Changes
CRTC
,/* HSYNC */
So even when a fully valid C-HSYNC pulse of 4µs is emitted, an HSYNC shorter than usual combined with a coloured border can induce a discolouration effect on those monitors.
=== Screen wobbling and Fine horizontal hardware scroll ===
These effects use invalid HSYNC signals, which are poorly supported by modern displays. However, these effects were used in commercial-era CPC games, so it's fine if you choose to use them.
When R2 increases by 1, the screen is shifted to the left by 16 mode2 pixels.
When R2 decreases by 1, the screen is shifted to the right by 16 mode2 pixels.
When R3l increases by 1 (and if <6), the screen is shifted to the left by 8 mode2 pixels.
When R3l decreases by 1 (and if >2), the screen is shifted to the right by 8 mode2 pixels.
The shift is not instantaneous but follows a logarithmic attenuation across several raster lines. This property can be used to get even finer horizontal control by modifying R2/R3 on each raster line.
[[Longshot]] has demonstrated [https://youtu.be/1q7RQykZoKY horizontal hardware scroll] with one-pixel precision in mode1 on all CRTCs. However, because you have to synchronize with each line, it takes a lot of CPU time if it's done in fullscreen (272 lines).
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