LM1236 National Semiconductor Corporation, LM1236 Datasheet - Page 20

• Manufacturer Part Number: LM1236
• Description: 150 Mhz I2c Compatible Rgb Preamplifier With Internal 254 Character Osd Rom, 512 Character Ram And 4 Dacs
• Manufacturer: National Semiconductor Corporation

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OSD Generator Operation
FOUR COLOR FONT AS TWO 2-COLOR
Using a 4 color character as two 2 color characters is
achieved by careful assignment of the four colors. When two
2 color characters are combined, there will be four pixel
colors:
Color 0: Those that are background pixels for both
Color 1: Those that are foreground pixels in character one
Color 2: Those that are foreground pixels in character two
Color 3: Those that are foreground pixels for both
In order to identify which pixels are which, both characters
should be drawn in one character cell using the same back-
ground color, and different background colors. In Figure 17 ,
both “A” and a “B” are drawn separately, then superimposed,
characters,
and background pixels in character two,
and background pixels in character one,
characters.
FIGURE 17. Four Color Character as a 2 x 2 Color
FIGURE 16. Four-Color Character
(Continued)
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with the final 4 color character on the right. Comparing it to
the list of colors, it is seen that white is color 0, black is color
1, blue is color 2 and red is color 3. (These particular four
colors were chosen for clarity).
Figure 18 shows the composite four color character in the
center and the palette choices on the left and the right which
result in the display of the two original characters.
To display character 1, which has a foreground color 1,
character 2 must be hidden by setting its foreground color
(color 2) to equal the background. Color 3 (common pixels)
must be set to the desired foreground (color 1). In this case,
color 0 and color 2 are black and color 1 and color 3 are
white.
To display character 2, set color 1 = color 0 (to hide charac-
ter 1) and color 3 = color 2. Other than this, there is no
restriction on the choice of the actual colors used.
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