ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

Page: 73 / 116

The
Guide
73
Bi
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Wh
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b
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?
Th
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As described earlier in the guide, predicting blocks consumes far less bits than
encoding them intra-blocks. Because B-frames can be both forwards and
backwards predicted in general B-frames will have a lower proportion of intra-
blocks than any other type of frame.
Since B-frames themselves are never referenced by any other frame it is
possible for the encoder to use a higher quantizer than is normal for their coding
since it need not be concerned with impacting upon the quality of other frames
due to compound errors.
These two key features of B-frames lead them to offer the greatest compression
ratio of any of the available frame types.
Backward prediction introduces issues relating to the sequence in which frames
must be encoded and decoded. Whereas forward prediction, as used by P-
frames, simply references the last frame that was decoded, a backwards
predicted block in a B-frame makes reference to some future frame that has not
yet been decoded. Therefore, in order to decode B-frames it is necessary to
decode future frames first. This is why frames may appear out-of-order in the
multipass log file (see
Bitrate mode—Multipass) .
Consider once again the above illustration. Here the blue arrows represent
forwards prediction in the P-frames and the red arrows represent either forward
(left) or backward (right) prediction in the B-frames.
It is clear from the diagram that the B-frames can make reference to future
frames because there are red arrows pointing to the right. The encoder must re-
order these frames so that they are in the correct order for decoding.
Frame 1 (I) Frame 3 (P) Frame 5 (P) Frame 2 (B) Frame 4 (B)
The ordering of frames