DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

When digital information is
processed or transmitted through
analog channels, bits are coded into
symbols that ideally remain independent.
Dispersion in this context is
called intersymbol interference (ISI).
k for Kell factor is unrelated to
K rating, sometimes called K factor,
which I will describe on page 542.
Kell, R.D., A.V. Bedford, and G.L.
Fredendall, “A Determination of the
Optimum Number of Lines in
a Television System,” in RCA Review
5: 8–30 (July 1940).
Hsu, Stephen C., “The Kell Factor:
Past and Present,” in SMPTE Journal
95 (2): 206–214 (Feb. 1986).
Figure 7.1 shows abstract input and output signals.
When bandwidth of an optical system is discussed, it is
implicit that the quantities are proportional to intensity.
When bandwidth of video signals is discussed, it is
implicit that the input and output electrical signals are
gamma-corrected.
Many digital technologists use the term bandwidth to
refer to data rate; however, the terms properly refer to
different concepts. Bandwidth refers to the frequency of
signal content in an analog or digital signal. Data rate
refers to digital transmission capacity, independent of
any potential signal content. A typical studio SDTV
signal has 5.5 MHz signal bandwidth and 13.5 MB/s
data rate – the terms are obviously not interchangeable.
Kell effect
Television systems in the 1930s failed to deliver the
maximum resolution that was to be expected from
Nyquist’s work (which I introduced on page 46). In
1934, Kell published a paper quantifying the fraction of
the maximum theoretical resolution achieved by RCA’s
experimental television system. He called this fraction k;
later, it became known as the Kell factor (less desirably
denoted K). Kell’s first paper gives a factor of 0.64, but
fails to give a complete description of his experimental
method. A subsequent paper (in 1940) described the
method, and gives a factor of 0.8, under somewhat
different conditions.
Kell’s k factor was determined by subjective, not objective,
criteria. If the system under test had a wide, gentle
spot profile resembling a Gaussian, closely spaced lines
on a test chart would cease to be resolved as their
spacing diminished beyond a certain value. If a camera
under test had an unusually small spot size, or a display
had a sharp distribution (such as a box), then Kell’s
k factor was determined by the intrusion of objectionable
artifacts as the spacing reduced – also a subjective
criterion.
Kell and other authors published various theoretical
derivations that justify various numerical factors;
Stephen Hsu provides a comprehensive review. In my
CHAPTER 7 RESOLUTION 67