. For each frame of prediction error Samples, 56) References Cited an entropy encoder Selects the one Huffman code table U.S. PATENT DOCUMENTS which will yield the Shortest encoded representation of the frame of prediction error Samples. The frame of prediction 4,396,906 8/1983 Weaver ................................... 340/347 CO Samples is then encoded using the selected Huffman 4,449,536 5/1984 Weaver ... ... 128/696 code table. A block Structure for the compressed data and a 4,503.510 3/1985 Weaver ... ... 364/715 decoder for reconstructing the original audio signal from the 4,535,320 8/1985 Weaver ........ ... 340/347 compressed data are also disclosed. 4,546,342 10/1985 Weaver et al. ... ... 340/347 4,754,483 6/1988 Weaver ..................................... 381/36 4,802,222 1/1989 Weaver ..................................... 381/35 14 Claims, 7 Drawing Sheets 1 O 4. COMPACT HUFFMAN TABLES DICTIONARY HUFFMAN BEST TABLE CODING AND OUTPUT, PREDICTOR SELECTOR FRAME STORAGE CODING COMPACT HUFFMAN WEIGHT TABLES U.S. Patent Mar 16, 1999 Sheet 1 of 7 5,884,269 104 COMPACT HUFFMAN TABLES DICTIONARY 1 O2 BEST té CODINGHUFFMAN AND OUTPUT PREDICTOR SELECTOR FRAME STORAGE CODING COMPACT HUFFMAN WEIGHT TABLES FIG. 1 204 HUFFMAN TABLES DCTIONARY 2O 2O5 FRAME HUFFMAN INVERSE INPUT, DECODING STORAGE BLOCK DECODING PREDICTOR FIG. 2 U.S. Patent Mar 16, 1999 Sheet 2 of 7 5,884,269 ETTVA(J>|E. G| „MULITIEVEJO?!c) U.S. Patent Mar 16, 1999 Sheet 3 of 7 5,884,269 401 NINIT ALL BIN HIT COUNTERS TO O, N=0 402- GET ERR SAMPLE IN 403 NFIND BIN HIT FOR ERR SAMPLE (N) 404 405 E EN YES 406-N INIT COST OF EACH TABLE TO O, TABLE POINTER T=0 4O7 COMPUTE COST FOR TABLET) 408 T=O OR COSTTKMINCOST 2 409 MINCOST = COSTT) BESTTABLE = T 41 O 411 T= LAST TABLE 2 YES FIG. 4 U.S. Patent Mar 16, 1999 Sheet 5 of 7 5,884,269 FROM 404 (FIG. 4) 522-N TAKE ABSOLUTE VALUE OF ERR SAMPLE ABS ERR(N) 523 LBYTE = ABS ERR(N) AND 00FF HBYTE = ABS ERR(N) AND FF00 HBYTE = HBYTE SHIFTED 8 BITS RIGHT 525 INC BINHITHIGH TABLEHBYTE) INC BINHITLOW TABLELBYTE)) ABS ERR x 255 ABS ERR < 256 TO 404 (FIG. 4) FIG. 5(B) U.S. Patent Mar 16, 1999 Sheet 6 of 7 5,884,269 601 N. COST = O, LEVEL POINTER L=0 6O2 COST = COST+(BINHITL) * WEIGHTL) FIG. 6 5,884,269 1 2 LOSSLESS COMPRESSION/ difference between the actual Sampled value and the pre DECOMPRESSION OF DIGITALAUDIO dicted value for that Sample, is then provided to an entropy DATA encoder, which can be based on a Huffman or arithmetic coding Scheme, to encode digital audio Samples into a FIELD OF THE INVENTION compressed form using code words of varying length. The present invention relates to an apparatus and method An object of the present invention is to provide a highly for digitally compressing and decompressing audio signals. efficient and compact way of mapping the Statistics of the More specifically, the present invention relates to an appa actual audio Signal, on a block basis, in order to Select the ratus and method for the lossleSS compression and decom optimum encoding table from a number of different prese pression of digital audio data. lected tables which have been tailored to different probabil ity density functions (PDFs) of the predictor error. It has BACKGROUND INVENTION been discovered that the long-term Statistics of the predic tion error of commonly used predictors follow a Laplacian Sampled digital audio data, from Sources Such as Speech distribution. For each block of audio data, the entropy or musical instruments, particularly in the form of linear 15 encoder of the present invention Selects one of a number of Pulse Code Modulation (PCM) samples, tends to include a encoding tables, each of which closely matches a Laplacian high degree of redundancy because of the high degree of distribution with a given variance. The encoding tables dependence between Successive Sample values. correspond to a Series of Laplacian distributions whose With the proliferation of multimedia applications, several Variances are in a geometric progression. compression/decompression algorithms have been pro While an ideal matching to each PDF would suggest a moted. U.S. Pat. No. 4,396,906 to Weaver describes a different probability for each value, Such an implementation, System which includes means for digitizing analog signals, for a 16-bit Signal, would require 64K entries for each for compression filtering the digital Samples, and for Huff encoding table. In accordance with the present invention, the man encoding the compressed digital Samples for recording range of possible values is divided into Sub-ranges or “bins' or transmission. The U.S. Pat. No. 4,396,906 patent also 25 with boundaries at integer powers of two, with each bin describes a receiving System which includes a Huffman having one entry in each encoding table. As a result, the decoder, a digital reconstruction filter and means for con number of entries per table is only n+1, where n is the Verting the decoded, reconstructed digital Signals back to number of bits of the predictor error signal to be encoded. analog form. A similar System is describe in an article by U. Each Huffman encoding table includes a prefix code for each E. Ruttimann et al., entitled “Compression of the ECG by of the n+1 bin entries. Each Huffman table will yield a Prediction or Interpolation and Entropy Encoding”, IEEE minimum coding length for a different bin and as Such will Transactions on Biomedical Engineering, Vol. BME-26, No. yield a different coding cost depending on the variance of the 11, November 1979, pp. 613-623. Another system is predictor error values over a frame. The Huffman table described in an article by K. L. Ripley et al., entitled “A having the lowest coding cost for each frame is Selected and Computer System for Capturing Electrocardiographic 35 used to encode the predictor error values for that frame. Data”, Pro. Comput. Cardiol., 1976, pp. 439-445. Another object of the present invention is to provide an To achieve constant compression rates, however, existing efficient and flexible compressed data file format. In accor Schemes have Sacrificed audio integrity, losing Some of the dance with the present invention, compressed audio data is information contained in the original audio signal. There are 40 organized in blocks having a header and a body of user data, lossleSS compression algorithms which have been used to i.e., the encoded audio data. The Start of each block is compress text and data files with completely accurate recov delimited by a unique Sync word in the header. Each of the ery of the primary data upon decompression. These variable-length codes in the encoding tables are of Such a techniques, however, are optimized for text and data and are format that they can be combined in any order without only marginally effective in compressing audio data. forming the same pattern as the unique code that is used for Some methods of audio compression are based on psy 45 the sync word in the block header. This feature adds a level choacoustics. Such perceptual coding algorithms drop psy of error detection making it possible to prevent the propa choacoustically imperceptible audio information. While gation of an error beyond the frame in which the error acceptable for most consumer audio delivery formats, Such occurred. The use of a unique Sync word also allows for easy as MiniDisc, DAT and CD-ROMs, such an approach is Synchronization of a random acceSS System fetch to an inadequate for professional audio production, where mate 50 arbitrary location in a compressed audio data file and for rial may go through multiple iterations of compression and Subsequently quickly initializing and Synchronizing the decompression before being mastered onto the final delivery decoding process of the following frame. medium. Any loss of audio information is compounded with The block header Structure in accordance with the present each iteration causing a Serious compromise in the audio 55 invention also provides a means of rapidly jumping forward integrity of the finished product.