manual/include/sidechaining.html

<p>
  Dynamic Processors &ndash; such as compressors &ndash; in general use the the original
  input signal for analysis and operate on the same signal.
  Side-chaining uses the signal level of <em>another input</em> to control
  the compression level of the original signal.
</p>
<p>
  Effect Processors which have a side-chain input (sometimes also called <em>key input</em>)
  have an additional input pin to receive a signal from an external input.
  In Ardour that extra input can be connected in the plugin's <kbd class="menu">Pin Configuration</kbd>
  dialog: the signal from one track can be tapped off and used as an input to a
  plugin on a different track. This dialog is
  accessed via the plugin's context-menu &gt; <kbd class="menu">Pin Connections&hellip;</kbd>.
</p>
<p>
  In case a plugin has a dedicated sidechain input, Ardour automatically
  creates a port for the input. This is a normal I/O port which can be fed by any
  external signal.
  The <kbd class="menu">Pin Configuration</kbd> dialog is not limited to
  processors with a dedicated sidechain input, it also allows to manually create
  (or remove) a sidechain input port and provides for flexible connection of the
  signal to plugin pins.
</p>
<p>
  The operational flow in the Ardour GUI starts at the processor which is to receive
  the signal: a sidechain source is selected, and Ardour creates a dedicated send-processor in the
  source processor box, the level of which can be adjusted either in the Pin Configuration window
  or directly on the source's send.
</p>

<h2>A simple example: Sidechain compression</h2>

<p>
  One example is the use of a bassdrum track to trigger
  the compression on a bass track. The sidechain compressor (a-Compressor) will be placed on
  the bass track, and will need to receive the signal from the bassdrum track as
  a way to trigger the compression.
</p>

<figure>
  <img src="/images/sc-comp-pin.png" alt="Sidechain compression: Pin configuration">
  <img src="/images/sc-comp-mixer.png" alt="Sidechain compression: Mixer view">
  <img src="/images/sc-comp-editor.png" alt="Sidechain compression: Editor view">
  <figcaption>
    A sidechain compression: Pin configuration, mixer view and editor view.
  </figcaption>
</figure>

<p>
  Here, on the bass track, an <em>a-Compressor</em> has been added, and the
  Drum track has been set as the sidechain source. The mixer reflects this
  by showing an <em>SC</em>-send processor in the drum track, very similar to a <a href="/signal-routing/aux-sends/">send</a>
  The bass track also shows an arrow as one of the a-compressor input.
</p>
<p>
  As a result, in the editor, each peak in the kick drum track triggers the compression on the bass track
  and the resulting track shows the compression kicking in on each kick drum peak, hence
  reducing the gain. The compression is applied to the bass, but only based on the level of the drum track.
</p>
<p>
  This is commonly used for <em>ducking</em> effect, when e.g. a radio speaker's voice triggers the
  compression on the audio playing.
</p>

<h2>MIDI Sidechaining</h2>

<p>
  Ardour allows the sidechain sources to be either audio or MIDI tracks/busses. This is
  particularly useful when a MIDI signal is used to control an audio effect, like a
  vocoder or an auto-tuner, like <a href="https://github.com/x42/fat1.lv2">fat1</a>,
  the LV2 port of Fons Adriaensen's <a href="http://kokkinizita.linuxaudio.org/linuxaudio/zita-at1-doc/quickguide.html">Zita AT1</a>
  by Robin Gareus:
</p>

<figure>
  <img src="/images/pin-connection-autotune.png" alt="MIDI Sidechain">
  <figcaption>
    MIDI sidechainig example: fat1.lv2.
  </figcaption>
</figure>

<p>
  Here, the MIDI track is inputed to the plugin's MIDI IN pin through a sidechain,
  indicating to the plugin what note should the source audio be corrected to.
</p>

<p class="note">
  Notice that in the example above, the output of the "Vocals" track is connected
  to the input of the "Corrected" track. We could have chosen to insert the "Vocals" track
  content as an audio sidechain too, totally disconnecting the input from the plugin, and
  connecting the plugin's input pin to the audio sidechain port.
</p>

<h2>Pre-processing the sidechained signal</h2>

<p>
  Sometimes, the effects of a sidechain signal on a plugin can be enhanced by
  pre-processing the signal.
</p>
<p>
  In the first example above, if the entire drum part is on one track,
  then compressing with this signal as a sidechain will result in every peak triggering the compression,
  be them bass drum kicks or snare, cymbals, etc.
</p>
<p>
  In this case, adding an EQ to the drum track with a low pass filter would filter out the peaks
  created by the high pitched instruments of the drum kit, and allow for a better triggering, though to
  avoid damaging the original drum track, a send to an intermediary track would be better suited
  to place the EQ on. This track won't be connected to the Master, as its content is of no musical
  interest except for it's use as a trigger, allowing for some extreme EQ.
</p>