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[ardour-manual] / include / timecode-generators-and-slaves.html

<p>
  Ardour supports three common timecode formats:
  <abbr title="Linear/Longitudinal Time Code"><dfn>LTC</dfn></abbr>,
  <abbr title="MIDI Time Code"><dfn>MTC</dfn></abbr>, and
  <dfn>MIDI Clock</dfn>, as well as
  <dfn>JACK-transport</dfn>, a JACK-specific timecode implementation.
</p>
<p>
  Ardour can generate timecode and thus act as timecode <dfn>master</dfn>,
  providing timecode information to other applications. Ardour can also be
  <dfn>slaved</dfn> to some external source in which case the playhead
  follows the incoming timecode.<br>
  Combining the timecode slave and generator modes, Ardour can also
  <dfn>translate</dfn> timecode. e.g create LTC timecode from incoming MTC.
</p>


<h2>Ardour Timecode Configuration</h2>

<p>
  Each Ardour session has a specific timecode frames-per-second setting which
  is configured in <kbd class="menu">session &gt; properties &gt;
  timecode</kbd>. The selected timecode affects the timecode ruler in the main
  window as well as the clock itself.
</p>

<p>
  Note that some timecode formats do not support all of Ardour's available
  fps settings. MTC is limited to 24, 25, 29.97 and 30 fps.
</p>

<p>
  The video pull-up modes change the effective samplerate of Ardour to allow
  for changing a film soundtrack from one frame rate to another. The concept is
  beyond the scope of this manual, but Wikipedia's entry on
  <a href="http://en.wikipedia.org/wiki/Telecine">Telecine</a>
  may be a good start.
</p>

<h2>Ardour Timecode Generator Configuration</h2>

<p>
  This is pretty straightforward: simply turn it on. The MTC and MIDI-Clock
  generator do not have any options. The LTC generator has a configurable
  output level. JACK-transport cannot be <em>generated</em>. Jack itself is
  always synced to its own cycle and cannot do varispeed&mdash;it will
  always be synced to a hardware clock or another JACK master.
</p>

<p>
  The relevant settings for timecode generator can be found in
  <kbd class="menu">Edit &gt; Preferences &gt; MIDI Preferences</kbd> (for MTC,
  MC) and
  <kbd class="menu">Edit &gt; Preferences &gt; Transport Preferences</kbd>
  (for LTC).
</p>

<p>
  The timecode is sent to jack-ports <code>ardour:MTC out</code>,
  <code>ardour:MIDI clock out</code> and <code>ardour:LTC-out</code>. Multiple
  generators can be active simultaneously.
</p>

<p class="note">
  Note that, as of Jan 2014, only the LTC generator supports latency
  compensation. This is due to the fact the Ardour MIDI ports are not
  yet latency compensated.
</p>

<p>
  In <kbd class="menu">Session &gt; Properties</kbd>, it is possible to
  define an offset between Ardour's internal time and the timecode sent.
  Currently only the LTC generator honors this offset.
</p>

<p>
  Both LTC and MTC are limited to 30&nbsp;fps. Using frame rates larger
  than that will disable the generator. In both cases also only 24, 25,
  29.97df (drop-frame) and 30&nbsp;fps are well defined by specifications (such as
  SMPTE-12M, EU and the MIDI standard).
</p>

<h3>MTC Generator</h3>
<p>
  The <dfn>MTC generator</dfn> has no options. Ardour sends full MTC
  frames whenever the transport is relocated or changes state (start/stop).
  MTC <dfn>quarter frames</dfn> are sent when the transport is rolling and
  the transport speed is within 93% and 107%.
</p>

<h3>LTC Generator</h3>
<p>
  The level of the <dfn>LTC generator</dfn> output signal can be configured
  in in the <kbd class="menu">Preferences &gt; Transport</kbd> dialog. By
  default it is set to -18&nbsp;dBFS, which corresponds to 0dBu in an EBU
  calibrated system.
</p>
<p>
  The LTC generator has an additional option to keep sending timecode even
  when the transport is stopped. This mode is intended to drive analog tape
  machines which unspool the tape if no LTC timecode is received.
</p>
<p>
  LTC is sent regardless of Ardour's transport speed. It is accurately
  generated even for very slow speeds (&lt;5%) and only limited by the
  soundcard's sampling-rate and filter (see
  <a
  href="http://en.wikipedia.org/wiki/Gibbs_phenomenon#Signal_processing_explanation">Gibbs phenomenon</a>)
  for high speeds.
</p>


<h2>Ardour Slave Configuration</h2>
<p>
  The timecode source can be switched with the button just right of
  Ardour's main clock. By default it is set to <kbd
  class="menu">Internal</kbd> in which case Ardour will ignore any external
  timecode. The button allows to toggle between Internal and the configured
  timecode source which is chosen in <kbd class="menu">Edit &gt; Preferences
  &gt; Transport</kbd>.
</p>
<p>
  When Ardour is <dfn>chasing</dfn> (synchronizing to) an external timecode
  source, the following cases need to be distinguished:
</p>
<ol>
  <li>the timecode source shares the clock</li>
  <li>the timecode source is independent (no wordclock sync)</li>
</ol>
<p>and</p>
<ol>
  <li>the timecode source uses the same FPS setting as Ardour</li>
  <li>the timecode source runs at different frames-per-second</li>
</ol>
<p>
  In both cases the first option is preferred: clock sync + same FPS setting.
</p>

<h3>Frames-per-second</h3>
<p>
  If the frames-per-second do not match, Ardour can either re-calculate
  and map the frames, or the configured FPS (<kbd class="menu">Session &gt;
  Properties</kbd>) can be changed automatically while the slave is active.
  The behavior is configured with the checkbox <kbd class="option">Edit
  &gt; Preferences &gt; Transport &gt; Match session video frame rate to
  external timecode</kbd>.
</p>
<p>
  When enabled, the session video frame rate will be changed to match that
  of the selected external timecode source. When disabled, the session video
  frame rate will not be changed to match that of the selected external
  timecode source. Instead the frame rate indication in the main clock will
  flash red, and Ardour will convert between the external timecode standard
  and the session standard.
</p>
<p class="warning">
  29.97 drop-frame timecode is another corner case. While the SMPTE 12M-1999
  specifies 29.97df as 30000/1001 frames per second, not all hardware devices
  follow that standard. The checkbox
  <kbd class="option">Lock to 29.9700 fps instead of 30000/1001</kbd> allows
  to use a compatibility mode for those devices.<br>
  When enabled, the external timecode source is assumed to use 29.970000 fps
  instead of 30000/1001. SMPTE 12M-1999 specifies 29.97df as 30000/1001. The
  <abbr title="specification">spec</abbr> further mentions that drop-frame
  timecode has an accumulated error of -86&nbsp;ms over a 24-hour period.
  Drop-frame timecode would compensate exactly for a NTSC color frame rate
  of 30 * 0.9990 (ie 29.970000). That is <em>not</em> the actual rate. However,
  some vendors use that rate&mdash;despite it being against the
  specs&mdash;because the variant of using exactly 29.97 fps yields zero
  timecode drift.
</p>

<h3>Clock Sync Lock</h3>
<p>
  As described in the
  <a href="@@on-clock-and-time">On Clock and Time</a>
  chapter, timecode and clock are independent. If the external timecode
  source is not in sample-sync with the audio hardware (and JACK), Ardour
  needs to run at varispeed to adjust for the discrepancy.
</p>
<p>
  The checkbox <kbd class="option">External timecode is sync locked</kbd>
  allows to select the behavior according to the setup. When enabled, it
  indicates that the selected external timecode source shares sync (Black
  &amp; Burst, Wordclock, etc) with the audio interface.
</p>
<p>
  In other words: if enabled, Ardour will only perform initial
  synchronization and keep playing at speed 1.0 instead of vari-speed
  adjusting to compensate for drift.
</p>
<p class="note">
  Note that vari-speed is unavailable when recording in Ardour, and all
  tracking happens at speed 1.0. So in order to record in sync with
  external timecode it must be sample-locked or it will drift over time.
</p>

<h3>MIDI Clock</h3>
<p>
  <dfn>MIDI Clock</dfn> is not a timecode format but tempo-based time. The
  absolute reference point is expressed as beats-per-minute and Bar, Beat
  and Tick. There is no concept of sample-locking for MIDI clock signals.
  Ardour will vari-speed if necessary to chase the incoming signal.
</p>
<p>
  Note that the MIDI Clock source must be connected to the
  <code>ardour:MIDI clock in</code> port.
</p>

<h3>LTC&mdash;Linear Timecode</h3>
<p>
  The <dfn>LTC</dfn> slave decodes an incoming LTC signal on a JACK audio
  port. It will auto-detect the frame rate and start locking to the signal
   once two consecutive LTC frames have been received.
</p>
<p>
  The incoming timecode signal needs to arrive at the
  <code>ardour:LTC-in</code> port. Port-connections are restored for each
  session and the preference dialog offers an option to select it for all
  sessions.
</p>
<p>
  Ardour's transport is aligned to LTC-frame start/end positions according
  to the SMPTE 12M-1999 specification, which means that the first bit of an
  LTC-Frame is aligned to different Lines of a Video-Frame, depending on the
  TV standard used. Only for Film (24fps) does the LTC-Frame directly match
  the video Frame boundaries.
</p>

<figure class="center">
  <img src="/images/ltc-transport-alignment.png" alt="LTC frame alignment">
  <figcaption>
    LTC frame alignment for the 525/60 TV standard
  </figcaption>
</figure>

<p>
  Ardour supports vari-speed and backwards playback but will only follow
  speed changes if the <kbd class="optoff">sync locked</kbd> option is
  disabled.
</p>
<p>
  While Ardour is chasing LTC, the main transport clock will display the
  received Timecode as well as the delta between the incoming signal and
  Ardour's transport position.
</p>
<p>
  A global offset between incoming timecode and Ardour's transport can be
  configured in <kbd class="menu">Session &gt; Properties</kbd>.
</p>
<p>
  The user-bits in the received LTC frame are ignored.
</p>

<h3>MTC&mdash;MIDI Timecode</h3>
<p>
  Ardour's MTC slave parses <dfn>full timecode messages</dfn> as well as
  MTC <dfn>quarter-frame messages</dfn> arriving on the
  <code>ardour:MTC in</code> port. The transport will only start rolling
  once a complete sequence of 8 quarter frames has been received.
</p>
<p>
  Ardour supports vari-speed and backwards playback but will only follow
  MTC speed changes if the <kbd class="optoff">sync locked</kbd> option
  is disabled.
</p>
<p>
  When Ardour is chasing MTC, the main transport clock will display the
  received Timecode as well as the delta between the incoming signal and
  Ardour's transport position.
</p>

<h3>JACK Transport</h3>
<p>
  When slaved to jack, Ardour's transport will be identical to
  JACK-transport. As opposed to other slaves, Ardour can be used to control
  the JACK transport states (stopped/rolling). No port connections need to
  be made for jack-transport to work.
</p>
<p>
  JACK-transport does not support vari-speed, nor offsets. Ardour does not
  chase the timecode but is always in perfect sample-sync with it.
</p>
<p>
  JACK-transport also includes temp-based-time information in Bar:Beats:Ticks
  and beats-per-minute. However, only one JACK application can provide this
  information at a given time. The checkbox
  <kbd class="option">Session &gt; Properties &gt; JACK Time Master</kbd>
  configures Ardour to act as translator from timecode to BBT information.
</p>