Summary of available attribute modes
Note that it is not possible to fully support 64 ink and 64 paper colours at an attributes
file resolution of 32 x 192, or to fully support 8 ink and 8 paper colours at an attributes
file resolution of 64 x 192. This is because these modes would require 2K more RAM
than is available for use by the attributes file. So instead, a composite display is
produced that consists of two areas of different resolutions. The details of this hybrid
display format and why there is a limit to the size of the attributes file are described
later in this chapter.
The new display modes are only available when enabled by setting configuration
switch 6 to the on position.
RGB picture generation
The most obvious approach to generating a RGB picture is to try and decode the YUV
colour difference signals available from the Spectrum’s expansion bus. However, these
signals are subject to so much electrical noise that it is impossible to reliably identify
all shades of colour produced by the Spectrum. A solution which bypasses the noise
is therefore required.
The Spectrum stores its picture display information at the beginning of the lower 16K
RAM bank. The SPECTRA interface listens to all writes to this RAM bank and keeps a
copy of the data in its own onboard RAM. It then uses this copy to independently
generate a TV picture that is identical to the one produced by the Spectrum. By
constructing the TV picture directly from raw display bytes, the issue of electrical noise
is completely circumvented.
1
Due to limited memory availability, this is a hybrid mode consisting of two regions of different line heights.
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