You adjust brightness during the fully
settled pixel period by varying the duty
cycle of time the backlight is switched on
during that period. This is called pulse-width modulation (PWM). There is a
default brightness level, with an associated PWM illumination period value,
that is less than the full period when the
pixels are stable. This allows for higher or
lower brightness levels by setting larger
or smaller PWM values. Conventional
PWM dimming turns the backlight on
at the start of the stable pixel period and
off after the PWM illumination period.
There is a danger that a large PWM value
may result in the backlight remaining
on into the next pixel-writing period for
that particular illumination zone (Figure 4).
To overcome this problem, the Reverse
PWM method aligns the end of the PWM
period to the end of the fully-settled pixel
period as seen at the bottom of Figure 2. It
is better to always use area B and to start
the illumination as early as necessary to
achieve the desired brightness.
It is important to reference the timing of
all illumination zones to the same VSYNC
that is associated with the start of that
frame’s pixel-writing process. Referencing
some zones to a subsequent VSYNC can
result in brightness banding due to variations in frame duration that can result
from video manipulation.
3-D TV, with left and right eye image, re-
duces the time available for backlight to be
on. There is a possibility of needing a large
PWM on-time that might spill over to the
next frame. In this case, it may be better
to illuminate before the pixels are settled
in the current frame itself. For 3-D applica-
tions with active shutter glasses, the timing
requirements are more stringent. The LCD
eyeglasses have slow response while chang-
ing from opaque to clear and vice-versa.
There should be no time overlap (crosstalk),
and this is guaranteed by inserting a black
frame during this transition (Figure 5).
LED backlighting for LCD TVs using edge-lit display panels has rapidly become the
technology of choice, providing both a
high-performance and cost-effective solution for the majority of the TV and desktop
Figure 3. Relationship of writing pixel rows
and illumination zones.
Figure 4. Reverse PWM avoids the next pixel
Figure 5. You should insert black frames for
operation as a 3-D TV.