The Blackmagic Design DeckLink Duo 2, DeckLink Quad 2 and DeckLink 8K Pro video devices require correct Connector Mapping Settings, to provide all available inputs (four or eight) to the just:in mac engine Just In Engine (Mac) application. Please open the macOS System Preferences and open the Blackmagic Desktop Video Preference Pane.
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Then switch to the Connectors Tab and set the Connector Mapping to SDI X, instead of SDI X & SDI Y for all available inputs.
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In case of the DeckLink 8K Pro, it should look like this (SDI X In or Out):
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