DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

COMPOSITE
NTSC
VIDEO
Y’/C
SEPARATOR
In NTSC Y’IQ system, on page 365,
I will explain why it is futile to
attempt to recover more than about
600 kHz of chroma bandwidth from
terrestrial VHF/UHF NTSC.
Even in NTSC, modulated chroma
inverts phase on alternate lines!
The A in PAL refers not to this
alternation, but to the line by line
alternation of phase of the V
chroma component.
Y’
C
SUBCARRIER
REGENERATOR
sin
cos
CHROMA
DEMODULATOR
QUADRATURE
DEMODULATOR
The decoder reconstructs the continuous-wave color
subcarrier that was used at encoding. I will describe
that process in Subcarrier regeneration, on page 344.
The separated modulated chroma is then multiplied
simultaneously by sine and cosine of the regenerated
subcarrier. The products are lowpass filtered to recover
the baseband U and V components. Luma is processed
through a matching delay.
Provided that U and V are limited in bandwidth to less
than half the subcarrier frequency, chroma modulation
is reversible without information loss. In practice,
chroma modulation itself introduces no significant
impairments, although the bandwidth limitation of the
color difference signals removes color detail.
PAL chroma modulation
Analog transmission is susceptible to differential phase
error, whereby the phase of modulated chroma is influenced
by luma (as I will describe on page 541). In
NTSC, these errors cause incorrectly decoded hue;
vision is quite sensitive to hue errors. PAL augments the
NTSC system with a V-axis inverter, which alternates the
phase of the modulated V component line-by-line. (PAL
CHAPTER 28 NTSC AND PAL CHROMA MODULATION 341
Y’
U
V
LPF, 1.3 MHz (studio),
600 kHz (consumer)
Figure 28.5 NTSC decoder block diagram. Y’/C separation is accomplished using a “notch” filter
or a comb filter. Subcarrier is regenerated from burst. Separated chroma is independently multiplied
by continuous sin and cos waves in quadrature; the products are lowpass filtered to recover U
and V. To recover wideband U and V in the studio, use 1.3 MHz filters. To recover narrowband U
and V in consumer applications, use 600 kHz filters. For VHS VCRs, use 300 kHz filters.