In Linux, Ubuntu, my VGA video card only gives 0.6volts out at peak white, instead of the expected 0.7volts. It is fine (0.7volts) in Windows, dual boot on the same computer. It is not distro specific, as it does the same on the System Rescue CD. The Nvidia driver is on its default settings.

The same SysRescueCD on my old computer MSI 6153 MB with Matrox graphics, gives 0.7volts, as expected.

The motherboard is a Gigabyte GA-M51GM with on-board graphics, NVidia GeForce 6100.

The above measurement was made at the VGA connector with an oscilloscope. Increasing brightness or contrast in the nvidia-settings boxes did not help. There appeared to be a "brick wall" at 0.6volts.

I realise this is all a bit weird, but has anyone ever come across this effect?

Strange
Did you use the same test image, resolution and vertical refresh rate on both systems?

Absolutely. Whatever I use, a greyscale, the white background in Firefox, a text editor, the white level will not go above 0.6volts, in spite of tweaks in the graphics card driver. I guess it is a problem in my motherboard/graphics combination, but it is fine in Windows! In fact the white level in Windows is/was slightly high (0.73volts).

Thank you for your interest, by the way. I realise the above is kind of obscure.

Any point in trying a separate graphics card, rather than the on-board, do you think? I would have to buy a new PCI-E to do that, unfortunately.