DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 3.2 Brightness control has the
effect of sliding the black-to-white
video signal scale left and right along
the 2.5-power function of the display.
Here, brightness is set too high;
a significant amount of luminance is
produced at zero video signal level.
No video signal can cause true black
to be displayed, and the picture
content rides on an overall pedestal
of gray. Contrast ratio is degraded.
Figure 3.3 Brightness control is set
correctly when the reference black
video signal level is placed precisely at
the point of minimum perceptible
light output at the display. In a
perfectly dark viewing environment,
the black signal would produce zero
luminance; in practice, however, the
setting is dependent upon the
amount of ambient light in the
viewing environment.
Figure 3.4 Brightness control set
too low causes a range of input
signal levels near black to be reproduced
“crushed” or “swallowed,”
reproduced indistinguishably from
black. A cinematographer might
describe this situation as “lack of
details in the shadows,” however, all
information in the shadows is lost,
not just the details.
When brightness is set as high as
indicated in Figure 3.2, the effective
power law exponent is lowered
from 2.5 to about 2.3; when set as
low as in Figure 3.4, it is raised to
about 2.7. For the implications of
this fact, see page 84.
The brightness control – more sensibly called black
level – effectively slides the black-to-white range of the
video signal along the power function of the display. It
is implemented by introducing an offset – in electrical
terms, a bias – into the video signal. Figure 3.3 (middle)
sketches the situation when the brightness control is
properly adjusted: Reference black signal level produces
zero luminance. Misadjustment of brightness is
a common cause of poor displayed-image quality. If
brightness is set too high, as depicted in Figure 3.2
(top), contrast ratio suffers. If brightness is set too low,
as depicted in Figure 3.4 (bottom), picture information
near black is lost.
26 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
Luminance
Luminance
Gray Pedestal
BLACK Video signal
WHITE
BLACK Video signal
WHITE
Luminance
Lost signal
BLACK Video signal
WHITE