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to broadcast using a format that most of the viewing audience could not see on their existing
sets. Both issues were resolved with the creation of a composite color signal. In the United
States, the specifications for the composite signal were created by the National Television Sys-
tems Committee, and this signal is often called an NTSC signal. The corresponding signals in
Europe are PAL (Phase Alternating Lines), developed in Germany, and SECAM (Séquential
Couleur avec Mémoire), developed in France. There is some (hopefully) good-natured rivalry
between proponents of the different systems. Some problems with color reproduction in the
NTSC signal have led to the name
Never Twice the Same Color
, while the idiosyncracies of the
SECAM system have led to the name
Système Essentiellement Contre les Américains
(system
essentially against the Americans).
The composite color signal consists of a
luminance
component, corresponding to the black-
and-white television signal, and two
chrominance
components. The luminance component is
denoted by
Y
:
114
B
(2)
where
R
is the red component,
G
is the green component, and
B
is the blue component.
The weighting of the three components was obtained through extensive testing with human
observers. The two chrominance signals are obtained as
Y
=
0
.
299
R
+
0
.
587
G
+
0
.
C
b
=
B
−
Y
(3)
C
r
=
R
−
Y
(4)
These three signals can be used by the color television set to generate the red, blue, and green
signals needed to control the electron guns. The luminance signal can be used directly by the
black-and-white televisions.
Because the eye is much less sensitive to changes of the chrominance in an image, the
chrominance signal does not need to have higher frequency components. Thus, lower band-
width of the chrominance signals along with a clever use of modulation techniques permits all
three signals to be encoded without need of any bandwidth expansion. (A simple and readable
explanation of television systems can be found in [
253
].)
The early efforts toward digitization of the video signal were devoted to sampling the com-
posite signal, and in the United States the Society of Motion Picture and Television Engineers
developed a standard that required sampling the NTSC signal at a little more than 14 million
times a second. In Europe, the efforts at standardization of video were centered around the
characteristics of the PAL signal. Because of the differences between NTSC and PAL, this
would have resulted in different “standards.” In the late 1970s, this approach was dropped
in favor of sampling the components and the development of a worldwide standard. This
standard was developed under the auspices of the International Consultative Committee on
Radio (CCIR) and was called CCIR recommendation 601-2. CCIR is now known as ITU-R,
and the recommendation is officially known as ITU-R recommendation BT.601-2. However,
the standard is generally referred to as recommendation 601 or CCIR 601.
The standard proposes a family of sampling rates based on the sampling frequency of
3.725MHz (3.725 million samples per second). Multiples of this sampling frequency permit
samples on each line to line up vertically, thus generating the rectangular array of pixels
necessary for digital processing. Each component can be sampled at an integer multiple of
3.725MHz, up to a maximum of four times this frequency. The sampling rate is represented
