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

<p>
  Ardour will only ever deal with a single <dfn>audio device</dfn>. When it is
  desired to use more than one audio device at the same time, there are two
  choices:
</p>

<ul>
  <li>Use Ardour to start JACK (which handles all audio I/O), and create a
  "fake" audio device which represents all the multiple devices to be used. 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. This drift cannot be stopped, although in some cases
  the effects may be insignificant enough that they might not be noticeable.
</p>

<p>
  Thus, in an ideal world, a single device with a single clock and all the inputs, outputs and other features needed should be used. Of course, there are those who like to point out that this is not 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, two eight-channel devices can be
  aggregated so that 16 channels can be recorded in Ardour.
</p>

<div class="note">
<p>
  When using a <em>single</em> typical 3rd party audio interface (such as those
  from Apogee, RME, Presonus, and many others), <em>or</em> using JackPilot or
  a similar application to start JACK, there is no need to worry about any of
  this. An aggregate device only needs to be set up if any of the following
  conditions are true:

<ul>
  <li>Two entirely separate devices are used <em>and</em> JACK is started using
  Ardour</li>
  <li>A <dfn>builtin audio device is used</dfn> <em>and</em> JACK is started
  using Ardour</li>
  <li>More than two entirely separate devices are used</li>
</ul>
</div>

<p>
  In the case of a builtin audio device, an aggregate device that combines
  "Builtin Input" and "Builtin Output" into one device needs to be created.
</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>