it would be nice to have a direct catalyst sdi output, but the hardware doesnt support such a thing.
The decklink input and outputs are totally separate, from the graphics card.
The graphics card does all the work.
The decklink card doesnt have a graphics chip.

It is not possible to frame grab the catalyst output and send it to the sdi output in software real-time internally.

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There is a professional range of graphics cards from nVidia which does have a direct SDI output from the graphics card- but these have never been released with a macintosh driver.