
L64005 MPEG-2 Audio/Video Decoder Technical Manual
8-7
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
8.3
Synchronization
Basics
The MPEG-2 encoded bit stream contains time stamps that indicate the
presentation and decoding times of video pictures and audio frames. The
video pictures and the audio frames are called Presentation Units (PUs).
The sets of coded bits that comprise the PUs and that are included in
the MPEG-2 bit stream are called Access Units (AUs).
Within an MPEG-2 bit stream, some Audio Access Units and Video
Access Units have Presentation Time Stamps (PTS) associated with
them. A PTS indicates the time at which the PU that results from decod-
ing the AU should be presented to the user. The L64005 samples the
audio PTS and video PTS with respect to the same clock reference, the
System Time Clock (STC). When the MPEG-2 bitstream transmits the
correct values of the audio and video PTS, the presented audio and
video need to be precisely synchronized with respect to the common
STC.
Figure 8.1 shows a block diagram of a MPEG-2 transport encoder and
the principle of sampling time stamps and reference clocks from the
same STC reference.
Figure 8.1
MPEG-2 Transport
Encoder
Video
Audio
Video ES
Audio ES
Video PES
Audio PES
Transport
Mux
Data
Data PES
Data ES
Program
Clock
Program Counter
27 MHz
SCR, PTS, DTS, ESCR
PCR
Transport
Stream
Program #n
Program #n+1
Program #n+2
Program #n+3
Video
Encoder
Audio
Encoder
Data
Encoder
Video
Packetizer
Audio
Packetizer
Data
Packetizer