DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

COMPOSITE
NTSC
VIDEO
In Consumer analog NTSC and
PAL, on page 579, I will discuss
the decoding of video from
consumer devices.
1H
sum of two vertically adjacent samples approximates
luma; their difference approximates modulated chroma.
In a decoder that uses a notch filter, NTSC chroma
inverts in phase line-by-line even if mistakenly decoded
as luma. When integrated vertically by large spot size at
the display, and when integrated vertically in the
viewer’s eye, luma tends to average (that is, to be reinforced)
and chroma tends to cancel. This visual filtering
is what allowed color to be retrofitted into black-andwhite
television: The newly added modulated chroma
component averaged out – and thereby was not very
visible – when viewed on a black-and-white receiver.
Proper operation of a comb filter depends upon the
video signal having stable timebase and coherent
subcarrier. In addition, comb filtering is only sensible if
the video signal has luma content above 3 MHz. All of
these conditions hold for broadcast signals, and
luma/chroma separation should use comb filtering. If
any of these conditions fails, as is likely with video originating
from a consumer device, then comb filtering is
likely to introduce artifacts and it should be defeated;
a notch filter should be used instead.
In later sections of this chapter, I will explain comb
filtering in the spatial frequency (two-dimensional)
domain and in the one-dimensional frequency domain.
Cross-luma and cross-color
If neither luma nor chroma changes vertically, vertical
summing and differencing in a comb filter can perfectly
separate luma and chroma! In a solid-colored area of an
CHAPTER 29 NTSC AND PAL FREQUENCY INTERLEAVING 353
+0.5
+0.5
–0.5
+0.5
∑
∑
3.0 … 4.2 MHz
Figure 29.6 NTSC 2-line comb filter separates Y’ and C using a 1H delay element. This is more
complex than a notch filter; however, cross-luma and cross-color artifacts are greatly reduced.
Y’
C