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Developement and Implementation of an MPEG1 Layer III Decoder on x86 and TMS320C6711 platforms Braidotti Enrico (Farina Simone)

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Presentation on theme: "Developement and Implementation of an MPEG1 Layer III Decoder on x86 and TMS320C6711 platforms Braidotti Enrico (Farina Simone)"— Presentation transcript:

1 Developement and Implementation of an MPEG1 Layer III Decoder on x86 and TMS320C6711 platforms Braidotti Enrico (Farina Simone)

2 What is MPEG1 Layer III ? Frequently referred to as “MP3” Method to store compressed audio (LOSSY ) Developed by Moving Pictures Expert Group (MPEG) Standard ISO/IEC (Audio Part 3), 1991 Compression rate w/out recognizeable quality loss up to 12x Last release of MPEG1 family: –Highest complexity –Provides best quality

3 Standard MPEG1 3 possible compression types (increasing complexity): –Layer I –Layer II –Layer III Sampling frequencies for Layer III: –32 kHz –44.1 kHz –48 kHz Bitrates: –Min 32 kbit/s –Max 320 kbit/s Compact Disc: 1.41 Mbit/s

4 BITSTREAM FORMAT Whole bitstream is divided into frames of defined length: framesize = 144· bitrate / sampling frequency + padding (bytes) Frames are divided in 2 granules and are composed by different parts: Header CRC (optional) Side Information Main data Ancillary data (optional)

5 FRAME HEADER Syncword = 12 bits put to ‘1’ ID = 1 for MPEG1 Audio (2 bits used for MPEG2 and 2.5) Padding = to adjust framesize (and effective bitrate of CBR files)

6 SIDE INFORMATION Length depends on number of channels 17 bytes for single channel 32 bytes for others Contains all necessary informations for decoding the Main data section Main structure is:

7 BIT RESERVOIR It is one of the most important features of Layer III format and it works as follows (use of main_data_end ):

8 MAIN DATA SCALEFACTORS informations in the Side Information section HUFFMAN CODED DATA extraction of scaled frequency lines (not ordered in some cases)

9 DECODING PROCESS

10 DECODING STEPS SYNCHRONIZATION HEADER DECODING SKIPPING CRC (if present) SIDE INFO DECODING SCALEFACTORS DECODING

11 HUFFMAN DECODING Lossless - type coding / decoding Fixed – variable Based on 18 Huffman Tables (specific for MPEG1) Codewords up to 19-bit long Tables up to 256 values

12 HUFFMAN DECODING Big Values Region 0 Region 1 Region 2 Count 1 RZero

13 HUFFMAN DECODING Couple of f. lines ( big-values ) Quadruple of f. lines ( count1 )

14 HUFFMAN DECODING CLUSTERED HUFFMAN DECODING (R. Hashemian ) Compromise between binary-tree and direct look-up decoding Custom made Huffman tables containing 16-bit words Structure of words depend on HIT / MISS:

15 HUFFMAN DECODING Example Huffman Table 1 x y len codeword Address HIT / MISS New Address Bits xy 0MISS101 1HIT00 10MISS HIT10 100HIT11 101HIT01 Clustered Table 1

16 REQUANTIZATION (DESCALING) The Huffman decoded frequency lines are restored to their original values according to the following formulas:

17 REQUANTIZATION (DESCALING) Use of large look-up table with all possible values of modulus of Huffman decoded data (0 → = 8206) pros: speed, accuracy cons: memory requirements (32 KByte with float precision) Reduced Look-up table pros: table is 87.5 % smaller (4 KByte with float precision) cons: speed (need to calculate is· 0.125), accuracy

18 REQUANTIZATION (DESCALING) Shift – based power computing (T. Uželac ) Requantization has to be done up to 2304 times each frame, direct computation of: would require too many clock cycles

19 REQUANTIZATION (DESCALING) shift operations 2 small look-up tables (total of 32 Bytes) scale = scalefac_scale + 1; a = global_gain (scalefac_long << scale); if (preflag) a -= (pretab << scale); if (a < -127) y = 0; if (a >= 0) y = tab[a&3]*(1 > 2)); else y = tabi[(-a)&3]/(1 > 2)); tab contains values: [2 0, 2 1/4, 2 1/2, 2 3/4 ] tabi contains values: [2 0, 2 -1/4, 2 -1/2, 2 -3/4 ]

20 STEREO PROCESSING INTENSITY STEREO In the critical bands higher than 2 kHz, the sensation of stereo is given mainly by the envelope of the signal. The encoder codes only one sum - like signal and the decoder extracts separate L and R with different scalefactors MIDDLE/SIDE STEREO Encoding of the Middle (L+R) and Side (L-R) signals for reducing redundant elements

21 STEREO PROCESSING There are 4 different typologies of transmission for stereophonic signals (according to mode_extension, found in the header ):

22 STEREO PROCESSING MIDDLE/SIDE STEREO Left and Right channels are simply reconstructed according to: INTENSITY STEREO Values are read from the Rzero part of Left channel and IS positions is_pos (sfb ) are read from scalefactors of right channel:

23 REORDERING It is performed only when using short blocks: this is due to the way the MDCT in the encoder arranges the output lines.


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