Background Oscar Z

[retro space]

I was looking some top-game lists, and you get tons of Youtubes, and all of them record emulation, never the real deal.
I was surprised to find the Oscar Z game look really different on my Sony CRT. The light purple background you see in emulation, is a blue to black fluent gradient in reality. Which makes the already tough as balls games even harder, but also makes me wonder how they did it. As if they break the palette limit. Anybody knows how the effect is achieved?

[Longshot]

May be the Hsync length (CRTC Register 3)

See the Compendium at 14.4, page 128 (https://shaker.logonsystem.eu)

"A short C-HSYNC may also have a discolouration effect on some non-CTM displays. This is for example the case if the CPC is connected to an ATARI SC1425 monitor (which was available with the ATARI 520STE).

If the HSYNC is cut to 6 μsec (R3=6), the GATE ARRAY sends black for approximately 2 μsec followed by the HSYNC signal of 4 μsec. If the colour of the BORDER has been defined with something other than black (which a HSYNC longer than 6 would generate) this has a discolouration impact on the line because each colour component is calibrated according to the colour present over the following 3 μsec (position 7, 8 and 9).

If the BORDER is not black and R3<9, the colour will be affected on the line. On this type of monitor, it is therefore possible to obtain more colours."