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

2. DECODING PROCESS

3. 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

4. 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

5. 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 original
positions
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 %

6. 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

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

8. 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

9. 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)

10. SYNTHESIS POLYPHASE FILTERBANK
• Konstantinides’ Algorithm

11. SYNTHESIS POLYPHASE FILTERBANK
• FCT Algorithm (Byeong-Gi Lee )
Using trigonometrical properties a 2
M
DCT can be performed by
2
M-1
2-point DCTs
Direct computation
× = N ²
+ = N· ( N-1 )
FCT
× = N/2 · log2
( N )
+ < 3· N/2 · log2
( N )

12. 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 di
campionamento
Canale
0 1 (Left)
2 (Right)
1 1 (Left)
2 (Right)
2 1 (Left)
2 (Right)

13. 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

14. 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.

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

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

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