DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

fSC,PAL-B/G/H/I ≈ fH,576i 1135
4
In PAL-M, the offset is absent.
See 576i PAL color subcarrier, on
page 375. PAL was developed in
Hannover [sic], Germany, at the
research labs of Telefunken. In
most video literature, the name of
the city is Anglicized and spelled
with a single n.
The introduction of cross-luma and cross-color artifacts
at an NTSC decoder can be minimized by using a comb
filter. However, encoders rarely perform any processing
(beyond bandpass filtering of modulated chroma) on
luma and chroma prior to their being summed. If luma
and chroma components overlap in spatial frequency,
they will be confused upon summation at the encoder,
and no subsequent processing in a decoder can possibly
repair the damage: The composite footprint is said to be
imposed on the signal by the first encoder.
Highly sophisticated encoders include comb filter
preprocessing to prevent the introduction, at the
encoder, of severe cross-color and cross-luma artifacts.
However, broadcasters have never deployed these
encoders in any significant numbers. This has placed an
upper bound on the quality of video delivered to those
consumers having high-quality receivers.
Frequency interleaving in PAL
In PAL chroma modulation, on page 341, I described the
V-axis switch, which introduces line-by-line phase
inversion of the V chroma component. PAL differs from
NTSC in two other significant ways that I will describe
in this section; other minor differences will be described
in 576i PAL composite video, on page 529.
In studio NTSC, subcarrier frequency is an odd multiple
of one-half the line rate. In PAL, subcarrier frequency is
based on an odd multiple of one-quarter the line rate.
On its own, this would lead to roughly a 90° delay of
subcarrier phase line-by-line; see Figure 29.9 overleaf.
In standard 576i (625/50) PAL, a +25 Hz frequency
offset is added to the basic subcarrier frequency to
reduce the visibility of a cross-luma artifact called
Hannover bars. The +25 Hz offset contributes a +0.576°
phase advance (0.0016 of a subcarrier cycle) to subcarrier
phase line-by-line. This phase advance leads to the
nonlinelocked characteristic, and the noninteger
number of samples per total line, of 576i, 4f SC PAL.
Since the offset adds exactly one subcarrier cycle over
the duration of a frame, it has no impact on the fourframe
sequence. Historically, some 576i PAL test signal
CHAPTER 29 NTSC AND PAL FREQUENCY INTERLEAVING 355