Developement and Implementation of an

MPEG1 Layer III Decoder on x86 and TMS320C6711

platformsBraidotti Enrico(Farina Simone)

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

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

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)

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)

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:

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

MAIN DATA• SCALEFACTORS

• informations in the Side Information section

• HUFFMAN CODED DATA

• extraction of scaled frequency lines (not ordered in some cases)

DECODING PROCESS

RetrievingFile

Information

HuffmanDecoding

Requantization

StereoProcessing

ReorderingAlias

Reconstruction

Hybrid Synthesis (IMDCT,

Windowing, Overlap-Add)

Frequency Inversion

Synthesis Polyphase Filterbank

Input File

PCM output samples

RetrievingFile

Information

HuffmanDecoding

Requantization

StereoProcessing

ReorderingAlias

Reconstruction

Hybrid Synthesis (IMDCT,

Windowing, Overlap-Add)

Frequency Inversion

Synthesis Polyphase Filterbank

Input File

PCM output samples

DECODING STEPS

• SYNCHRONIZATION

• HEADER DECODING

• SKIPPING CRC (if present)

• SIDE INFO DECODING

• SCALEFACTORS DECODING

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

HUFFMAN DECODING

• Big Values

• Region 0

• Region 1

• Region 2

• Count 1

• RZero

HUFFMAN DECODINGCouple of f. lines ( big-values ) Quadruple of f. lines ( count1 )

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:

HUFFMAN DECODINGExample

Huffman Table 1

x y len codeword0 0 1 10 1 3 0011 0 2 011 1 3 000

Address

HIT / MISS

New

Address

Bits x y

0 MISS 10 1

1 HIT 0 0

10 MISS 100 1

11 HIT 1 0

100 HIT 1 1

101 HIT 0 1

Clustered Table 1

REQUANTIZATION (DESCALING)

The Huffman decoded frequency lines are restored to their original values according to the following formulas:

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

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

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]

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

STEREO PROCESSING

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

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:

REORDERING

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