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S3C2410A LCD CONTROLLER
15-7
DITHERING AND FRAME RATE CONTROL
For STN LCD displays (except monochrome), video data must be processed by a dithering algorithm. The DITHFRC
block has two functions, such as Time-based Dithering Algorithm for reducing flicker and Frame Rate Control (FRC)
for displaying gray and color level on the STN panel. The main principle of gray and color level display on the STN
panel based on FRC is described. For example, to display the third gray (3/16) level from a total of 16 levels, the 3
times pixel should be on and 13 times pixel off. In other words, 3 frames should be selected among the 16 frames, of
which 3 frames should have a pixel-on on a specific pixel while the remaining 13 frames should have a pixel-off on a
specific pixel. These 16 frames should be displayed periodically. This is basic principle on how to display the gray
level on the screen, so-called gray level display by FRC. The actual example is shown in Table 15-2. To represent
the 14
th
gray level in the table, we should have a 6/7 duty cycle, which mean that there are 6 times pixel-on and one
time pixel-off. The other cases for all gray levels are also shown in Table 15-2.
In the STN LCD display, we should be reminded of one item, i.e., Flicker Noise due to the simultaneous pixel-on and
-off on adjacent frames. For example, if all pixels on first frame are turned on and all pixels on next frame are turned
off, the Flicker Noise will be maximized. To reduce the Flicker Noise on the screen, the average probability of pixel-
on and -off between frames should be the same. In order to realize this, the Time-based Dithering Algorithm, which
varies the pattern of adjacent pixels on every frame, should be used. This is explained in detail. For the 16 gray level,
FRC should have the following relationship between gray level and FRC. The 15
th
gray level should always have pixel-
on, and the 14
th
gray level should have 6 times pixel-on and one times pixel-off, and the 13
th
gray level should have 4
times pixel-on and one times pixel-off, ,,,,,,,, , and the 0
th
gray level should always have pixel-off as shown in Table
15-2.
Table 15-2. Dither Duty Cycle Examples
Pre-Dithered Data
(gray level number)
Duty Cycle Pre-Dithered Data
(gray level number)
Duty Cycle
15 1 7 1/2
14 6/7 6 3/7
13 4/5 5 2/5
12 3/4 4 1/3
11 5/7 3 1/4
10 2/3 2 1/5
9 3/5 1 1/7
8 4/7 0 0