The Livestream Studio HD51 and HD550 are each equipped with five Blackmagic DeckLink Mini Recorders; the HD31 has three cards installed.
Each card has one SDI input and one HDMI input.
The 4K editions of the HD51 and HD550 have 4K versions of these cards installed.
Each card can only handle one source at a time. You cannot use both the SDI and HDMI input on the same card.
For example, if you use the HDMI input on Mini Recorder 1, you cannot also use the SDI input on Mini Recorder 1 and would have to use an SDI input on one of the other Mini Recorder cards on your system.