Most of the time setting up an audio interface in macOS is pretty straightforward—after installing the interface (and any required drivers) you just select it in your DAW’s Preference or setup panel, and you’re good to go. But at times it can get a little more complicated. Sometimes you may want (or need) to combine more than one interface, or use an interface in conjunction with the computer’s built-in audio i/o. For those situations one of the available options is to create an Aggregate Device using the macOS’s included Audio MIDI Setup utility.
An Aggregate Device is a software pairing of inputs and outputs from more than one source. There are a number of reasons why you might want to do this—the most common is probably to expand the i/o (input/output) channel count.
Let’s say you have an audio interface with 8 ins & outs, and you need at least couple more for recording a band, or a multi-miked drumkit, for example. Of course you could buy an i/o expander and hook it up to the main interface via ADAT connections, but that might not be the ideal option. Maybe you have an extra interface sitting around—say, a smaller 4x4 box that would provide enough extra i/o without additional expense—but it doesn’t include ADAT connections. You could still combine the extra 4 audio channels with the main interface by combining them as an Aggregate Device, which would present them to the DAW as a single 12x12 interface.
Each interface would still be physically connected directly to the computer (via USB or Thunderbolt)—the combination device is created in software, using the Audio MIDI Setup utility. Here’s the process for doing that..
Audio MIDI Setup—AMS—can be found in the Utility folder within the root-level Application folder on your startup drive. It consists of two panels, one for setting up audio devices, and another for MIDI devices—here we’ll just look at the Audio panel.
If you have no audio interface connected to your computer the list of devices (on the left) will only include the computer’s own Built-in Microphone (input) and Built-in Output (sometimes labelled Headphone Out). If you do have one or more audio interfaces connected and installed then they’ll also show up in the list of devices, along with each’s i/o channel count.
To present, say, two interfaces to the DAW as a single larger interface, you can easily combine them into an Aggregate Device. You’d click the + button at the lower left of the device list. There are two options, an Aggregate device, and a Multi-Output device—to combine both the ins & outs of the two interfaces you’d choose Create Aggregate Device.
Now in the panel on the right you’ll see a list of all the available devices—you simply check the ones you want to include in the new Aggregate. For this example that would be the two audio interfaces. One of them has to serve as the Clock Source (to keep everything in digital sync)—choose the best-quality device (in this case the most high-end audio interface). Drift Correction should be checked for the other device—leave it checked. If you want, rename the new Aggregate Device, and you're done in Audio MIDI Setup.
To finish up you’d go to your DAW and select that Aggregate device for both Input and Output. Now when you click on an audio Input or Output setting for a channel strip, the popup should list all the channels for both interfaces, expanding your i/o capability.
Occasionally, some of the combined ins & outs may be labelled in a less-than-straightforward way—identified by number instead of by name. If that’s the case, you can check the Audio MIDI Setup panel for that Aggregate Device to see which numbers correspond to which interface connections. For example, my USB mic (in an Aggregate Device with my interface) shows up only as Input 15 (&16, for some reason), despite naming it in AMS. But once you’ve sussed out what’s what everything should work fine.
Besides expanding i/o, there are other reasons why you might want or need to create an Aggregate Device. One surprising use is when you want to use Pro Tools with the computer’s built-in i/o. This might be the case when you're on the road with a laptop and want to make some quick recordings with the built-in mic, and monitor off your headphones plugged into the laptop’s audio out. Many DAWs offer separate selections for input and output when you select the i/o for the DAW, but Pro Tools offers only a single option that includes both in & out. The problem with that is that macOS presents the Built-in Mic and the Built-in Output as two separate choices, so you can choose one or the other but not both!
But you can simply create an Aggregate Device that combines the Built-in Mic and Built-in Output into a single Aggregate Device, which can then be selected as the Playback Engine (audio interface) in Pro Tools, providing both system input and output.
Another application I’ve found for an Aggregate Device relates to streaming high-quality audio. I needed to broadcast an online class that included both my voice (from a USB mic) and high-quality audio directly from a DAW. To do that I needed to be able to route the DAW signal to two places—my regular interface outputs (for me to monitor it) and internally to the software providing the broadcast stream (OBS in this case). That required internal audio routing software to split and route the DAW audio as required. This could be done with software like Loopback (from Rogue Amoeba) or the free Soundflower driver—I used Soundflower.
After installing Soundflower it appeared in Audio MIDI Setup as another audio device. I was then able to create a Device that included my interface and Soundflower, but in this case I had to use the other option, and create a Multi-Output Device—with this selected as the DAW output device, DAW audio was routed to my interface for monitoring, and to OBS (via Soundflower) for broadcasting good-quality sound directly from the DAW.
Once you know how to set up an Aggregate Device, it can be a quick and easy way to expand i/o or solve other audio routing issues when working with audio in macOS.