Interlace
By alternating between two screens on every frame interrupt (50 Hz), it is possible to simulate a doubling of the vertical display resolution from 192 to 384 lines. The Timex clones and ZX Spectrum 128K implement a 'shadow' screen area which can be switched into place through the use of a single OUT command, and this is often utilised to rapidly switch between two images for this purpose (although this can also be achieved with a standard block copy, albeit not over the entire screen). When viewed on a television screen, the flicker is less noticeable than on a monitor. The image is linked to here (warning: the linked image flickers very rapidly, which could potentially cause seizures in people with photosensitive epilepsy).
The technique does not in fact achieve a true interlaced display, as the Spectrum lacks the ability to synchronise with the display hardware at such a low level. Rather, the effect is more akin to anti-aliasing, with certain pixels appearing at half intensity.
- Details:
- Pixels: 256 x 384
- Attributes: 32 x 24
- Colors: 15
Read more about this topic: ZX Spectrum Graphic Modes