manual/include/trackbus-signal-flow.html

<h2>Overview</h2>

<figure class="right">
  <img src="/images/track_signal_routing.png" alt="track signal routing">
  <figcaption>Typical signal routing in a channel strip.</figcaption>
</figure>

<p>
  In each individual Track or Bus the signal flow is top to bottom, as shown in
  the diagram on the right.
</p>

<p>
  Trim, Fader and Panner are provided by Ardour. The Processor Box can hold third
  party plugins or host-provided redirects (insert, aux-send, etc.).
</p>

<p>
  An important aspect is that the signal flow is multi-channel and not fixed
  throughout the track. For example, a track can have a mono input, a mono to
  stereo plugin (e.g. reverb) flowing into a surround panner with 6 outputs.
</p>
<p>
  The design of Ardour is that the width of the signal flow is defined by the
  passage through plugins in the processor box, followed by panning. The number
  of inputs to the panner is defined by the number of outputs of the last plugin
  in the chain. The number of panner outputs is equal to the track's outputs
  ports, which can be added and removed dynamically. This schema is called
  <em>Flexible I/O</em>. It is very powerful and a distinctive feature of
  Ardour.
</p>

<p class="note">
  The golden rule of processor signal flow: The number of outputs of one link
  of the process chain defines the number of inputs of the next, until the panner.
</p>

<p>
  Due to this rule there is one very common case that is hard to achieve:
  keeping a mono track mono. With <em>Flexible I/O</em>, if a stereo plugin is
  added on a mono track, the signal flow after that plugin becomes stereo.
</p>

<h2>Strict I/O</h2>

<p>
  Strict I/O enforces a simple rule: plugins have the same number of inputs as
  they have outputs. By induction the track will have as many output ports as
  there are input ports.
</p>

<p>
  Adding a plugin will not modify the signal flow. The number of plugin outputs
  is forced to the number of inputs present at the point of insertion. If a
  plugin pin is missing, it is ignored. If a plugin pin is not connected, it is
  fed with silence. Non-connected plugin outputs are ignored.
</p>

<p>
  Strict I/O enforces the number of output ports. The number of inputs to the
  panner (outputs of last plugin) defines the number of track outputs (after
  panner). Required ports are automatically added, excess ports are removed.
  The user cannot manually add or remove output ports.
</p>

<p>
  Strict I/O is set when creating the track and can later be enabled or
  disabled dynamically in the context menu of every mixer strip.
</p>

<figure class="center">
<img src="/images/strict_io_routing.png" alt="strict I/O routing" />
<figcaption class=center>Flexible vs. Strict I/O.</figcaption>
</figure>

<p>
  There are two exceptions to the above rule:
</p>

<ul>
  <li>Midi Synths. When adding a synth at a point where there is a Midi port
  only, the synthesizer plugin will add audio output ports, which trickle down
  the processor chain to all follow up plugins as inputs and in turn force
  their outputs to match</li>
  <li>Side chain inputs are not affected by Strict I/O</li>
</ul>

<h2>Customizing the Signal Flow: The Pin Connection window</h2>

<p>
  The signal flow though the mixer can be customized at every processor node
  via <kbd class=menu>Pin Configuration</kbd> in the context menu of every
  processor. User customization overrides all automatic (Flexible and Strict
  I/O mode) inferred output port settings.
</p>

<p>
  The Pin Connection window is made of three vertical sections:
</p>

<ul>
  <li>an I/O config column</li>
  <li>an interactive diagram</li>
  <li>a sidechain column</li>
</ul>

<p>
  By default, the I/O config is set to <em>Automatic</em>, i.e. the <kbd
  class="menu">Manual Config</kbd> LED light is turned off. In this mode, the
  diagram will display the standard input/outputs for this plugin, i.e. the
  number of ports (inputs &amp; outputs) is equal to the number of pins on the
  plugin, and a one-to-one connection is automatically created.
</p>

<p>
  Adding new instances of the plugin allows to apply this plugin to more inputs
  or outputs. E.g., a mono effect can be applied to each channel of a
  <em>n</em>-channels track by adding as many instances of the plugins as there
  are input channels (i.e. ports). This happens automatically when adding,
  e.g., a mono effect to a stereo track:
</p>

<ul>
  <li>Ardour creates two instances of the plugin</li>
  <li>the plugin gets a (2x1) label in the processor box</li>
  <li>its two input ports are each connected to one pin of an instance</li>
  <li>each mono output pin of the plugin is connected to one output port</li>
</ul>

<p>
  Output channels can also, in Manual Config mode, be added or removed, whether
  they are audio or MIDI.
</p>

<p class="note">
  Using the Pin Connection overrides the I/O config setting (Flexible vs.
  Strict). A processor <em>can</em>, even in Strict I/O mode, have a different
  number of outputs than inputs. Non-customized plugins downstream will follow
  suit depending on the selected route mode. e.g. adding an additional output
  to a plugin on a track set to Strict I/O will trickle down the process chain
  to the output and result in the addition of an output port. This is useful
  for example in case of a mono to stereo reverb.
</p>

<p>
  The window allows connection of the I/O ports to the plugin pins and other
  I/O ports, provided they are compatible (MIDI vs. audio), just by dragging
  and dropping the end connectors on top of one another. A dotted connector's
  line is a "<em>thru</em>" line that directly connects an input to an output
  without connecting to a pin on the plugin&mdash;hence without any audio
  modification. These "thru" connections are latency compensated, with respect
  to those being affected by the plugin, in order to avoid phasing issues.
</p>

<p>
  An example of using "thru" connections, shown below, is separate left/right
  channel equalization using two mono plugins on a stereo track:
</p>

<figure class=center>
<img src="/images/left_right_eq.png" alt="Separate left/right Eq">
<figcaption class=center>An example of using two mono plugins on a stereo track.</figcaption>
</figure>

<p>
  The only way to add inputs to a processor is via
  <a href="@@sidechaining">Sidechaining</a> from another signal.
  This is done by "tapping" the signal from another track or bus at any point.
</p>

<p>
  Adding a sidechain signal in Ardour is as simple as enabling the <kbd
  class="menu">Side Chain</kbd> button in the Pin Configuration window, and
  choosing an <kbd class="menu">Audio</kbd> or <kbd class="menu">MIDI</kbd>
  sidechain in the <kbd class="menu">Add Sidechain Input</kbd> lower right hand
  section. A new dropdown menu appears, which displays a list of the
  tracks/busses available to be sidechained, or, for a more complex setup (e.g.
  sidechaining from hardware directly), the
  <a href="@@patchbay">Routing Grid</a> (also accessible with a
  <kbd class="mouse">Right</kbd>-click on the dropdown menu).
</p>

<p>
  The sidechain ports can then be connected, as other inputs, to a pin of the
  plugin, or an output port as a "thru".
</p>