DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

CMs in a segment are
denoted a through e.
contains the DC term, and several AC terms, of each
block’s DCT. However, that’s not the whole story.
DV overflow scheme
When a portion of an image is devoid of detail, a few
low-frequency AC coefficients of its DCT may have
significant magnitude, but nearly all high-frequency AC
terms will have very small magnitude. When a portion
of an image has a lot of fine detail, many highfrequency
AC terms will have large magnitude. To
reproduce the image accurately requires that these
terms be recorded. Generally, increasing amounts of
detail require increased data capacity.
It is unusual for an image to contain detail everywhere;
typically, complexity is spatially concentrated in an
image. A compression algorithm should adapt to the
spatial distribution of image detail, by allocating bits
where they are needed. If fine detail is distributed
throughout an image – in a full-frame image of the
leaves of a tree, perhaps – then even quite large reconstruction
errors are likely to be imperceptible.
The requirement for picture-in-shuttle seems to
preclude allocation of data capacity to the regions
where more bits are required. However, DV implements
an overflow scheme whereby the bits resulting
from compression of a handful of macroblocks are
shared among a handful of CMs. Should the VLE-coded
AC coefficients for a complex block (augmented by
a 4-bit EOB) require more bits than the fixed capacity
assigned to that block in the CM, the overflow bits
“spill” into other blocks whose capacity was not filled.
Overflow data first spills into space that might remain
in other blocks of the same CM; any remaining bits spill
into available space in any of four other CMs associated
with diverse regions of the image. The set of five
coded macroblocks that share overflow space is called
a segment; a segment has a fixed capacity of 385 bytes.
The macroblocks of a segment are spatially distributed
throughout the frame: No two macroblocks are taken
from the same row or the same column of superblocks.
This distribution exploits the statistical likelihood that
only one or two of the macroblocks will be complex.
CHAPTER 39 DV COMPRESSION 465