I’m assuming that by “external video cards” you mean “external video devices” as referenced in this Knowledge Base article: https://helpcenter.steinberg.de/hc/en-us/articles/115000808250-New-video-engine-starting-with-Cubase-9-0-30-and-Nuendo-8
If not, then please forgive me for my incorrect assumption.
In this post I will be referring to them as “Video Interfaces” (a better term IMHO).
@JMaus
Unless they announced it and I didn’t notice it, currently only Blackmagic Design video interfaces are supported.
They are planning/working on adding support for AJA interfaces in the future (timeline unknown).
Please see the knowledge base article I linked above.
@zolhof
I assume that you have a Blackmagic Design video interface. If not then that’s the problem.
However, I believe that you’re mistaken in the way these work.
They’re not GPU’s, nor do they process video or offload video processing. They don’t appear to the operating system as a GPU.
They’re more of a video interface card (hence me referring to them as such). Their primary function is to allow a computer to output a “true” video signal that isn’t affected by the operating system’s color management (they were initially designed for video color grading work).
All video processing is done by software.
In Cubase (from what I gather) they allow for video to be viewed (on a separate monitor) without using the video window.
(the information above came from me browsing Blackmagic Design’s forums and reading the Davinci Resolve manual [a Blackmagic Design software]).
Hope this helps both of you.