LM1246AAA National Semiconductor, LM1246AAA Datasheet - Page 22

• 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

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OSD Generator Operation
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 19,
both “A” and a “B” are drawn separately, then superimposed,
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).
ATTRIBUTE TABLES
Each character has an attribute value assigned to it in the
page RAM. The attribute value is 4 bits wide, making each
character entry in the page RAM 13 bits wide in total. The
attribute value acts as an address, which points to one of 16
entries in either the two-color attribute table RAM or the
four-color attribute table RAM. The attribute word in the table
contains the coding information which defines which color is
represented by color 0 and color 1 in the two color attribute
table and color 0, color 1, color 2, color 3 in the four-color
attribute table. Each color is defined by a 9-bit value, with 3
bits assigned to each channel of RGB. A dynamic look-up
table defines each of the 16 different color ’palettes’. As the
and background pixels in character two,
and background pixels in character one,
characters.
FIGURE 20. Displaying Each Character Individually
FIGURE 19. Four Color Character as a 2 x 2 Color
(Continued)
22
Figure 20 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 character
1) and color 3 = color 2. Other than this, there is no restric-
tion on the choice of the actual colors used.
look-up table can be dynamically coded by the microcontrol-
ler over the I
assigned to any one of 2
maximum of 64 different colors to be used within one page
using the 4-color characters, with up to 4 different colors
within any one character and 32 different colors using the
2-color characters, with 2 different colors within any one
character.
TRANSPARENT DISABLE
In addition to the 9 lines of video data, a tenth data line is
generated by the transparent disable bit. When this line is
activated, the black color code will be translated as ‘trans-
2
C compatible interface, each color can be
9
(i.e. 512) choices. This allows a
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