On a CPC monitor, the HSYNC is rendered in "absolute black". It is darker than the black output by the Gate-Array.
The VSYNC is also modified before being sent to the monitor. It happens two lines after the VSYNC from the CRTC and stay two lines (same cut rule if VSYNC is lower than 4).
Using CRTC1, VSYNC width value 0 means a value of 16.
== The 6845 Registers ==