LM2467 datasheet(19/43 Pages) NSC

19 / 43 page

OSD Generator Operation (Continued)

OSD VIDEO TIMING

The OSD analog signal then goes to the switch, shown in

Figure 14 and Figure 1 where the timing control switches

from input video to OSD and back again as determined by

the control registers. This is also where horizontal and ver-

tical blanking are also inserted at their appropriate intervals.

CHARACTER CELL

Each character is defined as a 12 column by 18 row matrix of

picture elements, or “pixels”. The character font is shown in

Figure 15 through Figure 22. There are two types of charac-

ters defined in the character ROM:

Two-color: There are a total of 190 two-color characters.

Each pixel of these characters is defined by a single bit

value. If the bit value is 0, then the color is defined as

“Color 0” or the “background” color. If the bit value is 1,

then the color is defined as “Color 1”, or the “foreground”

color. An example of a character is shown in Figure 15.

The grid lines are shown for clarity to delineate individual

pixels and are not part of the actual displayed character.

Four-color: There are a total of 64 four-color characters

stored in the character ROM. Each pixel of the four-color

character is defined by two bits of information, and thus

can define four different colors, Color 0, Color 1, Color 2

and Color 3. Color 0 is defined as the “background”

color. All other colors are considered “foreground” col-

ors, although for most purposes, any of the four colors

may be used in any way. Because each four-color char-

acter has two bits, the LM1246 internally has a matrix of

two planes of ROM as shown in Figure 16. In that figure,

dark pixels indicate a logic “1” and light pixels which

indicate a logic “0”. The left side shows plane 0 which is

the least significant bit and the middle figure shows

plane 1 which is the most significant bit. The right side

composite character formed when each pixel is repre-

sented by its two bits formed from the two planes. The

color palette used in this example is “00” for white, “01”

for black, “10” for blue and “11” for red.

By appropriately selecting the color attributes, it is pos-

sible to have two 2-color characters in one four color

ROM location. If the required number of four color char-

acters is less than 128, the remaining characters can be

used to increase the number of two color characters

from 384 to 384 + 2*N, where N is the number of unused

four color characters. This is explained in the next

section.

20075729

FIGURE 14. Block Diagram of OSD DACs

20075730

FIGURE 15. Two-Color Character

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