CS9210 datasheet(18/39 Pages) NSC

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Revision 3.2

Functional Description (Continued)

3.8

COLOR GENERATION

Each pixel on an LCD panel consists of three primary

color components: red, green, and blue. Each primary

color component, for a given pixel, can be either turned on

or turned off. A total of eight colors can be generated for a

given pixel through different combinations of turning each

color component either on or off. In order to generate

more colors, frame rate modulation (FRM) and dithering

are used in the CS9210. The CS9210 is capable of gener-

ating 262,144 different colors based on the 18-bit RGB

pixel input from the pixel port interface. The following sec-

tions describe how frame rate modulation and dithering

are implemented.

3.8.1

Frame Rate Modulation (FRM)

The idea of frame rate modulation is to turn each primary

color component of a pixel on and off at a certain rate to

create the perception of various color intensities. The

intensity or brightness of each color component depends

on what percentage of time the color component is turned

on and what percentage of time the color component is

turned off.

For example, take a given pixel whose blue and green

color components are always off. If the pixel’s red color

component was also always off, the pixel would be black.

If the pixel’s red color component was always on, the pixel

would be bright red, as bright as the red could get. How-

ever, if the red color component were alternating between

being on and off, the pixel would look about half as bright

as the brightest red.

The CS9210 independently turns the red, green, and blue

pixel color components on and off on a per frame basis (a

frame is one entire screen of pixels). The FRM sequence

specifies which frames the color component will be on and

which ones it will be off. These sequences are 64 bits

long, with each bit representing one frame. Once the end

of a sequence has been reached, the CS9210 will go

back to the beginning of the sequence and start over.

Figure 3-5 illustrates how one color component of a given

pixel might be turned on and off over 64 frames to achieve

the perception of a given color component intensity.

The pixel port data of the CS9210 is comprised of six bits

for each of the three primary colors. Each of these 6-bit

color intensity values is dithered down to five bits (see

Section 3.8.2 “Dithering” on page 20 for a detailed

description of dithering). These 5-bit color intensity values

are then used to select one of the 32 FRM sequences

stored in the CS9210.

Figure 3-5. Sample FRM Sequence

3.8.1.1

Choosing FRM Sequences

Care must be taken when choosing FRM sequences to

reduce the effects of flickering (the low frequency varia-

tions) that can be detected by the human eye. Definition of

FRM sequences will also depend on the characteristics of

the LCD panel being used. For these reasons, generation

of an FRM sequence table involves lots of experimenta-

tion. Table 3-6 illustrates an FRM sequence table for a sin-

gle primary color component.

An FRM sequence of 1’s and 0’s is defined for each 5-bit

input color component intensity value. The frequency ratio

indicates the number of 0 to 1 transitions within the 64

frame sequence. This value multiplied by the screen

refresh rate will give the frequency of frame rate modula-

tion for the given color component intensity. The intensity

ratio indicates the fractional amount of time that the pixel

color component will be turned on.

Higher frame modulation frequencies result in better pic-

ture quality. Very low frequencies are more noticeable to

the human eye. It also seems that the human eye is less

responsive to differences in frequency at low intensities.

The relationship between input intensity and the resulting

intensity ratio of the FRM sequence is not necessarily lin-

ear. This relationship depends on the non-linear charac-

teristics of the LCD panel used.

Frame-count = 0

Frame-count = 1

Frame-count = 2

Frame-count = 3

Frame-count = 61

Frame-count = 62

Frame-count = 63

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