manual/include/stretching-regions.html

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<p>
The Stretch Mode tool can be switched to by selecting it in the
<a href="@@toolbox">Toolbox</a>, or simply by hitting the <kbd class="menu">T</kbd>
key.
</p>
<p>
It allows to extend or reduce the duration of a region, optionally maintaining
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its pitch. This is one of the few operations in Ardour that affect the underlying
audio data from a region, even if the original audio is kept safely&mdash;no data
is lost in the process.
</p>
<p>
This operation is usually used to fit an audio sequence with a different rhythm
into a session, but can be used in a wide area of cases, due to its ability to
maintain or alter the pitch.
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</p>
<figure>
<img src="/images/before-stretch.png" alt="region arrangement before the stretch" />
<img src="/images/while-stretch.png" alt="region arrangement while stretching" />
<img src="/images/after-stretch.png" alt="region arrangement after the stretch" />
<figcaption>
Using the Stretch Mode tool. Before stretching, while stretching, and after a stretch
</figcaption>
</figure>
<p>
The Stretch Mode tool is very similar in use to doing a trim in grab mode: the
boundary (start or end) is <kbd class="mouse">left</kbd>-clicked and dragged to
its wanted position. Notice a timer appearing, showing the new duration of the
region using the same <a href="@@editing-clocks">clock mode</a> as in the
<a href="@@transport-clocks">primary transport clock</a>.
</p>
<p>
Stretching is a complex operation (phase vocoding), involving resampling,
frequency analysis and synthesis. The parameters used to transform the audio
data are user tweakable, and exposed to the user as the <kbd class="mouse">
left</kbd> mouse button is released:
</p>
<figure>
<img src="/images/time-stretch-audio.png" alt="The Time Stretch Audio window" />
<figcaption>
The Time Stretch Audio window
</figcaption>
</figure>
<p>
The Time Stretch Audio window is made of:
</p>
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<table class="dl">
<tr><th><dfn>Duration</dfn></th><td>The target duration of the region, expressed using the
primary transport clock's mode</td></tr>
<tr><th><dfn>Percent</dfn></th><td>The target duration of the region, expressed as a percentage
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of the region's original length. Can be either higher than 100&#37; (to
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expand the region) or lower (to shrink it)</td></tr>
<tr><th><dfn>Contents</dfn></th><td>The type of audio the region is made of. Ardour will fine-tune
its algorithm based on this content, see below</td></tr>
<tr><th><dfn>Minimize time distortion</dfn></th><td>Tries to reduce the smearing
of the audio created by the phase vocoding process</td></tr>
<tr><th>a <dfn>Progress</dfn> bar</th><td>showing the operation in progress.</td></tr>
</table>
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<p>
The <em>Contents</em> should be selected to best fit the actual content of the region, amongst:
</p>
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<table style="text-align:center;">
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<tbody>
<tr> <td>Content</td>
<td>Disable phase resynchronisation at transients</td>
<td>Band-limit phase resync to extreme frequencies</td>
<td>Disable phase locking to peak frequencies</td>
<td>Use longer processing window (actual size may vary)</td>
<td>Use shorter processing window</td> </tr>
<tr><td><dfn>Mushy</dfn> </td><td>X</td><td></td><td>X</td><td>X</td><td></td></tr>
<tr><td><dfn>Smooth</dfn> </td><td>X</td><td></td><td>X</td><td></td><td></td></tr>
<tr><td><dfn>Balanced multitimbral mixture</dfn> </td><td>X</td><td></td><td></td><td></td><td></td></tr>
<tr><td><dfn>Unpitched percussion with stable notes</dfn> </td><td></td><td>X</td><td></td><td></td><td></td></tr>
<tr><td><dfn>Crisp monophonic instrumental</dfn> (<em>default</em>)</td><td></td><td></td><td></td><td></td><td></td></tr>
<tr><td><dfn>Unpitched solo percussion</dfn> </td><td></td><td></td><td>X</td><td></td><td>X</td></tr>
<tr><td><dfn>Resample without preserving pitch</dfn> </td><td colspan="5"><em>see below</em></td></tr>
</tbody>
</table>
<p>
While the table above details <em>how</em> the different kinds of audio material
alter the fine-tuning of the DSP, from an user point of view, the operation often
consists in trying different settings and listening to the result.
</p>
<p>
The best way to start experimenting is to consider the material itself:
</p>
<ul>
<li>If the material doesn't need its pitch to be preserved, the best choice is
<em>Resample without preserving pitch</em></li>
<li>For drum-type material, the best choice is (depending on the transients crispness,
stretching factor...) one of the two <em>percussion</em> types</li>
<li>For melodic mono-tonal material (bass, winds,&hellip;), the best (and default) choice is <em>Crisp
monophonic instrumental</em></li>
<li>For multi-tonal material (chords,&hellip;), either one of the three first choice, or the default
<em>Crisp</em>.</li>
</ul>