DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 29.17 Spatial
frequency spectrum of
PAL shows that chroma
is displaced vertically
compared to NTSC.
Modulated chroma interferes
with luma much
more severely than it
does in NTSC, unless
luma is spatially filtered
prior to encoding.
Vertical frequency, C/PH
In Filtering and sampling, on
page 141, I explained how a onedimensional
waveform in time
transforms to a one-dimensional
frequency spectrum.
288
144
MODULATED
C U , C V CHROMA
U, V SUBCARRIERS
LUMA
0
0 230 Horizontal frequency, C/PW
One-dimensional frequency spectrum of NTSC
The theory of spatial frequency hadn’t been invented
when the NTSC system was devised. In this section,
I recast the explanation of the NTSC spectrum into the
one-dimensional frequency domain that was familiar in
1953, at the time of the NTSC. In an image where every
scan line is identical, its one-dimensional waveform is
periodic at the line rate, and its power is thus concentrated
at multiples of the line rate: 0 (DC, or zero
frequency), fH (the line rate), 2fH, 3fH, and so on. In
a typical image, power tends to concentrate at low
frequencies, and diminishes toward higher frequencies.
If the content of successive scan lines varies, the effect
is to broaden the spectral lines in the one-dimensional
frequency spectrum: Power is centered at 0, f H , 2f H ,
3f H , and so on, but spreads into nearby frequencies.
A graph of the luma component of a typical image is
graphed at the top of Figure 29.18 overleaf.
Consider a video signal where every scan line contains
a sinewave of frequency f T chosen such that its phase
inverts on alternate lines. This signal is periodic at half
the line rate: All of the power in this signal lies at multiples
of f H ⁄ 2. Furthermore, owing to symmetry, the
power lies only at odd multiples.
In studio and broadcast NTSC, the color subcarrier
frequency is coherent with the line rate. Subcarrier has
227.5 cycles per total line, so subcarrier is located at
227.5f H . That frequency is an odd multiple of half the
line rate, so subcarrier phase inverts on alternate lines.
CHAPTER 29 NTSC AND PAL FREQUENCY INTERLEAVING 361