Page 34 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 2.3. Analog Video 11

Di6erent industries employ di6erent combinations of video parameters. For
example, the computer industry uses progressive scanning with a frame rate
of 72 frames=s. To reduce bandwidth requirements, the television industry uses
interlaced scanning. In this case, the eld rate is set to 50 or 60 elds=sto
2
avoid refresh /icker, while the frame rate (which, in interlaced video, is half
the eld rate) is 25or 30 frames=s to maintain smooth motion. Note that this
saving in bandwidth is at the expense of vertical resolution. There are two
main television scanning systems: 625=50 (625 scan lines and 50 elds=s) and
525=60.

2.3.2 Color Representation

The preceding discussion considered monochrome video. In practice, how-
ever, most videos are in color. According to the trichromatic theory of color
vision [11], color is perceived via three classes of cone cells, or photoreceptors,
in the eye. Consequently, a color video can be produced by the superposition
of three video signals. Each signal represents one of the three primary colors:
3
red, green, and blue (RGB). Practical television (TV) and video systems usu-
ally convert this RGB representation to a di6erent color space of luminance 4
5
(which is closely related to the perception of brightness ) and chrominance
6
7
(which is related to the perception of color hue and saturation ). This repre-
sentation serves two purposes. First, luminance ensures backward compatibility
with monochrome video. Second, this representation lends itself more easily
to video compression. This can be explained as follows. The human visual
system (HVS) has poor response to color (chrominance) spatial detail com-
pared to its response to luminance spatial detail [9]. Thus, the chrominance
signals can be bandlimited or subsampled to achieve compression.
There are three main analog color coding systems: Phase Alternation Line
(PAL), SEquential Couleur Avec Memoire (SECAM) and National
Television System Committee (NTSC). They di6er mainly in the way they

2 Originally, television refresh rates were chosen to match the local AC power line frequency.
3 The RGB is an additive color system. This means that when all the primaries are added
in equal maximum quantities, the color white is perceived. In printing and painting, the cyan,
magenta, and yellow (CMY) system is used. This is a subtractive color system since the total
absorbtion of all three primaries produces the color white.
4 Luminance is proportional to the light energy emitted per unit area of the source, but this
energy is weighted according to the spectral sensitivity of the eye [9].
5 Brightness is the attribute of a visual sensation according to which an area appears to emit
more or less light [9].
6 Hue is the attribute of a visual sensation according to which an area appears to be similar to
one of the perceived colors, red, yellow, green and blue, or a combination of two of them [9].
7 Saturation is the colorfulness of an area judged in proportion to its brightness [9].