LM1236 National Semiconductor Corporation, LM1236 Datasheet - Page 19

• 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

OSD Generator Operation
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
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:
1. Two-color: There are a total of 190 two-color characters.
2. Four-color: There are a total of 64 four-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.
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
FIGURE 14. Block Diagram of OSD DACs
(Continued)
FIGURE 15. Two-Color Character
19
the control registers. This is also where horizontal and ver-
tical blanking are also inserted at their appropriate intervals.
3. By appropriately selecting the color attributes, it is pos-
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.
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
four color characters. This is explained in the next
section.
20075730
*
N, where N is the number of unused
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