DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

50 1135
625 ×
2 4
4 433618750
⎛ ⎞ ⎛
⎜ Hz⎟
× ⎜ +
⎝ ⎠ ⎝
= .
MHz
1135
4
1 709379
+ =
625 2500
11× 64489
=
2 3
2 ×
5
1 ⎞
625
⎟
⎠
576i PAL color subcarrier
In 576i PAL, the color subcarrier frequency is based on
an odd multiple of one-quarter the line rate, using the
factor 1135 ⁄4 (or for PAL-N, 909 ⁄4). The odd multiple of
one-quarter, combined with the line-to-line alternation
of the phase of the V color difference component,
causes the U and V color components to occupy separate
parts of the composite signal spectrum. This makes
the PAL signal immune to the hue errors that result
when an NTSC signal is subject to differential phase
distortion.
In standard PAL-B, PAL-G, PAL-H, and PAL-I, an offset
of +25 Hz is added to the basic subcarrier frequency so
as to minimize the visibility of the Hannover bar effect.
The 25 Hz offset means that the phase relationship of
subcarrier to horizontal advances exactly +0.576° each
line. Consequently, subcarrier-locked sampling in PAL is
not line-locked: The subcarrier phase, modulo 90°, of
vertically aligned samples is not identical! The introduction
of the +25 Hz offset destroyed the simple integer
ratio between subcarrier and line rate: The ratio is quite
complex, as shown in the margin. The prime factor
64489 is fairly impenetrable to digital techniques.
4fSC sampling
The earliest digital television equipment sampled
composite NTSC or PAL video signals. It was convenient
for composite digital NTSC equipment to operate
at a sampling frequency of exactly four times the color
subcarrier frequency, or about 14.318 MHz, denoted
4fSC. This rate is sometimes sloppily referred to as D-2,
although that term properly refers to the corresponding
videotape format, not the sampling structure.
Any significant processing of a picture, such as repositioning,
resizing, rotating, and so on, requires that the
signal be represented in components. For this reason,
component video equipment is preferred in production
and postproduction. But 4f SC equipment has half the
data rate of Rec. 601 equipment; 4f SC equipment is
cheaper than component equipment, and continues to
dominate broadcast operations.
CHAPTER 31 FRAME, FIELD, LINE, AND SAMPLE RATES 375