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elementary streams (ES). Each digital program is composed of a
pair of audio and video ESs. That ES pair must then be delivered
together to the set top box for decoding and presentation. By
definition, compression suggests an end product that is smaller
than the original.To accomplish this reduction, MPEG compression
permanently removes data from the original file to get the smaller
output version. Much of the complex algorithms used in the
compression exist so the decoder can figure out how to rebuild all
the data that has been removed, and all of them require an ability
to make predictions of what the missing data is by looking at
video frames before and after the frame in question. For this to
work, the decoder must decode certain future video frames in
advance, and it must retain certain older frames to use as reference
in rebuilding intermediate video frames. This is accomplished by
the encoder generating two sets of time stamps for the video, one
for the presentation sequence (PTS) and another for the decode
sequence (DTS).
Presentation Order (coming out of encoder, and as seen on TV)
I1 B1 B2 P1 B3 B4 P2 B5 B6
I1 P1 B1 B2 P2 B3 B4 P3 B5 B6
Decode Order (going into set-top box buffer)
Chapter 6: Troubleshooting Video in the Headend
P3 B7 B8
Figure 6.2 MPEG compression removes data from the video frames, requiring the decoder to
make prections between reference I and P frames to rebuild the intermediary P and B frames.
This requires a different decode sequence then the presentation sequence.