
Audio Compression and Decompression Concepts
A-9
Figure A.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
synchronization purposes, and a system clock value. The system header
packet contains a variety of housekeeping data, in particular, a system
stream ID used to differentiate among multiple system streams. A
sequence of one or more packets contains either encoded audio or
encoded video stream data. The ISO 11172 end code is the final
element in an MPEG pack. For detailed definition of pack headers, refer
to the ISO/IEC 11172-1 system stream descriptions.
Any one MPEG packet carries either audio or video data, but not both
simultaneously. An MPEG Audio Packet contains an audio packet header
and one or more Audio Frames.
Figure A.6
shows the packet structure.
Figure A.6
MPEG Audio Packet Structure
(in audio frames)
Pack
Layer
Header
System
Header
Packet
Packet
(first)
Pack
Pack
Contains:
Pack Start Code (32 bits),
System Clock Reference
(128 bits)
Contains:
Various data, including
system stream ID
Packet
(last)
ISO
11172
End Code
. . .
Contains:
Audio stream data
More Packets
(variable #)
Packet
Header
Audio
Audio Packet
Contains:
Packet Start Code
Packet Length
Presentation Time Stamps
. . .
Audio
Frame
(first)
Audio
Frame
(last)
Audio Packet
Contains:
Audio Frame Header
Audio Frame CRC
Audio Data
Ancillary/User Data
Audio Frames
(quantity varies)
. . .