820 Chapter 28
resulting loss of color saturation. More serious still, if the phase of the signal suffers
from progressive phase disturbance, the color in the reproduced color is likely to
change. This remains a problem with NTSC where no means are taken to ameliorate
the effects of such a disturbance. The phase alternation line (PAL) system takes steps to
prevent phase distortion having such a disastrous effect by switching the phase of the
V subcarrier on alternate lines. This really involves very little extra circuitry within the
coder but has design ramifi cations, which means the design of PAL decoding is a very
complicated subject indeed. The idea behind this modifi cation to the NTSC system
(for that is all PAL is) is that, should the picture—for argument’s sake—take on a red
tinge on one line, it is cancelled out on the next when it takes on a complementary blue
tinge. The viewer, seeing this from a distance, just continues to see an undisturbed color
picture. In fact, things aren’t quite that simple in practice but the concept was important
enough to be worth naming the entire system after this one notion: phase alternation
line. Another disadvantage of the coding process illustrated in Figure 28.5 is because
of the contamination of luminance information with chrominance and vice versa.
Although this can be limited to some degree by complementary band-pass and band-
stop fi ltering, a complete separation is not possible, which results in the swathes of
moving colored bands (cross-color), which appear across high-frequency picture detail
on television—herringbone jackets proving especially potent in eliciting this system
pathology.
In the color receiver, synchronous demodulation is used to decode the color subcarrier.
One local oscillator is used and the output is phase shifted to produce the two orthogonal
carrier signals for the synchronous demodulators (multipliers). Figure 28.6 illustrates
the block schematic of an NTSC color decoder. A PAL decoder is much more
complicated.
Mathematically, we can consider the PAL and NTSC coding process, thus
NTSC colour signal Y B Y R Y
PAL colo 049 ωωtt088.( )sin .( )cos
uur signalY 049 BY ωωtt088.(RY )sin .( )cosNote that following the demodulators, the U and V signals are low-pass fi ltered to remove
the twice frequency component and that the Y signal is delayed to match the processing
delay of the demodulation process before being combined with the U and V signals in
a reverse color space conversion. In demodulating the color subcarrier, the regenerated
carriers must not only remain spot-on frequency, but also maintain a precise phase