Hi Daniel
Unfortunately, your Decklink HD Extreme is not capable of doing this feature, what is called a split down conversion. With the exception of the Decklink HD Pro DL PCI-X, all our cards utilise software down conversion, which means any processing is done by the host computer.
This also means, that when a video signal is passed to our board, it is already down converted, and the card can only accept one video signal for output.
However, the Decklink HD Pro DL PCI-X has the ability to do a split down conversion, which is hardware based on the card. This will allow you to send HD video on SDI out A, and SD video on SDI B.
https://www.blackmagic-design.com/products/hd/
We also do not have a convertor box that does a down conversion, the mini convertors only do a straight format conversion.
Both of your questions are very good ideas, and I will be passing them on to our development team as feature suggestions to raise awareness that our customers are looking for these features.
Callum McLay
Technical Support Consultant
Blackmagic Design EMEA