vMix Live Production Software Pro - Manual (Page 113)

vMix User Guide
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When using the Decklink Duo 2 or the Decklink Quad 2, port 1 will need to be configured in the Blackmagic
Desktop Video Utility
to group together SDI ports 1 and 2 as shown below:
External Output Configuration
When selecting a supported device in External Output settings in vMix, an Alpha Channel dropdown box will
appear as shown below:
None: No alpha channel, send the video output to the device as normal.
Straight: Output two channels over the device, Key + Fill on separate SDI ports. (This is also known as
Linear)
Premultiplied : Same as Straight except the Fill channel will be "Premultiplied" check with the receiving
device documentation to determine which of these options should be selected.