If the DeckLink 8K card works like the DeckLink Duo 2, each of the connectors can be set to full duplex (two connectors for a subdevice) or half duplex (one connector for a subdevice).
DeckLink cards support two types of keying:
- Internal which uses one SDI port as an input, overlays a key on the input video signal, sends it out over a second SDI port as an output.
- External which outputs a fill signal on one SDI port and a key signal on a second SDI port.
Both types of keying require two SDI connectors which mean they require a full duplex subdevice on the DeckLink card. Usually this means configuring each subdevice to use two connectors, one each for in and out (full duplex mode).
When the software you are using to generate the key signal initializes the keyer on the DeckLink subdevice, it will configure the subdevice for either internal or external keying. For internal keying, this means the two connectors assigned to the subdevice will be one input (first connector), one output (second connector). For external keying, this means the two connectors assigned to the subdevice will be fill output (first connector), key output (second connector).
The key point to keep in mind is that when the keying software enables the DeckLink keyer (in either internal or external mode), it may change how the SDI connectors assigned to the subdevice are used, but it does not automatically change which connectors are assigned to the subdevice, or whether the subdevice is configured for full duplex mode. So you usually need to make sure that each DeckLink subdevice is configured to use two connectors (in full duplex mode) in the Desktop Video setup utility first.
I'm guessing that if you configure the DeckLink 8K Pro as:
Decklink 8k Pro (1) SDI 1&2 in, SDI 3&4 out
You end up with 1 full duplex sub-device, where the single subdevice uses two connectors each for input and output (mostly intended for dual link SDI applications).