I’ve not clicked on Todds link, but I’m pretty sure it will explain everything.
There are two flavours of alpha, straight and premultiplied.
Premultiplied will look ‘normal’, and straight will look bad if the alpha is not applied.
(There could be a lot of garbage, but if the key signal says it’s not to be used, it does not matter of course. It’s the same that outside the set you see lights, flightcases and half empty coffee cups.)
However, both flavours contain the same data, only the math is different.
Most NLE’s accept only straight alpha. AE lets you choose.
So, don’t worry about it, just re-import in a NLE, overlay on ‘something’, then you can do QC.
Bouke
http://www.videotoolshed.com