AN1482 - APPLICATION NOTE

3/8

1 - OVERVIEW ON MAIN PROFILE DECODING

Figure 1 shows the block diagram of the Main Profile decoder [1]. The basic building blocks are the same

as for Simple Profile, but some additional features are supported in Main Profile. In particular, Main Profile

defines a new type of pictures, called B (bidirectionally interpolated), that are predicted from past and

future I/P images. This implies an increased complexity of the

bidirectional prediction.

A Main Profile decoder must support all the prediction modes (up to six) and the two possible picture

structures (frame pictures and field pictures).

Block

The corresponding bitstream parser, represented by the block

must be able to support the syntax corresponding to the above features.

Huffman decoding must support MPEG-1 backward compatibility (two special routines have been added

for this, since MPEG-1 syntax uses different escape codes) and two types of coefficients scan (zig-zag

and alternate).

The following chapters depict all these basic blocks, showing their performance on the optimized

assembly implementation for ST120. However, the reference C source is the one developed by the

MPEG Software Simulation Group [2].

Figure 1 : Main Profile decoder block diagram

IDCT

Temporal

FRAME BUFFER

B

Huffman

decoding/inverse

quantization/inverse

scan

Headers and motion

vectors decoding

I

T

S

T

R

E

A

M

Prediction

Add

Prediction