manual/include/monitor-signal-flow.html

<p>There are three basic ways to approach monitoring: </p>

<h3>External Monitoring</h3>
<img class="right"
src="/images/external-monitoring.png" />
<p>When using <dfn>external monitoring</dfn>, Ardour plays no role in
  monitoring at all. Perhaps the recording set-up has an external mixer which
  can be used to set up monitor mixes, or perhaps the sound-card being used
  has a "listen to the input" feature. This approach yields zero or near-zero
  latency.  On the other hand it requires external hardware, and the monitoring
  settings are less flexible and not saved with the session.</p>

<h3>JACK-Based Hardware Monitoring</h3>
<img class="right" src="/images/jack-monitoring.png" />
<p>Some sound cards have the ability
  to mix signals from their inputs to their outputs with very low or even zero
  latency, a feature called <dfn>hardware monitoring</dfn>.
  Furthermore, on some cards this function can be controlled by JACK.  This is a nice arrangement,
  if the sound card supports it, as it combines the convenience of having the
  monitoring controlled by Ardour with the low latency operation of doing it
  externally.
</p>

<h3>Software Monitoring</h3>
<img class="right" src="/images/ardour-monitoring.png" />
<p>With the <dfn>software monitoring</dfn> approach, all monitoring is
  performed by Ardour&mdash;it makes track inputs available at track
  outputs, governed by various controls. This approach will almost always have
  more routing flexibility than JACK-based monitoring. The disadvantage is
  that there will be some latency between the input and the output, which
  depends for the most part on the JACK buffer size that is being used.
</p>