Pod Geek

This is going to be for the Mac users, so if you are a PC user, sorry.  This functionality is exclusive to Macs (as far as I know, but I will investigate).

Lets say you started out your podcasting career needing only one or two inputs.  You were podcasting by yourself or a friend, or just needed enough inputs to for you to talk to someone on Skype.  Now you have grown a bit, or have a sudden need for more inputs, and you don’t want to just give up on your old device.

Mac OS X has a nifty setting that lets you combine multiple audio devices into one virtual audio device.  You tell your computer which audio devices you want to combine, and it makes a new single source that your computer sees, will all the inputs and outputs of both.

On this computer, I’m running OS X 10.6.8 as it’s a four year old computer and it works well for what I do.  In the newer versions of OS X, the steps are the same.

First, I have two devices connected to my computer.  One is a Firewire mixer and one is a USB interface, but the connections themselves don’t matter.  This is something you could do with several USB Microphones, or any combination of devices.

Mackie Onyx 1220i and Alesis io2

Next we need to open the Audio Midi Setup utility.  You can find it in your Applications folder under Utilities > Audio Midi Setup.  It is not the same as the Sound settings under System Preferences.

When you open Audio Midi Setup, you should see all your audio devices.  If you don’t, go to your toolbar and click Window > Show Audio Window.  You can see the two devices I’m going to combine, the Mackie Onyx mixer, and the Alesis io2.

Next, click the plus button in the lower left of the window.  You’ve just created an aggregate device.

On the right of the window is all the devices you can add.  You may notice that every devices listed on the left isn’t available.  Some of those devices on the left are virtual devices that aren’t available because the software that uses them isn’t running.  But Soundflower, a virtual audio device is available.

I’m just going to select the two devices I want to combine.  I could add more if I needed them, but I’m keeping it simple.

The aggregate device settings is going to ask you to select the Clock Source.  One of your devices has to act as a master clock to keep all the devices synced up.  I selected the Mackie mixer to be the clock, and the io2 will follow that.  The ‘resample’ box is checked, because the syncing of the two devices is happening inside the computer, rather than via external means, such as a word clock sync (if you don’t know what that means, don’t worry, it’s mostly stuff that happens in recording studios and higher level recording setups).

If you click on the individual devices within the aggregate device, you can set the sample rate and bit depth, as well as other options.

Make sure the settings of the two devices match.  The Mackie has more available sample rates than the Alesis, so I select it’s sample rate and bit depth first, then make the Alesis match that.

So now I have  a single 18 input, 4 output device for my DAW to use. Double clicking on the name of the device will allow you to rename it to something easier to remember.  I named mine Mackie and Alesis.

And now I can select it from my list of devices.

A few things to keep in mind about aggregate devices:

- Since these devices are combined in the computer, all of the audio has to be processed by your computer.  You can’t hear an audio input from one device on the other without latency or having to mix the sources in software.

- You can’t force a sample rate and bit depth on a device that the hardware isn’t capable of.  The Mackie mixer may be able to use am 88,200 Hz sample rate, but the Alesis simple can’t, and just because I combined the two devices, I can’t force it to.

Depending on your needs, it may make more sense for you to buy a larger capacity audio interface rather than use an aggregate device, but if you decide you just need to add something else to your existing rig, aggregate devices make it simple.

Have you used aggregate devices?  Have tips about using them?  Questions about what is posted here, or is there something I missed ?  The comments are open.