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MPEG1 Layer III Decoder Development on x86 and TMS320C6711

Explore the implementation of an MPEG1 Layer III (MP3) decoder on x86 and TMS320C6711 platforms, decoding steps, frame structure, Huffman decoding, requantization, and stereo processing.

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MPEG1 Layer III Decoder Development on x86 and TMS320C6711

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  1. Developement and Implementation of an MPEG1 Layer III Decoder on x86 and TMS320C6711 platforms BraidottiEnrico (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 11172-3 (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 Clustered Table 1 Huffman Table 1 xylencodeword 0 0 1 1 0 1 3 001 1 0 2 01 1 1 3 000

  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 → 15 + 213 = 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 - 210 - (scalefac_long << scale); if (preflag) a -= (pretab << scale); if (a < -127) y = 0; if (a >= 0) y = tab[a&3]*(1 << (a >> 2)); else y = tabi[(-a)&3]/(1 << ((-a) >> 2)); tab contains values: [20, 21/4, 21/2, 23/4] tabi contains values: [20, 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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