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PDF AN1482 Data sheet ( Hoja de datos )
| Número de pieza | AN1482 | |
| Descripción | MPEG2 Main Profile Video Decoding on ST100 | |
| Fabricantes | ST Microelectronics | |
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AN1482
APPLICATION NOTE
MPEG2 Main Profile Video Decoding on ST100
By Maurizio COLOMBO
ABSTRACT
This document reports the results of the porting of a MainProfile@MainLevel MPEG-2 decoder on ST120.
This is the standard notably used for DVD.
CONCLUSION
The optimized application needs 228.4 MCycles/s to decode 25 PAL frames/s in the case of M=3, N=12,
bitrate=15 Mbps. In this evaluation we assume that all the data are into the XY internal memories (ideal
DMA).
October 2001
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AN1482 - APPLICATION NOTE
Table 3 : Huffman decoding/Inverse quantization/Inverse scan
Function
Code Size (Bytes)
Cycles/Symbol
Decode_MPEG1_Intra_Block()
1272
24
Decode_MPEG1_Non_Intra_Block()
1040
24
Decode_MPEG2_Intra_Block()
1184
24
Decode_MPEG2_Non_Intra_Block()
896 24
All these functions are located in the module getblk.st1. The Huffman look-up tables needed for these
functions takes 1732 bytes (read-only data memory).
2.4 - Temporal Prediction
This function is basically made of three parts: the memory read from the external frame buffer, the
computation of horizontal and/or vertical interpolation and/or the average with the previously computed
predictor in case of bidirectional interpolation or dual prime prediction.
The first task is left to a DMA, that will be programmed with the address of the predictor just after the
macroblock header decoding, based on the prediction type information and on the motion vectors. For P
skipped macroblocks, the DMA will directly copy the predictor into the current frame buffer.
So, the profiling reported here only includes the computational cost of the interpolation and/or average.
An efficient implementation should envisage the use of a "powered" DMA, able to make itself this task, in
particular to cope with B skipped macroblocks. For these on, a simple memory copy is not enough, since
a B skipped macroblock inherit the prediction mode and the vectors from the previous one, whereas a P
skipped macroblock is simply patched with the corresponding one in the previous image.
Anyway, the software complexity of this task is computed here for the sake of completeness and for an
eventual implementation on a small video format. There are several possible combinations of
interpolations/average. The following Table 4 gives the number of clock cycles needed for a 16x16
luminance macroblock computation, in the different cases. The function is located in the module
motion_comp.st1 and its name is form_component_prediction().
Table 4 : Temporal prediction : the different cases
Operation
Clock Cycles For 16x16 Luminance
Simple copy
125
Copy with average
240
Vertical interpolation
274
Vertical interpolation + average
434
Horizontal interpolation
271
Horizontal interpolation + average
431
Vert+Hor interpolation
623
Vert+Hor interpolation + average
815
The code size for this function is of 5524 bytes. A read-only table of 48 bytes is needed for the jump at the
beginning (switch structure). The algorithm for the chroma components is the same, but the cost is
reduced by a factor 4 for each component.
2.5 - Bitstream Parsing
This block represents a set of small functions that are used basically to decode the macroblock headers
(containing information about the contents of the macroblock and the motion vectors). These functions
are located in the modules mb_headers.st1 (macroblock_modes(), Get_coded_block_pattern(),
Get_macroblock_address_increment()) and motion.st1(motion_vector() and motion_vectors()).
The code size of these functions amounts to 2336 bytes. The data memory needed for the Huffman
tables is of 584 bytes. The computational cost of these functions cannot be provided as standalone, since
it depends on the bitstream to be decoded.
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