
L64005 MPEG-2 Audio/Video Decoder Technical Manual
8-29
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
Real systems require ltering of the presentation
Errp. Providing hyster-
esis on the skip/repeat function prevents oscillating “skip-repeat-skip” in
a closed loop system.
Real systems have limits on available memory. Synchronization delays
result in additional bits that must be stored in the channel buffer. Under
normal conditions, the host may execute a video skip or repeat if the
video error is larger than the range
Errp = -15 ms to + 15 ms. If a system
has extremely tight channel buffering constraints, then the control soft-
ware can skew the presentation time more toward negative sync delay—
and present early—so that the skip/repeat boundary may occur at
Errp
= -25 ms to +5 ms after ltering. This causes a skip at even small sync
delays of 5 ms, preventing possible overow.
Real systems have overows, underows, channel changes and errors.
Each of these require exible mechanisms in the host processor control
loop for handling synchronization and re-synchronization. For example,
on a channel change, the host may clear all
Errp and SCRerr values and
clear all lters to restart synchronization control.
Real systems may perform clock recovery in the transport chip, while the
rest of the synchronization scheme is handled in the decoder chip. A
software-controlled closed loop can be implemented to easily manage
clock recovery across chip boundaries.
Real systems may have external presentation delays such as video-post-
processing chips for graphics, or external audio processors which can
skew the effective presentation time. A software-controlled system can
control the real presentation time by adding any presentation delay to the
skip/repeat limits, effectively delaying or advancing the audio or video in
time.
Real systems, or disk based systems, may chose to ignore the ESCR
and instead use the audio PTS as the system clock. Such “audio master”
systems have the advantage that they never introduce distortion in the
audio decode, but they require more exible software for video synchro-
nization. This system also requires a variable rate channel.