Q How would the image quality differ between converting Mini DisplayPort output to VGA or HDMI input? How good is software support for mirror or two-screen display? Can it also carry sound across?
A VGA is analogue, limited to a range of resolutions generally well below 1080p, and does not normally support widescreen displays well. It is a basic, old, and tatty semi-standard of the past.
HDMI is digital, and supports resolutions from 1080p upwards, particularly on widescreen displays. It includes HDCP rights protection, although that may not work fully through computer systems. It is the mainstream consumer HD standard of the present and future.
Macs with beefier graphics hardware give you the option of mirroring a built-in display or using the additional screen area as a contiguous desktop, provided that the Mac recognises the display properly.
Although the original Mini DisplayPort video output is video only, and models released before late 2009 do not support the streaming of audio through their ports, more recent models include support in their Thunderbolt output. You can select that in the Sound pane of System Preferences, although whether downstream devices such as a TV accept that input is another issue.
Comments Further details of display output formats is given here.
Updated from the original, which was first published in MacUser volume 28 issue 18, 2012.