
2004 Sep 03
80
Philips Semiconductors
Product specication
Enhanced TV microcontrollers with
On-Screen Display (OSD)
SAA56xx
23.4.19 CONTRAST REDUCTION
The device can act on the TV’s display circuit to reduce
contrast of the video by driving the COR output LOW.
Contrast reduction improves the readability of characters
in mixed mode.
TXT: Bits COR IN in SFRs TXT5 and TXT6 control when
the COR output of the device is activated. These bits allow,
for example, the display to be set-up so that the areas
inside Teletext boxes are contrast reduced when a subtitle
is displayed, leaving the rest of the screen displayed as in
normal conditions.
CC: Here, the contrast reduction is controlled by the
contrast reduction attribute (see Table
33). This attribute is
valid from the time set until the end of a row of a display
window, or otherwise modified if set with Serial Mode 0.
If set with Serial Mode 1, it is set from the next character
onwards.
23.5
Character and attribute coding
This section describes the character and attribute coding
for each mode.
23.5.1
CC MODE
Character coding is split into character oriented attributes
(parallel, see Table
32) and character group coding
(serial, see Table
33). The serial attributes take effect
either at the position of the attribute (set at), or at the
following location (set after) and remain effective until
either modified by a new serial attribute or until the end of
a row of a display window. A serial attribute is represented
as a space (the space character itself however is not used
for this purpose). The attributes that are still active,
e.g. overline and underline, are visible during the display
of the space.
The default setting at the start of a row is:
1× size
Flash off
Overline off
Underline off
Italics off
Display mode = superimpose
Fringing off
Background colour duration = 0
End of row = 0.
The coding is done in 15-bit words. The codes are stored
sequentially in the Display memory. A maximum of
768 character positions can be defined for a single display.
23.5.2
TXT MODE
Character coding is in a serial format, with only one
attribute being changed at any single location. The serial
attributes take effect either at the position of the attribute
(set at), or at the following location (set after). The attribute
remains effective until either modified by new serial
attributes or until the end of a row of a display window.
The default settings at the start of a row are:
Foreground colour white (CLUT address 7)
Background colour black (CLUT address 8)
Horizontal size ×1, vertical size ×1 (normal size)
Alphanumeric on
Contiguous Mosaic Graphics
Release Mosaics
Flash off
Box off
Conceal off
Twist off.
The attributes have individual codes which are defined in
23.5.3
PARALLEL CHARACTER CODING
Table 32 Parallel character coding
BITS
DESCRIPTION
0 to 7
8-bit character code
8 to 10
three bits for eight foreground colours
11
mode bit: 0 = parallel code
12 to 13 character set selection; see Section
23.11.214
special graphics; see Section
23.4.17