DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Active lines (vertically) encompass
the picture height. Active samples
(horizontally) encompass not only
the picture width, but also up to
about a dozen blanking transition
samples.
compression is now so important that a count of
480 lines has become de rigeur for 525-line MPEG
video. In 576i scanning, a rigid standard of 576 picture
lines has always been enforced; fortuitously for MPEG
in 576i, this is a multiple of 16.
MPEG-2 accommodates the 1920×1080 image format,
but 1080 is not a multiple of 16. In MPEG-2 coding,
eight black lines are appended to the bottom of each
1920×1080 picture, to form a 1920×1088 array that is
coded. The extra 8 lines are discarded upon decoding.
Traditionally, the image array of 480i and 576i systems
had halflines, as sketched in Figures 11.3 and 11.4 on
page 98: Halfline blanking was imposed on picture
information on the top and bottom lines of each frame.
Neither JPEG nor MPEG provides halfline blanking:
When halfline-blanked image data is presented to
a JPEG or MPEG compressor, the blank image data is
compressed. Halflines have been abolished from HDTV.
Studio video standards have no transition samples on
the vertical axis: An instantaneous transition from
vertical blanking to full picture is implied. However,
nonpicture vertical interval information coded like
video – such as VITS or VITC – may precede the picture
lines in a field or frame. Active lines comprise only
picture lines (and exceptionally, in 480i systems, closed
caption data). L A excludes vertical interval lines.
Computer monitor interface standards, such as those
from VESA, make no provision for nonpicture (vertical
interval) lines other than blanking.
Choice of SAL and SPW parameters
In Scanning parameters, on page 54, I characterized two
video signal parameters, samples per active line (SAL )
and samples per picture width (SPW ). Active sample
counts in studio standards have been chosen for the
convenience of system design; within a given scanning
standard, active sample counts standardized for
different sampling frequencies are not exactly proportional
to the sampling frequencies.
CHAPTER 27 VIDEO SIGNAL PROCESSING 325