z86131 ZiLOG Semiconductor, z86131 Datasheet - Page 14

• Manufacturer Part Number: z86131
• Description: Ntsc Line 21 Decoder
• Manufacturer: ZiLOG Semiconductor

Z86129/130/131
NTSC Line 21 Decoder
Z86129/130/131 FUNCTIONAL DESCRIPTION (Continued)
Character width is 42.37 µs also centered on the screen, re-
sulting in a leading and trailing 1.32 µs black border.
An optional Caption display mode, Drop Shadow, can be
selected by the user through the serial port. This display
mode eliminates the black box around the characters and
places a 2-dot black shadow to the right and below the char-
acter luminance dots when in the 15 scan line per row mode.
This display mode is usable in Captions, Text and OSD dis-
plays. Figure 8 illustrates the characters with shadowing
added.
Extended Features
EIA–608 defined new extended features such as optional
Background and Foreground display attributes and optional
Extended Characters. The Z86129 always responds to the
Extended Characters but the Extended Background/Fore-
ground response can be controlled by the user. The Back-
ground and Foreground attributes add codes for background
colors, black foreground as well as transparent, opaque and
semi-transparent background. The BOX signal output pin
is set to a tri-state condition whenever one of the semi-trans-
parent attribute codes is active. The external keying circuits
can then use this condition to implement the intended video
display.
The font for the Extended Characters are illustrated in Fig-
ure 9. The accented capital letters have been implemented
by placing the accent marks above the character cell. When
selected, this mode results in the accent marks being written
into the character cell space of the row above. In some op-
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P R E L I M I N A R Y
erating modes, the Z86129 expands the size of the overall
box height by adding two additional scan lines at the top
and one additional line at the bottom. This addition makes
room for the accent marks in the topmost row and add a
black line below the descenders of any lowercase characters
in the last row.
This approach is desirable because shrinking the capitals to
make room for the accent mark within the character cell
makes poor quality characters and in some cases there
would be no differentiation between the capital and lower
case letter. It also has the advantage of minimizing the ROM
size and providing a good readable font that closely matches
what is normally seen in print.
In the unlikely case of a conflict between an accented capital
letter in one row and a lower case descender in the same
character position in the row above, the descender is given
priority. The improved readability of this approach over
shrunk capital letters far outweighs this potential conflict
and results in a cost-effective compromise for providing a
full, extended features implementation.
The Extended Characters share their address space with the
OSD Graphics Characters. When a BOX display is used the
Extended Character set is in force. However, if a Drop Shad-
ow display is used the Graphics Characters are in force. For
Caption and Text display modes, if Drop Shadow is set, the
user must also command the Z86129 to switch back to Ex-
tended Characters.
DS007200-TVX0199
ZiLOG