Image Processing Reference
In-Depth Information
process is computationally quite expensive and to achieve the necessary performance, the
coder may have a range limit on where it looks for blocks to motion estimate. There are a
variety of techniques that codec implementers use to speed this up, and these will be the
differentiating factors in their coder designs. Noticeably better results may be obtained
with one encoder when compared with another, even though they purport to code the
same standard model. As long as the bit stream they create is strictly standards-compliant,
the decoder will be able to understand it. You may find that paying more for the encoder
turns out to be money well spent in the end.
The concept of tools is introduced in MPEG-1 with there being just the one tool available
to control the quantization scale factor (QSF). The QSF tool is used to manage the loading
of the outgoing bit stream and is discussed in Chapter 14.
Tools are a way of describing components within the implementation of the standard
that is used to define how much of the standard is implemented. In general, these tools
describe techniques that are engineered into the encoding process.
MPEG-1 Caveats
Note that MPEG-1 has no capability to deal with interlaced content. Any processing or
management of interlaced fields must be carried out prior to using the MPEG-1 encoder.
Your application code has to work somewhat harder to de-interlace the content before
MPEG-1 encoding starts, which is an additional processing overhead. The consequence is
that the MPEG-1 coding efficiency is compromised.
Note also that the bit-rate specification for MPEG-1 places severe limits on how much
data can be transmitted. The limits are defined in bits per second (bps) but if this is con-
verted to macroblocks per second, there is not enough bandwidth to deliver a full-resolu-
tion TV signal at the full frame rate. It is only a little bit smaller than the 525-line raster used
in the United States but it is significantly smaller than the 625-line raster required in Europe.
For these reasons, MPEG-1 has some continued use as a legacy format but for televi-
sion applications it has been superseded by the MPEG-2 standard.
The Next Generation
While the MPEG-1 codec did a fine job in the past, the limitations forced the experts to
develop an improved coding standard. Understanding MPEG-1 is important because all
the advancements in MPEG coders after that are based on a lot of the same concepts. Yes,
they are more complex and sophisticated but the underlying principles are the same.
MPEG-2 uses the same divide-and-conquer techniques as we will see when we examine it
in the next chapter.
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