DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

See Scanning parameters, on
page 54, and Transition
samples, on page 323.
In 480i line assignment, on
page 500, I will detail how 480i
studio standards provide up to
487 picture lines. In 576i line
assignment, on page 520, I detail
how 576i studio standards
provide 574 full lines and two
halflines.
filtering operations are frequently cascaded, particularly
in the studio, and it is unacceptable to repeatedly
narrow the image width upon application of a sequence
of FIR filters. A strategy is necessary to deal with
filtering at the edges of the image.
Some digital image-processing textbooks advocate
considering the area outside the pixel array to contain
replicated edge samples. I consider this to be quite
unrealistic, because a small feature that happens to lie
at the edge of the image will exert undue influence into
the interior of the pixel array. Other digital imageprocessing
textbooks consider the image to wrap in
a cylinder: Missing samples outside the left-hand edge
of the image are copied from the right-hand edge of the
image! This concept draws from Fourier transform
theory, where a finite data set is treated as being cyclic.
In practice, I consider the wrapping strategy to be even
worse than edge replication.
In video, we treat the image as lying on a field of black:
Unavailable samples are taken to be zero. With this
strategy, repeated lowpass filtering causes the implicit
black background to intrude to some extent into the
image. In practice, few problems are caused by this
intrusion. Video image data nearly always includes
some black (or blanking) samples, as I outlined in the
discussion of samples per picture width and samples
per active line. In studio standards, a region lying within
the pixel array is designated as the clean aperture, as
sketched in Figure 6.4, on page 55. This region is
supposed to remain subjectively free from artifacts that
originate from filtering at the picture edges.
Picture lines
Historically, the count of picture lines (image rows, LA) has been poorly standardized in 480i systems. Various
standards have specified between 480 and 487 picture
lines. It is pointless to carry picture on line 21/284 or
earlier, because in NTSC transmission this line is
reserved for closed caption data: 482 full lines, plus the
bottom halfline, now suffice. With 4:2:0 chroma
subsampling, as used in JPEG, MPEG-1, and MPEG-2,
a multiple of 16 picture lines is required. MPEG
324 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES