DVD Demystified

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Chapter 3
encoders only create I frames. This is less efficient but may be necessary for
very inexpensive real-time encoders that must process 30 or more frames a
second.
MPEG-2 encoding can be done in real time (where the video stream
enters and leaves the encoder at display speeds), but it is difficult to produce
quality results, especially with variable bit rate (VBR). VBR allows varying
numbers of bits to be allocated for each frame depending on the complexity.
Less data is needed for simple scenes, whereas more data can be allocated
for complex scenes. This results in a lower average data rate and longer
playing times but provides room for data peaks to maintain quality. DVD
encoding frequently is done with VBR and is usually not done in real time,
so the encoder has plenty of time for macroblock matching, resulting in
much better quality at lower data rates. Good encoders make one pass to
analyze the video and determine the complexity of each frame, forcing I
frames at scene changes and creating a compression profile for each frame.
They then make a second pass to do the actual compression, varying quantization
parameters to match the profiles. The human operator often
“tweaks” minor details between the two passes. Many low-cost MPEG
encoding hardware or software for personal computers uses only I frames,
especially when capturing video in real time. This results in a simpler and
cheaper encoder, since P and B frames require more computation and more
memory to encode. Some of these systems can later reprocess the I frames
to create P and B frames. MPEG also can encode still images as I frames.
Still menus on a DVD, for example, are I frames.
The result of the encoding process is a set of data and instructions (Figure
3.5). These are used by the decoder to recreate the video. The amount of
compression (how coarse the quantizing steps are, how large a motion estimation
error is allowed) determines how closely the reconstructed video
resembles the original. MPEG decoding is deterministic—a given set of
input data always should produce the same output data. Decoders that
properly implement the complete MPEG decoding process will produce the
same numerical picture even if they are built by different manufacturers. 7
This does not mean that all DVD players will produce the same video picture.
Far from it, since many other factors are involved, such as conversion
from digital to analog, connection type, cable quality, and display quality.
Advanced decoders may include extra processing steps such as block filter-
7 Technically, the inverse discrete cosine transform (IDCT) stage of the decoding process is not
strictly prescribed, and is allowed to introduce small statistical variances. This should never
account for more than an occasional least significant bit of discrepancy between decoders.