VGA to TV driver architechture
The DOS drivers for using VGA to TV drivers have to change VGA register to make VGA card to send sync frequencies which are compatible with normal TV. For changing of the those frequencies the following thing needs to be done:
- Set suitable clock frequency by reprogramming Sequencer registers
- Set CRT Controller timign register to make sync signals match TV standard
- VGA screen mode change is handled in following way
- The driver calls the original mode setup routine in BIOS
- The driver modifies the necessary VGA registers to make VGA card to output at TV frequencies
- VGA text mode font size changes
- Keep the VGA fonts the driver want to use always active
The VGA text modes are:
Mode Type Resolution Chars Colours (Hex) 0,1 text 360x400 40x25 16 2,3 text 720x400 80x25 16 7 text 720x400 80x25 mono
Normally VGA text mode has 25 lines. The screen font resolution is 9x16 pixels which leads to 720x400 effective resolution. Showing this resolution in TV would need interlacing, which is not a good idea in text mode because it causes too much flickering to text on TV screen. Getting stable text display on TV involves the following things:
- Setting the font size to 9x8 pixels and keeping the 25 text lines
- Setting the pixel clock frequency to half speed
- Adjust horizonal frequency using CRTC timing registers
- Adjust vertical frequency using CRTC timing registers
The VGA graphics modes are:
Mode Type Resolution Chars Colours (Hex) 4,5 gfx 320x200 40x25 4 6 gfx 640x200 80x25 2 D gfx 320x200 40x25 16 E gfx 640x200 80x25 16 F gfx 640x350 80x25 mono 10 gfx 640x350 80x25 16 11 gfx 640x480 80x30 2 12 gfx 640x480 80x30 16 13 gfx 320x200 40x25 256
Graphics with vertical resolution of less than 240 lines
VGA card is set to output non-interlaced pcture. The pixel frequency is set to half of the normal VGA pixel rate using CRT controller. Horizonal frequency and sync signal width is adjustested by changing CRTC timing registers. Vertical frequency is adjusted to suitable rate by adjusting the number of total vertical lines in te picture (black border lines are added to get 50 Hz or 60 Hz picture rate).
Vertical resolution of 480 lines
There are two ways to show 480 line display: vertical scaling and interlacing. In vertical scaling the VGA card is set to show only every other vertical line of the picture. In this way the effective number of scan lines is 240 and it can be handled as lower resolution graphics mode and shown as non-interlaced picture to TV.
To show the full vertical resolution needs use od interlacing. Interlacing in not a standard feature in standard VGA chipset but has been inmpelemented in many modern VGA chipsets. The methods how to set the interlace mode on are VGA chipset specific.
Vertical resolution of 600 lines
This resolution can be shown only on PAL TVs (actually only 580 lines are visible). This screen mode can be handled in the same way as 480 line modes.
Availability of source code
The driver is written fully in assembler and can be compiled with Turbo Assembler. The souce code of the VGA to TV driver is available only to selected group of registered devellopers. If you want to become a develloper e-mail to me for more details.
Tomi Engdahl <[email protected]>