I have finally managed to get some time to pin this problem down and look at this de-interlacing problem. Now I have my own Blackmagic card I have been able to do some extensive testing.

I have now discovered that the problem is a bug in the DirectX drivers for the DeckLink cards. The issue is that the card drivers are reporting the image to already be de-interlaced. They are flagged as being a progressive image. Whatever I do in Screen Monkey the software will always read the image to be progressive and not bother to de-interlace the image. I have tried many things to forcibly override this behaviour but none have succeeded.

When I contacted Blackmagic about this problem I got the following response.

At this time, I could not say if we are looking to update our DirectShow support. We now have the Decklink API for interfacing with Decklink cards, which offers greater control and access to the Blackmagic hardware, and it is also a multi platform interface for Blackmagic cards (Mac, Windows, Linux). I am afraid this does also mean an application needs to be designed to use the Decklink API, rather than the DirectShow interface.

I will try and find time to work on a dedicated DeckLink plugin but for the moment there is no easy solution to this problem.