LM1246AAA National Semiconductor, LM1246AAA Datasheet - Page 21

• Manufacturer Part Number: LM1246AAA
• Description: 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM/ 512 Character RAM and 4 DACs
• Manufacturer: National Semiconductor

OSD Generator Operation
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 17 through Figure 24. There are two types of charac-
ters defined in the character ROM:
1. Two-color: There are a total of 384 two-color characters
2. Four-color: There are a total of 128 four-color charac-
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
in 6 banks (banks 0, 1, 2, 4, 5 and 6). 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 “back-
ground” color. If the bit value is 1, then the color is
defined as “Color 1”, or the “foreground” color. An ex-
ample of a character is shown in Figure 17. The grid
lines are shown for clarity to delineate individual pixels
and are not part of the actual displayed character.
ters, in two banks of 64 (banks 3 and 7). Each pixel of
the four-color character is defined by two bits of infor-
mation, 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 “fore-
FIGURE 18. Four-Color Character
(Continued)
FIGURE 17. Two-Color Character
21
3. By appropriately selecting the color attributes, it is pos-
colors:
Color 0: Those that are background pixels for both
Color 1: Those that are foreground pixels in character one
ground” 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 LM1247 internally
has a matrix of two planes of ROM as shown in Figure
18. 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 represented 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.
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.
characters,
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