LM1237 datasheet(16/42 Pages) NSC

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OSD Generator Operation (Continued)

OSD VIDEO TIMING

The OSD SELECT signal switches the source of video infor-

mation within the preamplifier from external video to the

internally generated OSD video.

WINDOWS

Two separate windows can be opened, utilizing the data

stored in the page RAM. Each window has its own horizontal

and vertical start position, although the second window

should be horizontally spaced at least two character spaces

away from the first window, and should never overlap the

first window when both windows are on. The OSD window

must be placed within the active video.

CHARACTER CELL

Each character is defined as a 12 wide by 18 high matrix of

picture elements, or ‘pixels’. The character font is shown in

Figure 25. There are two types of characters defined in the

character ROM:

Two-color: There are 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 1’

or the ‘background’ color. If the bit value is 1, then the

color is defined as ‘Color 2’, or the ‘foreground’ color. An

example of a character is shown in Figure 14.

Four-color: There are 64 four-color characters stored in

the character ROM. Each pixel of the four-color charac-

ter is defined by two bits of information, and thus can

define four different colors, Color 1, Color 2, Color 3, and

Color 4. Color 1 is defined as the ‘background’ color. All

other colors are considered ‘foreground’ colors, although

for most purposes, any of the four colors may be used in

any way. Because each four-color character has two

bits, the LM1237 internally has a matrix of two planes of

ROM as shown in Figure 15.

20023420

FIGURE 13. Block Diagram of OSD DACs

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