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18 © 2008 VBrick Systems, Inc.
past frame (for example, it is likely that the sky will still be on the top of the frame and that it
will still be blue), it is possible to predict portions of a future frame. P-Frames (Predictive)
reference the nearest previous I-frame. Video data that contains only I and P frames requires
less bandwidth than video that contains only I-Frames for comparable quality.
With I-Frames and P-Frames being generated, it is possible to place the P-Frame some
number of frames away from its reference I-Frame, and calculate the contents of the frames
in between using both the I-Frame and the P-Frame as references. These frames are called
B-Frames and are bi-directional (i.e. they are based on a predicted future frame and on a past
frame). Also note that B-Frames are based only on I-Frames and P-Frames, never on past B-
Frames, so errors are not propagated. VBricks can be configured to produce I-Frames only, I
and P-Frames only, or I, B and P-Frames. In terms of bandwidth usage, I-Frames use the
most, B-Frames require the least. It follows then that I-Frames contain the most information
and B-Frames contain the least information.
Group of Pictures (GOP)
The collection of I, B, and P frames makes up a Group of Pictures (GOP). The selection of
GOP parameters is a trade-off between bandwidth usage, picture quality, sensitivity to
transmission errors, and delay. An extended GOP will be more sensitive to transmission
errors than a small GOP because any error can affect the entire GOP. It is possible to
configure the GOP several ways, for example, I only, IP, IBP, IBBP, etc.
Reference Distance
Reference Distance and Intrapicture Distance are set in
Configuration: Encoder > Video. The
Reference
Distance
defines the number of consecutive B pictures
there are between an I or P picture and then next I or P
picture:
1. No B picture
2. One B picture between I or P pictures
3. Two B pictures between I or P pictures
The
Intrapicture Distance can be set between 1 and 19 (default is 15) for MPEG-2. The
value refers to the number of P or B frames between I frames and defines the Group of
Pictures (GOP).
Interrelationship
Table 2 Reference Distance Settings
illustrates various combinations Reference Distance
settings (up to 3) and GOP Length (up to 19). The GOPs are shown in display order. More P
and B frames increase the compression ratio; however, B frames increase the overall delay. In
general, it is best to chose the highest compression ratio and then try lower ratios (e.g. lower
reference distance) if the video experiences transmission problems.
In operation, a P-Frame will be generated and transmitted before the B-Frame. An MPEG
decoder must, therefore, reorder the incoming frame to display the I-B-P frames in the
correct order. This is done using "presentation time stamps" created by the MPEG encoding
and multiplexing process. This reordering process requires one or more frames be held in the
decoder buffer, waiting it's turn for display, therefore increasing the delay. At 30 frames per
second, each frame requires approximately 33 milliseconds. Synchronized audio is also
compressed using MPEG, and is multiplexed with the video to create an MPEG Transport
Stream.