Triple-Play Service Deployment

Chapter 6: Troubleshooting Video in the Headend
Reference Frames
Another critical element in this decode process is the use of a true
reference frame. The predictions the decoder must make will be
limited by the accuracy of its reference points. MPEG compression
creates three types of video frames, based on the level of
compression applied to each frame.
– I-Frames (Intra-coded Frame): These undergo minimal compression,
and the decode of an I-frame has no dependence on any
other frame. This is a true frame, a complete reference for the
decoder to use.
– P-Frames (Predicted Frames): These undergo moderate compression,
and the decoder must look backward to a previous I or P
frame to apply the differentials and rebuild these.
– B-Frames (Bi-directionally predicted Frames): These undergo
extensive compression, and the decoder rebuilds them in a similar
fashion from both previous and future I and P frames.
The encoder will divide the entire video file into groups of these
frames, each called a Group of Pictures (GOP). Each GOP starts with
a new full reference I-Frame. This division of frames helps mitigate
the propagation of errors as they are limited to one GOP. With the
next GOP comes a new I-frame, and the picture sequence begins
again. Traditionally, the GOP would be 12-15 frames, but the GOP
size is variable; there can be different settings for more or less
frames in the GOP. Adjusting the GOP settings directly affects the
presentation picture quality by adjusting the level of compression,
and indirectly affects the user quality of experience. The GOP size
affects the subscriber’s set top box workload, impacting factors
including latency and channel change time. For example, the set
top box must wait for an I-Frame to start a program presentation.
This wait time is a significant factor in channel change time.
Therefore, the GOP settings will impact that aspect of the user
experience.
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