ST Video Modes
ST Video Modes
The ST has three main video modes: two colour modes which are designed for display on a CRT television or colour monitor; and one high-resolution but monochrome video mode designed for display on a monochrome monitor. The electrical signal characteristics of the colour modes and the monochrome mode are different enough that there was no monitor originally available that could display all three. These days, some VGA monitors or televisions are capable of displaying all the modes.
Available modes depends on which monitor is connected to the video socket. If a monochrome monitor is detected then the machine will display monochrome mode only. Otherwise, the two colour modes are available (selectable in software).
Frame rate (50Hz or 60Hz) is switchable in software, but initially set on bootup by TOS - with the frequency depending on the TOS region. System clock speed is also slightly different depending on the region where the machine was sold: a 32.084988 MHz crystal is installed for PAL machines, and a 32.215905 MHz crystal for NTSC.
Every video mode displays, by default, exactly 32,000 bytes of data, with a trade-off of more displayed pixels with a smaller colour palette, or fewer pixels but a greater colour palette, as per the table below.
All video modes are displayed with a border on all four sides. This border is not accessible for displaying pixels the usual way, but software techniques make this possible.
| Mode name | Compatible monitor | Resolution | Displayable colours | Frame rate |
|---|---|---|---|---|
| Low | Colour / TV | 320x200 | 16 (4 bits) | 50/60Hz |
| Medium | Colour / TV | 640x200 | 4 (2 bits) | 50/60Hz |
| High | Monochrome / VGA | 640x400 | 2 (1 bit) | 71.2 Hz |
| Mode | Pixel clock rate | Scanlines per frame | CPU clocks / scanline | HSync Freq |
|---|---|---|---|---|
| Low (50Hz) | 8MHz | 313 | 512 | 15.666 KHz |
| Low (60Hz) | 8MHz | 262 | 508 | 15.666 KHz |
| Medium (50Hz) | 16MHz | 313 | 512 | 15.666 KHz |
| Medium (60Hz) | 16MHz | 262 | 508 | 15.666 KHz |
| High (71.2Hz) | 32MHz | 501 | 224 | 35.809 KHz |
HSync timing is given for PAL machines, for NTSC machines they will be about 100Hz faster due to the faster crystal.
TODO
- Talk about VBL's, HBL's, Video shifter
- See also Videl
- How to set video mode in desktop
- How to set video mode in asm, GFA etc
- How to connect an ST up to a monitor, or a modern VGA alternative