manual/include/signal-routing.html

<h2>Routing for audio tracks</h2>

<p>
  Ardour exposes multiple ports for various parts of the signal chain to link
  those parts: track inputs and outputs, bus inputs and outputs, sends and
  inserts, monitor section outputs. When using the JACK audio backend, these
  ports are also accessible by other applications and can be routed externally.
</p>

<figure class="right">
  <img src="/images/general-signal-flow-audio.png" height="700px">
  <figcaption>General signal flow for audio</figcaption>
</figure>

<p>
  The chart on the right demonstrates a common signal flow for recording
  an instrument: a guitar is plugged into a front input of an audio interface,
  the signal then goes directly into the track output, passes the <a
  href="@@processor-box">processor box</a> with plugins, fader, and panner,
  connects to the input of the master bus, passes its processor box, the goes
  into the monitor section, then finally connects to physical outputs like
  studio monitors or headphones.
</p>

<p>
  This configuration can have multiple variations, such as:
</p>

<ul>
  <li>
    There can be a DI box sitting between the guitar and the front input,
    if the guitar has a passive pickup.
  </li>
  <li>
    The signal from a guitar can pass a DI box and feed into one track, but
    another path can go through preamp/amp/cabinet/mic and feed into another
    track, so that the musician has both processed sound and dry sound that can
    be re-amped later on.
  </li>
  <li>
    The same can be achieved by creating an input I/O plugin (a guitar amp/cab
    simulator), passing a copy of guitar's DI'ed signal through it and feeding
    the I/O plugin's output to another track.
  </li>
  <li>
    Monitoring could be done with hardware, so that there would be no monitor
    section, and thus the master bus would be connected to physical output ports
    directly.
  </li>
</ul>

<p>
  When Ardour creates multiple tracks and/or busses at once, this is what
  happens.
</p>

<ul>
  <li>
    <dfn>Track inputs</dfn> are optionally auto-connected to hardware inputs,
    in the round-robin order. In the example below where an audio interface
    only has two inputs and 8 new tracks have been created, Ardour connects
    the first input to the first track, then the second input to the second
    track, then the first input to the third track, and repeats it until all
    tracks have an input assigned for them. The exact configuration will
    depend on how many channels have been chosen for each new track in the
    <a href="@@newopen-session-dialog"><kbd
    class="menu">Session &gt; New Session</kbd> dialog</a>. 
    <figure>
      <img src="/images/round-robin-connections.png" width="60%">
      <figcaption>Round-robin assignment of connections</figcaption>
    </figure>
  </li>
  <li>
    <dfn>Bus inputs</dfn> are left disconnected.
  </li>
  <li>
    The number of <dfn>track and bus outputs</dfn> are equal to the number
    of inputs of the master bus.
  </li>
  <li>
    Track and bus outputs are always auto-connected to the master bus inputs.
  </li>
  <li>
    Master bus outputs are connected to hardware outputs if new session don't
    have a monitor section by default (this is set when the user runs Ardour
    for the first time and can be changed on the <kbd class="title">Monitoring
    Page</kbd> of the <kbd class="window">Preferences</kbd> dialog).
  </li>
</ul>
<p>
  This configuration is sufficient to do basic tracking and playback of many
  sessions without any adjustment by the user. Changing these connections
  is generally not necessary and often leads to problems.
</p>
<p>
  However, for many workflows during mixing, more complicated signal routing
  is required. Ardour offers many possibilities for connecting things to fit any
  particular workflow.
</p>

<h2>Routing for MIDI tracks</h2>

<p>
  Typical routing for MIDI tracks is very similar to that of audio tracks.
</p>

<figure class="right">
  <img src="/images/general-signal-flow-midi.png" height="700px">
  <figcaption>General signal flow for MIDI</figcaption>
</figure>

<p>
  A MIDI keyboard output goes into MIDI IN port of an audio interface, then
  MIDI events are transmitted over USB to a MIDI track where they are sent to
  a software synthesizer. The synthesizer plugin outputs two or more audio
  channels that are automatically connected to the master bus, and master bus
  outputs are connected to studio monitors or headphones.
</p>

<p>
  Notably, the processor box for MIDI tracks and busses always has a MIDI
  THROUGH port that carries a copy of all events coming through MIDI IN.
</p>

<p>
  There are also some variations here possible:
</p>

<ul>
  <li>
    The first plugin in the track can be a MIDI plugin that somehow transforms
    incoming events, e.g. transposes them by two octaves or builds arpeggios and
    <em>then</em> send the resulting notes to a software synthesizer or a
    sampler.
  </li>
  <li>
    The MIDI output from the audio interface can be connected to a MIDI bus with
    an arpeggiator that sends resulted MIDI events to a MIDI track for capturing
    and to a hardware synthesizer for playback.
  </li>
  <li>
    The MIDI keyboard can be also connected directly to a laptop or a desktop via
    a USB port.
  </li>
</ul>

<p>
  Ardour uses the same round-robin logic to connect MIDI ports to MIDI tracks
  when multiple MIDI tracks are created. However, when no MIDI device is
  connected, Ardour will connect the newest created track to its own internal
  virtual MIDI keyboard and keep the other MIDI tracks not connected.
</p>