
L64005 MPEG-2 Audio/Video Decoder Technical Manual
1-9
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
Coding and quantization are techniques that are applied to data that has
been mapped into the frequency domain and ltered into subbands.
Psychoacoustic modeling is a technique that determines the best alloca-
tion of data within the available data channel bandwidth based on human
perception.
The general structure of an MPEG audio encoder is shown in
Figure 1.4.Figure 1.4
Audio Encoding
Process
(Simplied)
Once audio data has been coded, it may be stored or transmitted digi-
tally. MPEG provides a framework for use of packet-oriented transmis-
sion of compressed data. In particular, ISO CD 11172 denes formats
for digital data streams for both video and audio. The ISO System
Stream format is designed to accommodate both audio packets and
video packets within the same framework for transmission. The data may
be physically delivered in parallel form or serial form. The System Stream
is composed of a sequence of packs, as shown in
Figure 1.5.Figure 1.5
ISO System
Stream
An MPEG pack is composed of a
pack layer header, a system header
packet, a sequence of packets, and ends with an ISO 11172 end code.
The pack layer header contains a pack start code used for synchroniza-
Bit Allocation
Processor
(Among Subbands,
Coding, Quantizing)
Frequency
Filter Bank
Digitized
Audio
Input
Psychoacoustic
Model
Bitstream
Formatter
(Mapping)
(in audio frames)
Pack
Layer
Header
System
Header
Packet
Pack
Contains:
Pack Start Code (32 bits),
System Clock Reference
Contains:
Packet
ISO
11172
End Code
. . .
Various data, including
system stream ID
Contains:
Audio stream data
More Packets
(rst)
(last)
(variable #)
(128 bits)