Q: Pro Tools on my Mac has an audio engine option for Aggregate I/O; what is this?
A: On the Mac using Core Audio, in the Pro Tools Audio Engine, when you choose the built-in input as the sound source, you don’t have access to any outputs. Likewise, when selecting the built-in output for Pro Tools sound play back, you don’t have access to any inputs. Clearly this is a major limitation!
The Aggregate I/O setting in the Mac version of Pro Tools was introduced in Pro Tools 9 and designed to allow the software to combine the Mac’s built-in input and output into one “virtual” interface so that you could access both simultaneously — and that’s really as far as Avid officially supports the feature.
However, in many cases you can use Aggregate I/O to use more than one audio interface from within Pro Tools at the same time. Since the combination of other audio interfaces (besides the Mac’s built-in input and out) is not officially supported, you are on your own as far as compatibility and tech support. In our experience many other combinations of interfaces will work — trial and error is the only way to find out for sure what works and what doesn’t.
See this SweetCare Knowledge Base article for more information.