293 lines
11 KiB
HTML
293 lines
11 KiB
HTML
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<p>
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Ardour supports three common timecode formats:
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<abbr title="Linear/Longitudinal Time Code"><dfn>LTC</dfn></abbr>,
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<abbr title="MIDI Time Code"><dfn>MTC</dfn></abbr>, and
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<dfn>MIDI Clock</dfn>, as well as
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<dfn>JACK-transport</dfn>, a JACK-specific timecode implementation.
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</p>
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<p>
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Ardour can generate timecode and thus act as timecode <dfn>master</dfn>,
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providing timecode information to other applications. Ardour can also be
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<dfn>slaved</dfn> to some external source in which case the playhead
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follows the incoming timecode.<br>
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Combining the timecode slave and generator modes, Ardour can also
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<dfn>translate</dfn> timecode. e.g create LTC timecode from incoming MTC.
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</p>
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<h2>Ardour Timecode Configuration</h2>
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<p>
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Each Ardour session has a specific timecode frames-per-second setting which
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is configured in <kbd class="menu">session > properties >
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timecode</kbd>. The selected timecode affects the timecode ruler in the main
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window as well as the clock itself.
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</p>
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<p>
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Note that some timecode formats do not support all of Ardour's available
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fps settings. MTC is limited to 24, 25, 29.97 and 30 fps.
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</p>
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<p>
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The video pull-up modes change the effective samplerate of Ardour to allow
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for changing a film soundtrack from one frame rate to another. The concept is
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beyond the scope of this manual, but Wikipedia's entry on
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<a href="http://en.wikipedia.org/wiki/Telecine">Telecine</a>
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may be a good start.
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</p>
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<h2>Ardour Timecode Generator Configuration</h2>
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<p>
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This is pretty straightforward: simply turn it on. The MTC and MIDI-Clock
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generator do not have any options. The LTC generator has a configurable
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output level. JACK-transport cannot be <em>generated</em>. Jack itself is
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always synced to its own cycle and cannot do varispeed—it will
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always be synced to a hardware clock or another JACK master.
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</p>
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<p>
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The relevant settings for timecode generator can be found in
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<kbd class="menu">Edit > Preferences > MIDI Preferences</kbd> (for MTC,
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MC) and
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<kbd class="menu">Edit > Preferences > Transport Preferences</kbd>
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(for LTC).
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</p>
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<p>
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The timecode is sent to jack-ports <code>ardour:MTC out</code>,
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<code>ardour:MIDI clock out</code> and <code>ardour:LTC-out</code>. Multiple
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generators can be active simultaneously.
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</p>
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<p class="note">
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Note that, as of Jan 2014, only the LTC generator supports latency
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compensation. This is due to the fact the Ardour MIDI ports are not
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yet latency compensated.
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</p>
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<p>
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In <kbd class="menu">Session > Properties</kbd>, it is possible to
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define an offset between Ardour's internal time and the timecode sent.
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Currently only the LTC generator honors this offset.
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</p>
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<p>
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Both LTC and MTC are limited to 30 fps. Using frame rates larger
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than that will disable the generator. In both cases also only 24, 25,
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29.97df (drop-frame) and 30 fps are well defined by specifications (such as
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SMPTE-12M, EU and the MIDI standard).
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</p>
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<h3>MTC Generator</h3>
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<p>
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The <dfn>MTC generator</dfn> has no options. Ardour sends full MTC
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frames whenever the transport is relocated or changes state (start/stop).
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MTC <dfn>quarter frames</dfn> are sent when the transport is rolling and
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the transport speed is within 93% and 107%.
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</p>
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<h3>LTC Generator</h3>
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<p>
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The level of the <dfn>LTC generator</dfn> output signal can be configured
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in in the <kbd class="menu">Preferences > Transport</kbd> dialog. By
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default it is set to -18 dBFS, which corresponds to 0dBu in an EBU
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calibrated system.
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</p>
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<p>
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The LTC generator has an additional option to keep sending timecode even
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when the transport is stopped. This mode is intended to drive analog tape
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machines which unspool the tape if no LTC timecode is received.
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</p>
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<p>
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LTC is sent regardless of Ardour's transport speed. It is accurately
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generated even for very slow speeds (<5%) and only limited by the
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soundcard's sampling-rate and filter (see
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<a
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href="http://en.wikipedia.org/wiki/Gibbs_phenomenon#Signal_processing_explanation">Gibbs phenomenon</a>)
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for high speeds.
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</p>
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<h2>Ardour Slave Configuration</h2>
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<p>
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The timecode source can be switched with the button just right of
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Ardour's main clock. By default it is set to <kbd
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class="menu">Internal</kbd> in which case Ardour will ignore any external
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timecode. The button allows to toggle between Internal and the configured
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timecode source which is chosen in <kbd class="menu">Edit > Preferences
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> Transport</kbd>.
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</p>
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<p>
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When Ardour is <dfn>chasing</dfn> (synchronizing to) an external timecode
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source, the following cases need to be distinguished:
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</p>
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<ol>
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<li>the timecode source shares the clock</li>
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<li>the timecode source is independent (no wordclock sync)</li>
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</ol>
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<p>and</p>
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<ol>
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<li>the timecode source uses the same FPS setting as Ardour</li>
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<li>the timecode source runs at different frames-per-second</li>
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</ol>
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<p>
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In both cases the first option is preferred: clock sync + same FPS setting.
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</p>
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<h3>Frames-per-second</h3>
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<p>
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If the frames-per-second do not match, Ardour can either re-calculate
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and map the frames, or the configured FPS (<kbd class="menu">Session >
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Properties</kbd>) can be changed automatically while the slave is active.
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The behavior is configured with the checkbox <kbd class="option">Edit
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> Preferences > Transport > Match session video frame rate to
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external timecode</kbd>.
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</p>
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<p>
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When enabled, the session video frame rate will be changed to match that
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of the selected external timecode source. When disabled, the session video
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frame rate will not be changed to match that of the selected external
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timecode source. Instead the frame rate indication in the main clock will
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flash red, and Ardour will convert between the external timecode standard
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and the session standard.
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</p>
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<p class="warning">
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29.97 drop-frame timecode is another corner case. While the SMPTE 12M-1999
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specifies 29.97df as 30000/1001 frames per second, not all hardware devices
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follow that standard. The checkbox
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<kbd class="option">Lock to 29.9700 fps instead of 30000/1001</kbd> allows
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to use a compatibility mode for those devices.<br>
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When enabled, the external timecode source is assumed to use 29.970000 fps
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instead of 30000/1001. SMPTE 12M-1999 specifies 29.97df as 30000/1001. The
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<abbr title="specification">spec</abbr> further mentions that drop-frame
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timecode has an accumulated error of -86 ms over a 24-hour period.
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Drop-frame timecode would compensate exactly for a NTSC color frame rate
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of 30 * 0.9990 (ie 29.970000). That is <em>not</em> the actual rate. However,
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some vendors use that rate—despite it being against the
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specs—because the variant of using exactly 29.97 fps yields zero
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timecode drift.
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</p>
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<h3>Clock Sync Lock</h3>
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<p>
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As described in the
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<a href="@@on-clock-and-time">On Clock and Time</a>
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chapter, timecode and clock are independent. If the external timecode
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source is not in sample-sync with the audio hardware (and JACK), Ardour
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needs to run at varispeed to adjust for the discrepancy.
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</p>
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<p>
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The checkbox <kbd class="option">External timecode is sync locked</kbd>
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allows to select the behavior according to the setup. When enabled, it
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indicates that the selected external timecode source shares sync (Black
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& Burst, Wordclock, etc) with the audio interface.
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</p>
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<p>
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In other words: if enabled, Ardour will only perform initial
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synchronization and keep playing at speed 1.0 instead of vari-speed
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adjusting to compensate for drift.
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</p>
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<p class="note">
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Note that vari-speed is unavailable when recording in Ardour, and all
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tracking happens at speed 1.0. So in order to record in sync with
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external timecode it must be sample-locked or it will drift over time.
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</p>
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<h3>MIDI Clock</h3>
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<p>
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<dfn>MIDI Clock</dfn> is not a timecode format but tempo-based time. The
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absolute reference point is expressed as beats-per-minute and Bar, Beat
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and Tick. There is no concept of sample-locking for MIDI clock signals.
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Ardour will vari-speed if necessary to chase the incoming signal.
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</p>
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<p>
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Note that the MIDI Clock source must be connected to the
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<code>ardour:MIDI clock in</code> port.
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</p>
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<h3>LTC—Linear Timecode</h3>
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<p>
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The <dfn>LTC</dfn> slave decodes an incoming LTC signal on a JACK audio
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port. It will auto-detect the frame rate and start locking to the signal
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once two consecutive LTC frames have been received.
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</p>
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<p>
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The incoming timecode signal needs to arrive at the
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<code>ardour:LTC-in</code> port. Port-connections are restored for each
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session and the preference dialog offers an option to select it for all
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sessions.
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</p>
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<p>
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Ardour's transport is aligned to LTC-frame start/end positions according
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to the SMPTE 12M-1999 specification, which means that the first bit of an
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LTC-Frame is aligned to different Lines of a Video-Frame, depending on the
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TV standard used. Only for Film (24fps) does the LTC-Frame directly match
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the video Frame boundaries.
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</p>
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<figure class="center">
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<img src="/images/ltc-transport-alignment.png" alt="LTC frame alignment">
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<figcaption>
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LTC frame alignment for the 525/60 TV standard
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</figcaption>
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</figure>
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<p>
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Ardour supports vari-speed and backwards playback but will only follow
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speed changes if the <kbd class="optoff">sync locked</kbd> option is
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disabled.
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</p>
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<p>
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While Ardour is chasing LTC, the main transport clock will display the
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received Timecode as well as the delta between the incoming signal and
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Ardour's transport position.
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</p>
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<p>
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A global offset between incoming timecode and Ardour's transport can be
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configured in <kbd class="menu">Session > Properties</kbd>.
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</p>
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<p>
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The user-bits in the received LTC frame are ignored.
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</p>
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<h3>MTC—MIDI Timecode</h3>
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<p>
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Ardour's MTC slave parses <dfn>full timecode messages</dfn> as well as
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MTC <dfn>quarter-frame messages</dfn> arriving on the
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<code>ardour:MTC in</code> port. The transport will only start rolling
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once a complete sequence of 8 quarter frames has been received.
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</p>
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<p>
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Ardour supports vari-speed and backwards playback but will only follow
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MTC speed changes if the <kbd class="optoff">sync locked</kbd> option
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is disabled.
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</p>
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<p>
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When Ardour is chasing MTC, the main transport clock will display the
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received Timecode as well as the delta between the incoming signal and
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Ardour's transport position.
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</p>
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<h3>JACK Transport</h3>
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<p>
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When slaved to jack, Ardour's transport will be identical to
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JACK-transport. As opposed to other slaves, Ardour can be used to control
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the JACK transport states (stopped/rolling). No port connections need to
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be made for jack-transport to work.
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</p>
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<p>
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JACK-transport does not support vari-speed, nor offsets. Ardour does not
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chase the timecode but is always in perfect sample-sync with it.
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</p>
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<p>
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JACK-transport also includes temp-based-time information in Bar:Beats:Ticks
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and beats-per-minute. However, only one JACK application can provide this
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information at a given time. The checkbox
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<kbd class="option">Session > Properties > JACK Time Master</kbd>
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configures Ardour to act as translator from timecode to BBT information.
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</p>
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