Chapter 4
30
In half cell mode with extra colours and using double byte colour, the first half of the
attributes file holds the colour for the right half of each cell and the second half of the
attributes file holds the colour for the left half for each cell.
Colour byte formats
The SPECTRA interface drives its SCART socket using 6 colour lines, two for each of
the red, green and blue channels. This yields 4 levels for each channel, and they
correspond to approximately 0%, 33%, 66% and 100% of full brightness. When the
channels are combined, a total of 64 different colours can be produced. Of these 15
match the basic range of colours achievable on the standard Spectrum display. It is
the state of the basic / extra colours bit of the display mode register that selects
whether a basic colour palette or the larger 64 colour palette is used.
The basic colour palette, including each colour’s index number and bright variation, is
shown below. Note that some attribute modes can only show the non-bright colours
due to a lack of resources in the SPECTRA interface to add support for the full range.
Basic colour palette
The 64 colour palette, including each colour’s index number, is shown below.
64 colour palette
The state of the single / double byte colour bit of the display mode register
determines whether the ink and paper colour values for an attribute cell are held in
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