C-HSYNC and C-VSYNC are composited using the XNOR function. The resulting CSYNC signal produced by the Gate Array is 1 when inactive and 0 when active.
On a CPC monitor, the CSYNC is rendered in "absolute black". It is darker than the palette colour black output by the Gate Array. The electron beam is basically turned off. Turning up the brightness level won't make it any brighter.
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