Durango colour palette
The Durango-X and Durango-S colour computers have a colour display mode with 128 x 128 pixels. Using 4 bits per pixel, it allows the simultaneous use of 16 colours on the screen. Unlike some old computers like the ZX-Spectrum, Commodore 64, MSX etc there are no restrictions about how many different colours may appear in a certain area.
Palette colour list
||Deep Sky Blue||#00AAFF|
This palette is designed in a way that changing the Least Significant Bit will produce a noticeable luminance change, thus providing adequate contrast.
Depending on your display and component tolerances, some colours may differ in practice -- dark green in particular will usually look darker, thus minimising the difference between colours where this bit changes.
You may download a GIMP palette file with the colours available on Durango-X for convenient artwork creation.
In all video modes, Durango-X uses a linear framebuffer. Its dimensions being powers of 2, any screen position is easily computed into the corresponding memory address.
Display memory follows the "chunky" model -- all bits defining a pixel are packed together into the same byte. At 4 bpp, two pixels are encoded on every byte:
|left B||left g||left R||left G||right B||right g||right R||right G|
where R=red, B=blue, G=high green (67%) and g=low green (33%)
Whenever the colour mode is active, the four bits for each pixel are sent to the video mixer thru a crude Digital-to-Analog Converter (DAC). This mode can be displayed on a SCART-equipped TV by setting the
FAST BLANKING pin low, which is done thru software by setting low bit
D3 in the video mode register
This control bit is write only, thus the current mode cannot be checked. Software is expected to keep this bit set all the time, as will be ignored in HIRES mode anyway.
The bits on every byte are now interpreted as follows:
Note the reverse bit order, intended to closely (but not exactly) match the luminance values of the colours.
This mode was intended as an emergency resource for SCART-equipped devices that do not support RGB input (e.g. VCRs) and won't be available in most configurations, like the Sync-on-Green or PAL composite option.
RGB to Component video Converter
For improved compatibility with modern TV sets, this option will be included in Revision 2 of the Durango-X board; however, a standalone converter is also available and suitable for SCART-equipped v1 Durangos.
The Component Video input expects three signals (besides Audio):
The two first signals are generated inside the converter, and the last one is taken straight from the composite video output of Durango-X. But this signal is not really Luminance, but a greyscale signal instead -- close, but not a perfect match. For better colour accuracy when using the Component Video converter, the following changes are suggested in the Durango-X simple DAC:
|Designator||Value for greyscale||Value for Luminance|
This improved colour accuracy comes with the cost of impairing greyscale linearity but, as mentioned, this is rarely an issue.
Since the aforementioned mod has no effect whatsoever on the RGB output, it is highly recommended to build every Durango-X v1 DAC this way, just in case the Component Video converter is ever used.