CS9210 datasheet(20/39 Pages) NSC

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Revision 3.2

Functional Description (Continued)

3.8.1.2

Removal of Flickering

One side effect of frame rate modulation is flickering.

When a large group of pixels on an LCD panel are the

exact same color, and all of the pixels in this large group

are blinking on and off together in synchronization, the

flickering effect is detectable by the human eye. The

CS9210 removes detectable flickering by de-synchroniz-

ing adjacent pixels so that they do not turn on and off at

thesametime.

This reduction of flickering due to FRM is achieved in the

CS9210 through the use of one pair of linear feedback

shift registers (LFSRs) for each pixel color component to

introduce screen position dependent randomization. For

each color component, one 15-bit LFSR, which is

advanced each pixel, is used to generate global random-

ization, and one 9-bit LFSR, which is advanced each hori-

zontal line, is used to generate local randomization. Both

LFSRs are reset to their seeded value at the beginning of

each frame. The lower six bits of each LFSR is added to

theframe countand theresulting valueisused toindex

the FRM sequence table. The addition of the lower six bits

of these two LFSRs gives each pixel location on the

screen a fixed random offset into the FRM sequence table

so that adjacent pixels of the same color are not on the

same frame count in the 64-bit FRM sequence.

3.8.2

Dithering

The idea behind dithering is to achieve intermediate color

intensities by allowing the human eye to blend or average

the intensities of adjacent pixels on a screen. Intensity

resolution is gained by sacrificing spatial resolution.

For example, consider just the red color component of a

2x2 square of pixels. If the only two options for the red

color component were to be turned on or off, then there

would only be two colors, black and the brightest red.

However, if two of the pixels’ red color components in the

2x2squarewere turnedonand twowereturnedoff,the

human eye would blend these adjacent pixels and the 2x2

pixel square would appear to be half as bright as the

brightest red. The drawback is that fine details and bound-

aries between regions of differing color intensities become

slightly blurred.

The CS9210 supports dithering patterns over a 4x4 pixel

area. A 4x4 pixel area supports 16 different dithering pat-

terns. This means that the 6-bit input intensity for a given

pixel primary color component can be reduced to its two

most significant bits by using the four least significant bits

to select a 4x4 pixel pattern whose average intensity is

equal to the original 6-bit input intensity value.

For example, consider a display screen (not a DSTN

panel) which is capable of producing four different intensi-

ties of the red color component for each pixel. Given a 6-

bit red intensity value, "010110", the problem is to come

up with a 4x4 pixel pattern using only the four available

red pixel intensities that, when averaged together, yields

the value of the original 6-bit intensity.

Figure 3-6 shows a potential dither pattern for this color

intensity. As the computer starts to update the screen, the

X[1:0] and Y[1:0] values will both be 00. According to the

4x4 pattern in Figure 3-6, the value "100000" will be sent

to the screen. After that pixel has been sent, the next pixel

in the display line will be processed, incrementing X[1:0]

to 01 and leaving Y[1:0] untouched. Looking at the dither

pattern, the value for this pixel is "010000", which is sent

to the display screen. The dither pattern is traversed in

this manner, X increments after each pixel and Y incre-

ments after each display line, until the whole screen has

been rendered. If all sixteen values of this dither pattern

were averaged, the result would match the original value

of "010110".

The actual dithering pattern is a 4x4 pattern of 1’s and 0’s.

A "0" in a given position of the pattern indicates that the

truncated value of the input color component intensity be

used. A "1" means use the next higher truncated value.

Since, in the previous example, only four different intensi-

ties are capable of being generated, only the upper two

bits are sent to the display screen, the rest are dropped.

For an intensity value of "010110"; the truncated value is

"01", and the next higher truncated value is "10".

Figure 3-6. Dithered 4x4 Pixel Pattern

X[1:0]

Y[1:0]

100000

010000

100000

010000

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