SAA5X9X Datasheet, PDF (42/68 Pages) NXP Semiconductors – Economy teletext and TV microcontrollers

Philips Semiconductors
Economy teletext and TV microcontrollers
Preliminary specification
SAA5x9x family
When teletext box control characters are present in the
page memory, whichever is relevant of the ‘Boxes On
Row 0’, ‘Boxes On Row 1 to 23’ and ‘Boxes On Row 24’
SFR bits in TXT17 must be set if the display mode is to
change in the box. These bits are present to allow boxes
in certain areas of the screen to be disabled so that teletext
boxes can be used for the display of OSD messages
without the danger of subtitles in boxes, which may also be
in the page memory, being displayed. The use of teletext
boxes for OSD messages has been superseded in this
device by the OSD box concept, described later, but these
bits remain to allow teletext boxes to be used, if required.
The COR bits in the TXT5 and TXT6 SFRs control when
the COR output of the device is activated (i.e. pulled
down). This output is intended to act on the TV’s display
circuits to reduce the contrast of the video display when it
is active. The result of contrast reduction is to improve the
readability of the text in a mixed text and video display.
The bits in the TXT5 and TXT6 SFRs allow the display to
be set up so that, for example, the areas inside teletext
boxes will be contrast reduced when a subtitle is being
displayed but that the rest of the screen will be displayed
as normal video.
Setting the shadow TXT4.SHADOW ENABLE bit will add
a ‘south east’ shadow to the text, significantly enhancing
its readability in mix mode. Shadowing is illustrated in
Fig.13.
The readability of text can also be enhanced using
‘meshing’. Meshing causes the VDS signal to switch so
that when the text background colour should be displayed
every other pixel is displayed from the video picture. Text
foreground pixels are always displayed.
The TXT4.BMESH bit enables meshing on areas of the
screen within the text display area with black as the
background colour. The TXT4.CMESH bit has the same
effect on areas with other background colours. Meshing
can only be invoked in areas displayed in text mode i.e.
where the TXT5.TEXT IN and TXT5.BKGND IN bits are
both set to logic 1s, and in OSD boxes. Meshed text can
also be shadowed. Meshing is illustrated in Fig.13.
The TXT4.TRANS bit causes areas of black background
colour to become transparent i.e. video is displayed
instead of black background. Black background
transparency can also only be invoked in areas displayed
in text mode i.e. where the TXT5.TEXT IN and
TXT5.BKGND IN bits are both set to a logic 1, and in OSD
boxes.
Table 19 Display control bits
PICTURE ON
0
0
0
1
1
1
TEXT ON
0
1
1
0
1
1
BACKGROUND ON
EFFECT
X
text mode, black screen
0
text mode, background always black
1
text mode
X
TV mode
0
mixed text and TV mode
1
text mode, TV picture outside text area
1997 Jul 07
42