DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

In composite NTSC and PAL video, color information is
subject to quadrature modulation that imposes color
information onto the color subcarrier, as I described in
NTSC and PAL chroma modulation, on page 335.
(Quadrature modulation suppresses the unmodulated
subcarrier.) An encoder then sums luma and modulated
chroma. Low-frequency chroma information lies very
near the subcarrier frequency; higher-frequency color
detail causes the modulated information to spread out
in frequency around odd f H ⁄ 2 multiples, in the same
manner that luma spreads from f H multiples. The
middle graph of Figure 29.18 shows the spectrum of
modulated NTSC chroma in a typical image.
One way to accomplish separation of luma and chroma
in an NTSC decoder is to use a band reject (notch) filter
operating in the horizontal domain. The a band reject
filter rejects signals in the region of subcarrier
frequency. In Figure 16.14, at the bottom of page 153,
I showed the response of a [1, 0, 1] filter, which has
a notch at one-quarter of the sampling frequency. In a
system sampled at four times the color subcarrier
frequency (4f SC ), this filter would reject chroma.
Although a notch filter eliminates chroma, luma that
happens to occupy similar horizontal frequencies is
eliminated as well: A notch filter reduces picture detail.
In Comb filtering, on page 190, I described a filter that
produces notches at regular intervals of the one-dimensional
frequency spectrum. If a comb filter such as the
one in Figure 16.19, on page 156, has a total delay
corresponding to exactly one line time, the peaks will
pass luma and the notches will reject modulated
chroma. A comb filter rejects chroma as effectively as
a notch filter, but it has the great advantage that luma
detail in the range of subcarrier frequencies is retained.
If luma that has been separated from a composite signal
is subtracted from the composite signal, chroma
remains! Prior to subtraction, the composite signal
must be delayed to compensate the delay of luma
through the separation filter. The bottom graph of
Figure 29.18 shows the chroma response of a comb
filter cascaded with a bandpass filter.
CHAPTER 29 NTSC AND PAL FREQUENCY INTERLEAVING 363