DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

It is unfortunate that the formulation
used in video is reversed
from the convention established
by Euler in 1758, and ubiquitous
in mathematics, that
θ
2
= cos θ + sin θ,
= −1
where is associated with the
y-axis. The video formulation also
opposes convention in communications
theory, where the carrier
is considered to be a cosine wave,
not a sine wave.
Chroma subcarrier
Phase (encodes Hue)
Amplitude
(encodes Saturation)
Figure 28.3 Quadrature
modulation can be viewed
as simultaneous phase and
amplitude modulation.
The exact subcarrier frequency is not important to
quadrature modulation, but it is critical to frequency
interleaving, as I will detail in NTSC and PAL frequency
interleaving, on page 349. The spectrum of the modulated
chroma is sketched at the right of Figure 28.2.
The sum of the sine and cosine terms is then bandpass
filtered, producing the modulated chroma spectrum
sketched at the right of Figure 28.2. Modulated chroma
is centered on the subcarrier. With U and V bandwidth
of 1.3 MHz, the lower sideband extends 1.3 MHz below
the subcarrier frequency, and the upper sideband
extends 1.3 MHz above. (In NTSC and PAL frequency
interleaving, on page 349, I will discuss how much
chroma bandwidth is preserved in transmission.)
If you transform the two color differences U and V from
rectangular to polar coordinates, you can think of U and
V as being conveyed by a combination of phase and
amplitude modulation, as suggested by Figure 28.3 in
the margin. Consider the point in the chroma plane
plotted at coordinates [U, V]. The angle from the x-axis
to the point relates to the hue attribute of the associated
color; this quantity effectively modulates subcarrier
phase. The distance from the origin to the point
[U, V] relates to the saturation of the color; this quantity
effectively modulates subcarrier amplitude.
It is standard to sample digital composite video at four
times the subcarrier frequency, 4f SC . Early digital NTSC
systems sampled on the [B’-Y’, R’-Y’] axes – that is,
sampling took place at the 0°, 90°, 180°, and 270°
phases of subcarrier, so the sine subcarrier took values
chosen cyclically from {0, 1, 0, -1}; the cosine subcarrier
took values chosen cyclically from {1, 0, -1, 0}.
In early 4f SC NTSC systems, multiplying sine by U and
cosine by V, and adding the two products, gave digital
modulated chroma samples {V, U, -V, -U}. However,
when the SMPTE 244M standard was established for
4f SC NTSC, it called for sampling on the [I, Q] axes –
that is, at the 33° phase of subcarrier. In modern 4f SC
NTSC equipment, digital modulated chroma samples
take values chosen cyclically from {I, Q, -I, -Q}. In the
CHAPTER 28 NTSC AND PAL CHROMA MODULATION 339