Video Synchronization 819low-pass fi ltered to a bandwidth of 0.5 MHz.^2 These two signals are used to control the
amplitude and phase of a high-frequency subcarrier superimposed onto the luminance
signal. This chrominance modulation process is implemented with two balanced
modulators in an amplitude-modulation-suppressed-carrier confi guration—a process that
can be thought of as multiplication. A clever technique is employed so that U modulates
one carrier signal and V modulates another carrier of identical frequency but phase
shifted with respect to the other by 90°. These two carriers are then combined and result
in a subcarrier signal that varies its phase and amplitude dependent on the instantaneous
value of U and V. Note the similarity between this and the form of color information
noted in connection with the dot-sequential system: amplitude of high-frequency carrier
dependent on the depth—or saturation—of the color and phase dependent on the hue
of the color. (The difference is that in NTSC, the color subcarrier signal is coded and
decoded using electronic multiplexing and demultiplexing of YUV signals rather than the
spatial multiplexing of RGB components attempted in dot-sequential systems.) Figure 28.5
illustrates the chrominance coding process.
While this simple coding technique works well, it suffers from a number of important
drawbacks. One serious implication is that if the high-frequency color subcarrier is
attenuated (for instance, due to the low pass action of a long coaxial cable), there is a
(^2) In NTSC systems or 1 MHz in PAL systems.
Delay
Composite
video
90 Degrees
Subcarrier phase-shift
oscillator
Y
B – Y
R – Y
90
Figure 28.5 : NTSC color-coding process.