manual/include/using-more-than-one-audio-device.html

<p>
  Ardour will only ever deal with a single <dfn>audio device</dfn>. If you
  want to use more than one, you have two choices:
</p>
<ul>
  <li>
    If you want to use Ardour to start JACK (which handles all
    audio I/O) you will need to create a "fake" audio device on your
    computer the represents all the multiple devices you wish to
    use. How to do this is platform dependent and described below.
  </li>
  <li>
    Use a different tool to start JACK and manage all the devices.
  </li>
</ul>
<p>
  Ardour is fundamentally designed to be a component in a
  pro-audio/music creation environment. Standard operating practice
  for such setups involves using only a single digital <dfn>sample
  clock</dfn> (something counting off the time between audio samples).
  This means that trying to use multiple independent soundcards is
  problematic, because each soundcard has its own sample clock, running
  independently from the others. Over time, these different clocks
  <dfn>drift</dfn>
  out of sync with each other, which causes glitches in the audio. You
  cannot stop this drift, although in some cases the effects may be
  insignificant enough that some people might not care about them.
</p>
<p>
  Thus in an ideal world you should not use multiple independent
  soundcards but instead use a single device with a single clock and all
  the inputs, outputs and other features that you need.
</p>
<p>
  Of course, a lot of people don't live in an ideal world, and believe
  that software should make up for this.
</p>

<h2>OS X</h2>
<p>
  In CoreAudio, <dfn>aggregate devices</dfn> provide a method to use
  multiple soundcards as a single device. For example, you can
  aggregate two 8-channel devices so that you can record 16 channels
  into Ardour.
</p>
<div class="note">
<p>
  If you are using a <em>single</em> typical 3rd party
  audio interface (such as those from Apogee, RME, Presonus, and many
  others), <em>or</em> you are using JackPilot or a similar
  application to start JACK, you do not need to worry about this.<br>
  You will need to set up an aggregate device only if either
  of the following conditions are true:
</p>
<ul>
  <li>You want to use two entirely separate
    devices <em>and</em> want to start JACK using Ardour.</li>
  <li>You want to use your <dfn>builtin audio device</dfn> <em>and</em>
    want to start JACK using Ardour.</li>
  <li>You want to use more than two entirely separate devices</li>
</ul>
</div>
<p>
  In the case of your builtin audio device, you will need to create
  an aggregate device that combines "Builtin Input" and "Builtin
  Output" into one device.
</p>
<p>
  The precise instructions for creating an aggregate device on OS X
  have varied from one released to another. Please read <a href="https://support.apple.com/en-us/HT202000">https://support.apple.com/en-us/HT202000</a>
</p>

<h2>Linux</h2>
<p>
  Please see the instructions at <a href="http://jackaudio.org/faq/"
  title="http://jackaudio.org/faq/">http://jackaudio.org/faq</a>
</p>