DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

I introduced twitter on page 57.
Mitsuhashi, Tetsuo, “Scanning Specifications
and Picture Quality,” in
Fujio, T., et al., High Definition television,
NHK Science and Technical
Research Laboratories Technical
Monograph 32 (June 1982).
opinion, such numerical measures are so poorly defined
and so unreliable that they are now useless. Hsu says:
Kell factor is defined so ambiguously that individual
researchers have justifiably used different theoretical
and experimental techniques to derive widely varying
values of k.
Today I consider it poor science to quantify a Kell
factor. However, Kell made an important contribution
to television science, and I think it entirely fitting that
we honor him with the Kell effect:
In a video system – including sensor, signal processing,
and display – Kell effect refers to the loss of resolution,
compared to the Nyquist limit, caused by the spatial
dispersion of light power. Some dispersion is necessary to
avoid aliasing upon capture, and to avoid objectionable
scan line (or pixel) structure at display.
Kell’s 1934 paper concerned only progressive scanning.
With the emergence of interlaced systems, it became
clear that twitter resulted from excessive vertical detail.
To reduce twitter to tolerable levels, it was necessary to
reduce vertical resolution to substantially below that of
a well-designed progressive system having the same
spot size – for a progressive system with a given k, an
interlaced system having the same spot size had to have
lower k. Many people have lumped this consideration
into “Kell factor,” but researchers such as Mitsuhashi
identify this reduction separately as an interlace factor
or interlace coefficient.
Resolution
SDTV (at roughly 720×480), HDTV at 1280×720, and
HDTV at 1920×1080 all have different pixel counts.
Image quality delivered by a particular number of pixels
depends upon the nature of the image data (e.g.,
whether the data is raster-locked or Nyquist-filtered),
and upon the nature of the display device (e.g.,
whether it has box or Gaussian reconstruction).
In computing, unfortunately, the term resolution has
come to refer simply to the count of vertical and hori-
68 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES