manual/include/audiomidi-busses-mixer-strips.html

<figure class="right">
    <img src="/images/bus-mixer-strip.png" alt="A bus mixer strip">
</figure>

<p>
  An Ardour bus can be considered a virtual track, as in a track that doesn't
  have a playlist (so, no regions). Its use is to "group" some audio signals to
  be treated the same way. One simple use case is to group all the audio tracks
  containing the different drums of a drumkit. Routing all the drums tracks
  outputs to a bus allows, once the different levels amongst the drums have been
  set, to adjust the global level of the drumkit in the mix.
</p>
<p>
  Bus usage goes way beyond this simple example though: busses, as tracks, can
  receive plugins for common audio treatment, and be routed themselves as needed.
  This makes for a very useful tool that is very commonly used both for musical
  purposes and computing ones: instead of using e.g. ten discrete delay plugins on
  ten different tracks, busses are often used as receivers of <a
  href="@@aux-sends">sends</a>, and only one delay plugin is used on this bus,
  reducing the processing power needed.
</p>

<h2>Audio Busses vs MIDI Busses</h3>

<p>
	Ardour supports two types of busses: Audio and MIDI. A MIDI bus differs from an
  audio bus just by:
<p>
<ul>
  <li>its input (which is midi, as shown by the red signal lines in the processor box) instead of <em>n</em> audio</li>
  <li>the fact that an instrument can be placed on it at creation time, whereas it can't easily be done for an audio bus</li>
  <li>as for tracks, the MIDI bus doesn't have a trim knob or invert phase button(s).</li>
</ul>
<p>
  MIDI busses provide a particularly efficient workflow for virtual drumkits where
  the arrangement uses different MIDI tracks. Moreover, busses with both Audio and
  MIDI inputs are well suited for vocoders and similar plugins, where a MIDI
  signal controls an audio one.
</p>
<p class="note">
  Adding any audio input to a MIDI bus transforms it into an audio bus.
</p>

<h2>Description</h2>
<p>
  Busses look and behave exactly like tracks, so they share nearly <a href="@@audiomidi-mixer-strips">
  all of their controls</a>. The differences are:
</p>
<ul>
  <li>as the busses dont have a playlist (and cannot host any media), they can't be recorded on. The recording controls are not present</li>
  <li>an <kbd class="menu">Aux</kbd> button replaces these controls.</li>
</ul>
<p>
  Clicking the <kbd class="menu">Aux</kbd> button makes every track that sends a
  signal to this bus through <a href="@@aux-sends">Aux sends</a> blink in
  turquoise. Right clicking this button brings up a menu:
</p>
<table class="dl">
  <tr><th>Assign all tracks (prefader)</th><td>Creates an Aux Send in every track, to this bus. The send is placed just before the fader</td></tr>
  <tr><th>Assign all tracks and busses (prefader)</th><td>Creates an Aux Send in every track and every bus, to this bus. The send is placed just before the fader</td></tr>
  <tr><th>Assign all tracks (postfader)</th><td>Same as above, but the send is placed just after the fader</td></tr>
  <tr><th>Assign all tracks and busses (postfader)</th><td>Same as above, with tracks and busses</td></tr>
  <tr><th>Assign selected tracks (prefader)</th><td>Same as for all tracks, but only applies to the selected tracks</td></tr>
  <tr><th>Assign selected tracks and busses (prefader)</th><td>Same as for all tracks and busses, but only applies to the selected tracks and busses</td></tr>
  <tr><th>Assign selected tracks (postfader)</th><td>Same as above, but the send is placed just after the fader</td></tr>
  <tr><th>Assign selected tracks and busses (postfader)</th><td>Same as above, with tracks and busses</td></tr>
  <tr><th>Set sends gain to -inf</th><td>For all the sends to this bus, put the send fader to &minus;&infin; so no signal is sent</td></tr>
  <tr><th>Set sends gain to 0dB</th><td>For all the sends to this bus, put the send fader at the default position, 0dB (100&percnt; of th signal is sent)</td></tr>
</table>

<h2>Connecting a track to a bus</h2>
<p>
  Depending on the user's workflow and the way busses are used, two possibilities exists:
</p>

<h3>Connecting a track to a bus via its outputs</h3>

<img class="left" src="/images/connecting_bus_output.png" alt="Connecting a bus through a track's outputs">
<p>
	Connecting the output(s) of a track to the input(s) of the bus sends
	<em>all</em> the audio/MIDI to the bus. In the mixer strip, select (at the
	bottom) the OUTPUT button (often, by default, "Master"), and in the list,
	choose the input of a bus. Note that only the bus able to receive this output
	will show up, e.g. a mono bus won't be able to be connected to the output of a
	stereo track).
</p>
<p>
	Obviously, doing so will (by default) disconnect the output from the Master's
  input, which means all the audio/MIDI will be routed to the bus. For more
  complex routing, the OUTPUT button allows to show the <kbd class="menu">Routing
  Grid</kbd> that allows to plug the output of the track to multiple outputs at
  once, be it busses, tracks, Master&hellip; The button will then reflect these
  multiple connections by showing a <em>*number*</em>, number being the number of
  connections made in the routing grid.
</p>

<h3>Connecting a track to a bus via Sends</h3>

<img class="left" src="/images/connecting_bus_send.png" alt="Connecting a bus through a send">
<p>
	This allows not to interrupt the natural flow of the signal, i.e. the track will
  still output to what its connected to (e.g. Master). The signal is "tapped" at
  the point of insertion of the send, to be sent to the bus, by <kbd class="mouse">right</kbd>
  clicking where in the signal flow the signal should be tapped, and selecting <kdb class="menu">
  New Aux Send&hellip; &gt; name_of_the_bus</kbd>.
</p>
<p>
  By <kbd class="mouse">left-clicking</kbd> the send meter, it is possible to
  adjust the amount of signal sent to the bus. This is often the way tracks are
  connected to an effect bus, like a Delay bus.
</p>

<p class="note">
  Busses can be plugged to other busses, through outputs or sends. Both example
  workflows discussed previously, i.e. busses for grouping tracks and busses for
  effects, can both coexist, as e.g. a "grouping" drum bus can have a send to a
  reverb bus, and be connected to a compressor bus.
</p>