DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

COMPOSITE
NTSC
VIDEO
Saturation
Hue
Y’/C
SEPARATOR
Y’
C
SUBCARRIER
REGENERATOR
sin
cos
CHROMA
DEMODULATOR
CHROMA
GAIN
QUADRATURE
DEMODULATOR
LOWPASS FILTER
LOWPASS FILTER
Figure 28.10 NTSC decoder controls. Saturation alters chroma gain. Hue rotates the [U, V]-plane
as viewed on a vectorscope. These controls were necessitated by the instability of analog receivers.
Modern circuits are sufficiently stable that user adjustment of these controls is no longer necessary.
Figure 28.11 Saturation
control in a decoder alters
chroma gain. The comparable
control in processing
equipment is chroma gain.
Figure 28.12 Hue control
is usually implemented in a
decoder by altering phase
of regenerated subcarrier,
so as to rotate the demodulated
[U, V] components.
The comparable control in
processing equipment is
chroma phase.
Figure 28.10 shows the block diagram of an NTSC or
PAL decoder, augmented to reflect the places in the
signal flow where the these controls are effected.
The saturation control, whose icon is shown in
Figure 28.11, alters the gain of modulated chroma prior
to demodulation. (Alternatively, the control can be
implemented by altering the gain of both decoded
color difference components.) This control is sometimes
called color, but saturation is preferred because
it is ambiguous whether color should adjust which
color (hue) or the amount of color (saturation). This
control is found in both NTSC and PAL decoders.
The hue control, whose icon is shown in Figure 28.12,
rotates the decoded colors around the [U, V] plot of
Figure 28.1; it is implemented by altering the phase of
regenerated subcarrier. This control is sometimes called
tint. That name is misleading and should be avoided,
because to an artist, tint refers to adding white to
increase lightness and decrease saturation – to an artist,
to tint a color preserves its hue! As I mentioned on
page 342, PAL has inherent immunity to hue errors
arising from differential phase errors; consequently, the
hue control is usually omitted from PAL decoders.
CHAPTER 28 NTSC AND PAL CHROMA MODULATION 347
Y’
U
V