Pretty much. I don't think Pro Tools recognizes any I/O other than Digi unless you use ADAT or S/PDIF.

So it sounds like you really looked into equipment carefully before your initial purchase????

You can only use Digi design authorized equipment within ProTools of any variety. You can however add additional microphone preamps of a generic nature, provided they have compatible digital ports, that are compatible with other ProTools interfaces, that have provisions for external digital input devices to expand track counts.

You can also utilize your other equipment to record within other software and then be able to import to ProTools, but that doesn't help you much if you need something like 16 to 24 simultaneous inputs if you're thinking of utilizing your ProTools rig as a convenient multitrack recorder.

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If you are still interested in PT, you can go to M-Powered: they have many more interface options.