L64005 MPEG-2 Audio/Video Decoder Technical Manual
8-9
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
The actual values encoded in the PTS elds are dened in terms of the
System Target Decoder (STD). The STD is a mathematical model of an
idealized decoder and is a fundamental normative constraint on all sys-
tem bit streams. In the STD model, the decoder is always synchronized
with the data source, and audio and video decoding and presentation are
exactly synchronized. The STD is a simplied model with respect to real
decoder implementation. In particular, the STD model assumes decoding
is performed instantaneously In a real decoder the video and audio
decoding are not instantaneous, and their delays must be taken into
account.
Another parameter in the system bit stream is the Decode Time Stamp
(DTS). The DTS refers to the time that an AU is to be extracted from the
decoder buffer and decoded in the STD model. Because the STD model
dictates instantaneous audio and video decoding time, the DTS and PTS
are identical. The DTS and PTS are not identical when video pictures are
reordered—for example, when B-frames are used.
Because not every Audio Access Unit and Video Access Unit requires
PTS and DTS values, the decoder may interpolate these values when
they are not encoded. PTS values are required with intervals not exceed-
ing the 700ms in each elementary audio and video stream. In properly
locked systems, there is a xed difference in STC counts between the
decoding of adjacent PUs. For example, video PUs coded at a 29.97 Hz
frame rate have a difference of 3003 cycles, or 90 KHz, between pic-
tures.
Table 8.3 lists an example of the DTS and PTS values for audio and
video AUs that the upstream encoder generates and inserts in the
MPEG-2 bit stream. In the example, the Video Access Unit is 25-Hz
CCIR-601, with 2 B-frames between I- and P-frames, and 12 frames
between I-frames. The Audio Access Unit has a 48-KHz sampling rate,
with 1152 samples per audio frame. The initial delay in the presentation
time stamps supports the Video Buffer Verier (VBV) delay.