4
Known limitations
The ZX-HD is not compatible with the (Eastern European) Pentagon ZX Spectrum clone;
The ZX-HD is 99.99% synchronous to the ZX Spectrum ULA, but some very fast so-called ‘border
effects’ may be just as bit out of sync;
The ZX-HD does not have audio over HDMI –check the “Adding sound” chapter for more info;
Although the latest ULAplus™status register can be read, it is currently not possible to read the
ULAplus™palette values, but there is no software that uses that feature (the games that use
ULAplus™only set the 64 colour palette) - we will integrate the feature in the future;
The ZX-HD ULAplus™implementation currently does not yet support Timex modes and HAM256
and HAM8x1 modes (which are all optional ULAplus™features);
The ZX-HD synchronous ULA functionality has not yet been tested or adapted to NTSC models of
the ZX Spectrum or Harlequin kits;
The ZX-HD seems to have issues with an Interface 1 attached; we’re analyzing the problem.
Interlaced Mode
The ZX Spectrum 128 models (the ‘toastrack’, grey +2, black +2A and +3) have 2 video memory
pages. The ZX-HD supports using these two video memory pages.
The ZX-HD even supports software that very quickly switches between these video pages to achieve
a so-called ‘interlacing’of two screen pages.
When the ZX-HD detects that screen pages change with every of 4 consecutive screen updates, it
automatically switches to ‘Interlaced Mode’. If no screen page change is detected during four
consecutive updates it will switch back to normal mode.
In the ZX-HD Interlaced Mode the number of displayed screen lines is doubled, hence the resolution
of the HDMI framebuffer is doubled, and the odd or the even screen lines are updated one after the
other. This results in the two different screen pages being displayed simultaneously.
One reason why the Interlaced Mode is not used permanently, is that the odd/even screen line
updates only refresh half of the screen content at a time: if there is a quickly moving object on the
screen, two different phases of the movement can be seen simultaneously which may result in a bit
odd look.
You can override the automatic Interlaced Mode in the cmdline.txt file on the micro SD card of the
ZX-HD containing the ZX-HD firmware. More information is found in that file.
DivMMC EnJOY! / DivMMC EnJOY! users
When connecting the ZX-HD with a DivMMC EnJOY! to a ZX Spectrum 128 ‘toastrack’, DIP switch 4
of the DivMMC EnJOY! needs to be switched OFF.
When connecting the ZX-HD with a DivMMC EnJOY! to a ZX Spectrum 128 ‘toastrack’, DIP
switch 3 of the DivMMC EnJOY! needs to be switched OFF.
Keeping this DIP switch ON will prevent the system from starting correctly with this setup.
