L64005 MPEG-2 Audio/Video Decoder Technical Manual
1-5
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
of the series. The MPEG-2 standard includes additional block scanning
orders.
After block scanning has been performed, the encoder performs
run-
length coding on the AC coefcients. This process reduces each 8 x 8
block of DCT coefcients to a number of events represented by a non-
zero coefcient and the number of preceding zero coefcients. Because
many coefcients are likely to be zero after quantization, run-length cod-
ing increases the overall compression ratio.
The encoder then performs
variable-length coding (VLC) on the resulting
data. VLC is a reversible procedure for coding that assigns shorter code-
words to frequent events and longer codewords to less frequent events,
thereby achieving video compression. Huffman encoding is a particularly
well-known form of VLC that reduces the number of bits necessary to
represent a data set without losing any information.
The nal compressed video data is now ready for transmission to either
a local storage device from which a video decoder may later retrieve and
decompress the data, or to a remote video decoder via cable or direct
satellite broadcast, for example.
1.1.2
Bitstream
Syntax
The MPEG standard species the syntax for a compressed bitstream.
The video syntax contains six layers, each of which supports either a sig-
nal processing or a system function. The layers and their functions are
Table 1.1
MPEG
Compressed
Bitstream Syntax
The MPEG syntax layers correspond to a hierarchical structure. A
sequence is the top layer of the video coding hierarchy and consists of
a header and some number of
groups-of-pictures (GOPs). The sequence
header initializes the state of the decoder, which allows the decoder to
decode any sequence without being affected by past decoding history.
Syntax Layers
Function
Sequence Layer
Random Access Unit: Context
Group of Pictures Layer
Random Access Unit: Video
Picture Layer
Primary Coding Unit
Slice Layer
Resynchronization Unit
Macroblock Layer
Motion Compensation Unit
Block Layer
DCT Unit