
7-6
Audio Decoder
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
7.2.5
Determining
the
Presentation
Time
Once the decoder is running, the presentation time for the audio Presen-
tation Unit (PU) is deterministic. This is because the length of the audio
PU is constant. It is therefore of interest to know only the difference
between the actual presentation time of the PU and the time that is
requested by the encoder and transmitted in the audio presentation time
stamp (PTS). Ideally, the audio PTS will match the actual presentation
time. If it does not match, the audio play speed must be increased or
decreased.
The audio decoder provides an Audio Sync Detect interrupt synchro-
nously with the start of decode of the access unit. The system controller
can use this interrupt to determine the error between the audio PTS and
the presentation time of the PU. If the decoder is repeating a frame sec-
tion, the Audio Sync Detect interrupt does not occur until the repeat is
nished and the next new frame has entered the decoder. If the decoder
is skipping a frame, the interrupt occurs earlier than normal—as soon as
the new frame starts to be decoded.
Note that there is a deterministic delay from when the Audio Sync Detect
interrupt is generated and when the audio enters the PCM output lter.
This delay is 1/12 of an audio frame for Layer 1 and 1/36 of a frame for
Layer 2.
7.2.6
Ancillary
Channel Data
The L64005 parses ancillary audio channel data, but the user cannot
access this data.
7.2.7
Error Detection
The L64005 detects certain errors in the coded MPEG data, as well as
other error conditions.
Table 7.2 lists the maskable interrupts that are
used to indicate errors and their location. Refer to
Chapter 2 for specic
interrupt descriptions.