saa5288 NXP Semiconductors, saa5288 Datasheet - Page 24

• Manufacturer Part Number: saa5288
• Description: Microcontroller With Full Screen Screen Display
• Manufacturer: NXP Semiconductors

Philips Semiconductors
7.6.5
The device signals the TV’s display circuits to display the
R, G and B outputs of the device, rather than the video
picture, by outputting a logic 1 on the VDS output. The way
in which this signal is switched is controlled by the bits in
the TXT5 and TXT6 SFRs. There are 3 control functions:
text on, background on and picture on. Separate sets of
bits are used inside and outside teletext boxes so that
different display modes can be invoked. Also, different
SFRs are used depending on whether the newsflash (C5)
or subtitle (C6) bits in row 25 of the basic page memory are
set (SFR TXT6) or not (SFR TXT5). This allows the
software to set up the type of display required on
newsflash and subtitle pages (e.g. text inside boxes, TV
picture outside) this will be invoked without any further
software intervention when such a page is acquired.
When teletext box control characters are present in the
page memory, whichever is relevant of the ‘Boxes On Row
0’, ‘Boxes On Row 1 - 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OSD messages has been superseded in this device by the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 (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.
Table 13 Display control bits
1997 Jun 24
TV microcontroller with full screen
On Screen Display (OSD)
PICTURE ON
D
0
0
0
1
1
1
ISPLAY MODES
TEXT ON
0
1
1
0
1
1
BACKGROUND ON
X
X
0
1
0
1
24
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 shown in Fig.6.
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 1, and in
OSD boxes. Meshed text can also be shadowed. Meshing
is illustrated in Fig.6.
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 logic 1, and in OSD
boxes.
text mode, black screen
text mode, background always black
text mode
TV mode
mixed text and TV mode
text mode, TV picture outside text area
EFFECT
Preliminary specification
SAA5288