
L64005 MPEG-2 Audio/Video Decoder Technical Manual
1-21
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
Reduced Memory Mode, capable of decoding and displaying a full
resolution PAL with 16-color OSD with only 2 MBytes of external
DRAM.
Addition of on-chip PLL provides higher bandwidth that triples the
DRAM interface clock frequency from 27 MHz to 81 MHz.
1.6.2
System Layer
Decoding
The L64005 performs MPEG-2 system layer decoding of the Pack and
PES layer. The primary purpose of Pack and PES decoding is to support
the extraction of the video and audio timestamps, which are then used
for synchronization purposes. The system layer is parsed bit serially
before the data is written to the channel buffers.
The L64005 parses MPEG-2 Pack Layer and video and audio PES pack-
ets only. Other types of PES packets are discarded. The video and audio
streams are separated into header and payload streams and written to
independent buffers in the DRAM. There are therefore four buffers: video
payload, video PES header, audio payload and audio PES header. The
video and audio payload buffers are the video and audio channel buffers.
The PES header buffers queue pending header information until decode
time. The external microprocessor may read the contents of the PES
header buffers at any time. The external processor can synchronize
audio and video by reading the presentation timestamps from the PES
header buffers, adjusting them for system dependent delays, and then
writing the adjusted values back to the L64005. The adjusted values are
compared with a local copy of the system clock reference at the presen-
tation time of the respective audio and video presentation units. The error
between the actual presentation time and the desired presentation time
can then be determined. Depending on the sign and magnitude of the
error, the audio and video decoders may then be independently
instructed to skip or repeat a frame. This process continues until the
decoders are synchronized to the system clock reference.
Other parameters carried in the PES headers may be read on demand
and limited to the length of the PES header buffers—which is program-
mable.
1.6.3
Video Output
Features
The decoder outputs video through an 8-bit interface clocked at the
device clock. It is expected that in a given implementation, the L64005
will be operated at the nominal clock frequency of 27 MHz. This clock
provides both the computational timebase as well as the video timing ref-