Premultiplication violates the PNG spec [link]. PNG alpha is always supposed to be straight.
What is the problem you're having? Can you show the issue and describe your workflow?
I wasn't aware of that, thanks. I've done a little digging and it appears the Blackmagic ATEM doesn't interpret PNG's correctly. Looks like I'll be switching to targa, for the ATEM. I thought I was having problems with CasparCG, but that has turned about to be ok.