PCs use 1-255, where 1 is black and 255 is white. Video uses 16 as black and 235 as white, with below black and above white reserved for video processing.

The display has to know which is being used, if it's wrong you'd get a washed picture or a picture which is too dark and high in contrast. I was wondering how HDMI and DVI tell the display which is being used? Is there a flag, does it expect the display to automatically detect it by measuring the levels, or are there rules based on which colour space is being used?

Thanks

Depends on the device in question. My Sony HS10 projector allows me to select either "Computer" or "Video GBR" mode on the DVI port. On an HTPC, you can use Overlay or VMR9, which use computer and video levels, respectivelty. Other devices may or may not let you select.

IT's possible that there is a flag specified somewhere, but like many flags it's not utilized. The bottom line, the display doesn't "know," anything, you just have to align it to one or the other. You can tell by using appropriate test patterns.

My TV, a new Samsung LCD, has a setting called HDMI Black Level with the options Low and Normal. The manual says it only affects DVI over HDMI. Low appears to be the correct setting for computer graphics when i've got my computer plugged in with a DVI -> HDMI cable. With my Sony PS3 it's available (Low) when playing games but greyed out when playing a DVD. It is greyed out all the time for my Toshiba HD-A1 and DVD player. Black level seems to be constant between all of these devices.