Z86129 Datasheet, PDF (10/50 Pages) Zilog, Inc. – NTSC LINE 21 DECIDER

Z86129/130/131
NTSC Line 21 Decoder
PRELIMINARY
Z86129/130/131 FUNCTIONAL DESCRIPTION (Continued)
The default setting of the Z86129 places the center of the
Box at about 13.5 µs to allow for some delay in the normal
video path. However, the Box horizontal position can be
adjusted by the user in 330 ns increments. The display will
be approximately within the safe title area for NTSC
receivers. Character width is 42.37 µs also centered on the
screen, resulting 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
character luminance dots when in the 15 scan line per row
mode. This display mode is usable in Captions, Text and
OSD displays. Figure 5 shows 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 will always
respond to the Extended Characters but the Extended
Background/Foreground response can be controlled by
the user. The Background and Foreground attributes add
codes for background colors, black foreground as well as
transparent, opaque and semi-transparent background.
The BOX signal output pin will be set to a tri-state condition
whenever one of the semi-transparent 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 shown in Figure
6. The accented capital letters have been implemented by
placing the accent marks above the character cell. When
selected, this mode will result in the accent marks being
written into the character cell space of the row above. In
some operating modes the Z86129 will expand the size of
the overall box height by adding two additional scan lines
at the top and one additional line at the bottom. This will
make 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 Shadow 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 Extended Characters.
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