Developement and Implementation of an

MPEG1 Layer III Decoder on x86 and TMS320C6711

platformsFarina Simone(Braidotti Enrico)

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

ALIAS RECONSTRUCTION

It is performed only when using long blocks: this means only when using pure long blocks or mixed blocks.

Lets see what long/short blocks are

Not encoded signal

Same signal encoded using long blocks

Same signal encoded using short blocks

HYBRID SYNTHESIS

• IMDCT (Inverse Modified Discrete Cosine Transform)

Subbands are backward transformed separately depending on block length.

• 6-point IMDCT

When short blocks are used (pre-echoes masking)

• 18-point IMDCT

When long blocks are used

HYBRID SYNTHESIS

• Fast IMDCT algorithm (Szu-Wei Lee )

Based on simmetric properties of cosine function

It needs a rearranging stage to restore values to their originalpositions

Drastically reduces number of operations if compared to direct

implementation

× (short/long) + (short/long)

Direct Implementation 216 / 648 180 / 612

Fast IMDCT (Szu-Wei Lee) 33 / 43 69 / 115

Improvement 84.7 % / 93.3 % 61.7 % / 81.2 %

HYBRID SYNTHESIS

• Windowing

Once transformed, subbands are windowed according to value of block_type (subbands with short blocks are separately transformed for each window and then overlapped)

• Overlap-adding

First half of transformed blocks is overlapped with second half of the corresponding blocks in the previous granule

FREQUENCY INVERSION

Every second sample in every second subband has to be multiplied by -1.

SYNTHESIS POLYPHASE FILTERBANK

This process produces 32 PCM audio samples.

• 576 / granule

• 1152 / frame (equal to 26 ms of audio @ 44,1 kHz)

Composed of several steps, it turns out to be the most time-consuming stage of the overall decoding process

SYNTHESIS POLYPHASE FILTERBANK

• Polyphase Matrixing

It is a cosine-like transform (non standard )

The direct computation involves a 64×32 matrix and requires almost ¼ of decoding time

Needs optimization to perform real-time decoding

• K. Konstantinides’ algorithm

• 32-point Fast DCT (B.G.Lee)

SYNTHESIS POLYPHASE FILTERBANK

• Konstantinides’ Algorithm

SYNTHESIS POLYPHASE FILTERBANK

• FCT Algorithm (Byeong-Gi Lee )

Using trigonometrical properties a 2M DCT can be performed by 2M-1 2-point DCTs

Direct computation

× = N ²

+ = N· ( N-1 )

FCT

× = N/2 · log2 ( N )

+ < 3· N/2 · log2 ( N )

WAVE STANDARD

• Individuated by a 44-byte header, holds information about:

• sampling frequency• number of channels• . . .

• Uncompressed PCM audio samples (normally with 16 bits/sample resolution) stored in following way:

Istante dicampionamento

Canale

0 1 (Left)

2 (Right)

1 1 (Left)

2 (Right)

2 1 (Left)

2 (Right)

PERFORMANCE ANALYSIS• PC Performances

The decoder, without optimization, works in real time on the following CPUs:

The decoder, with optimization , reaches 17,5× on Pentium IV CPU

PERFORMANCE ANALYSIS• C6711 DSK Performances

The decoder, without optimization, doesn’t work in real time on the board

• Parallel port is used for data transfer and it’s very very slow

• Most algorithms need optimization (only Huffman Decoding is optimized)

• Code needs some ASM optimization to use the full-potential of the board architecture

Whole decoding process (except data transfer TO external hard disk) takes about 10 times more than needed to work in real time. With optimization it is an easy goal to reach.

PERFORMANCE ANALYSIS• Time-occupation of optimized processes on C6711 DSK:

PERFORMANCE ANALYSIS• Time-occupation of optimized processes on C6711 DSK:

PERFORMANCE ANALYSIS• Time-occupation of other processes on C6711 DSK: