DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Bellamy, John C., Digital
Telephony, Second Edition
(New York: Wiley, 1991),
98–111 and 472–476.
For engineering purposes, we
consider R’, G’, and B’ to be
encoded with identical transfer
functions. In practice, encoding
gain differs owing to white
balance. Also, the encoding
transfer functions may be
adjusted differently for artistic
purposes during image capture
or postproduction.
Excursion in analog 480i
systems is often expressed in IRE
units, which I will introduce on
page 327.
As I have mentioned, CD audio is coded linearly, with
16 bits per sample. Audio for digital telephony usually
has just 8 bits per sample; this necessitates nonlinear
coding. Two coding laws are in use, A-law and µ-law;
both of these involve decoder transfer functions that
are comparable to bipolar versions of Figure 2.3.
In video (including motion-JPEG and MPEG), and in
digital photography (including JPEG/JFIF), R’G’B’
components are coded in a perceptually uniform
manner. Noise visibility is minimized by applying
a nonlinear transfer function – gamma correction – to
each tristimulus value sensed from the scene. The
transfer function standardized for studio video is
detailed in Rec. 709 transfer function, on page 263. In
digital still cameras, a transfer function resembling that
of sRGB is used; it is detailed in sRGB transfer function,
on page 267. Identical nonlinear transfer functions are
applied to the red, green, and blue components; in
video, the nonlinearity is subsequently incorporated
into the luma and chroma (Y’C B C R ) components. The
approximate inverse transfer function is imposed at the
display device: A CRT has a nonlinear transfer function
from voltage (or code value) to luminance; that function
is comparable to Figure 2.3 on page 21. Nonlinear
coding is the central topic of Chapter 23, Gamma, on
page 257.
Headroom and footroom
Excursion (or colloquially, swing) refers to the range of
a signal – the difference between its maximum and
minimum levels. In video, reference excursion is the
range between standardized reference white and reference
black levels.
In high-quality video, it is necessary to preserve transient
signal undershoots below black, and overshoots
above white, that are liable to result from processing by
digital and analog filters. Studio video standards provide
footroom below reference black, and headroom above
reference white. Headroom allows code values that
exceed reference white; therefore, you should distinguish
between reference white and peak white.
22 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES