Documenting the Rhythm Ferret

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Ed Ward 2017-02-18 19:49:59 +01:00
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<p class="fixme">Add content</p>
<figure>
<img src="/images/rhythm-ferret.png" alt="The Rhythm Ferret window">
<figcaption>
The Rhythm Ferret window
</figcaption>
</figure>
<p>
The Rhythm Ferret is a dedicated tool to speed up the usually labor intensive
task to slice and adjust a sound region to match a specific time grid. It is
especially usefull for drum tracks, either to match a different tempo, or to
adjust a slightly out of tempo performance.
</p>
<p>
It is not limited to this use though, as it supports both percussive and note
type detection, and can be used on melodic material too.
</p>
<h2>Accessing the Rhythm Ferret</h2>
<p>
The Rhythm Ferret window can be accessed by <kbd class="mouse">Right</kbd> clicking
any audio region, then <kbd class="menu"><em>Name_Of_The_Region</em> &gt; Edit
&gt; Rhythm Ferret</kbd>.
</p>
<p>
Once the window is open, selecting any region will make it the focus of the
Rhythm Ferret's detection, hence allowing to process multiple regions sequentially
without reopening the window each time.
</p>
<p>
The window itself is made of:
</p>
<ul>
<li>a "mode" selection</li>
<li>some parameters for this mode</li>
<li>an operation selection, that for now only allows to <kbd class="menu">Split
regions</kbd>.</li>
</ul>
<h2>The "Mode" selection</h2>
<p>
As the Rhythm Ferret is able to detect both percussive hits and melodic notes,
it is important to choose the best suited mode for the considered material,
so that Ardour can perform the detection with the greatest accuracy :
</p>
<ul>
<li><dfn>Percussive Onset</dfn> will detect the start of each hit based
on the sudden change in energy (&eq; volume) of the waveform</li>
<li><dfn>Note Onset</dfn> will detect the start of each note based on the changes
in the frequency domain.</li>
</ul>
<h2>The Percussive Onset mode</h2>
<p>
In this mode, only two parameters are active:
</p>
<dl>
<dt><dfn>Sensitivity</dfn> (%)</dt><dd>The proportion of the samples that must exceed the
energy rise threshold in order for an onset to be detected (at frames in which
the detection function peaks). This roughly corresponds to how "noisy" a percussive
sound must be in order to be detected.</dd>
<dt><dfn>Cut Pos Threshold</dfn> (dB)</dt><dd>The rise in energy amongst a group of samples
that is required for that to be counted toward the detection function's count.
This roughly corresponds to how "loud" a percussive sound must be in order to
be detected.</dd>
</dl>
<p>
As those parameters are very material-related, there is no recipe for a perfect
match, and a good peak detection is a matter of adjusting those two parameters
by trial and error, and trying using the <kbd class="menu">Analyze</kbd> button
after each try.
</p>
<p>
Vertical grey markers will appear on the selected region, showing where Ardour
detects onsets as per the parameters. This markers can be manually adjusted, see
bellow.
</p>
<h2>The Note Onset Mode</h2>
<p>
In the Note Onset mode, more parameters are active:
</p>
<dl>
<dt><dfn>Detection function</dfn></dt><dd>The method used to detect note changes. More on
this bellow.</dd>
<dt><dfn>Trigger gap (postproc)</dfn> (ms)</dt><dd>Set the minimum inter-onset interval,
in milliseconds, i.e. the shortest interval between two consecutive onsets.
</dd>
<dt><dfn>Peak threshold</dfn></dt><dd>Set the threshold value for the onset peak picking.
Lower threshold values imply more onsets detected. Increasing this threshold
should reduce the number of incorrect detections.</dd>
<dt><dfn>Silence threshold</dfn> (dB)</dt><dd>Set the silence threshold, in dB, under which
the onset will not be detected. A value of -20.0 would eliminate most onsets
but the loudest ones. A value of -90.0 would select all onsets.</dd>
</dl>
<p>
The Detection function, used in Note Onset mode to choose the mathematical strategy
used to detect the note changes, is user-selectable:
</p>
<dl>
<dt><dfn>Energy based</dfn></dt><dd>This function calculates the local energy of the input
spectral frame</dd>
<dt><dfn>Spectral Difference</dfn></dt><dd>Spectral difference onset detection function
based on Jonhatan Foote and Shingo Uchihashi's "The beat spectrum: a new
approach to rhythm analysis" (2001)</dd>
<dt><dfn>High-Frequency Content</dfn></dt><dd> This method computes the High Frequency
Content (HFC) of the input spectral frame. The resulting function is efficient
at detecting percussive onsets. Based on Paul Masri's "Computer modeling
of Sound for Transformation and Synthesis of Musical Signal" (1996)</dd>
<dt><dfn>Complex Domain</dfn></dt><dd>This function uses information both in frequency and
in phase to determine changes in the spectral content that might correspond
to musical onsets. It is best suited for complex signals such as polyphonic
recordings.</dd>
<dt><dfn>Phase Deviation</dfn></dt><dd>This function uses information both energy and in
phase to determine musical onsets.</dd>
<dt><dfn>Kullback-Liebler</dfn></dt><dd>Kulback-Liebler onset detection function based on
Stephen Hainsworth and Malcom Macleod's "Onset detection in music audio
signals" (2003)</dd>
<dt><dfn>Modified Kullback-Liebler</dfn></dt><dd>Modified Kulback-Liebler onset detection
function based on Paul Brossier's "Automatic annotation of musical audio for
interactive systems" (2006)</dd>
</dl>
<p>
Ardour defaults to Complex domain, which usually gives good result for harmonic
material.
</p>
<h2>Manual adjustment</h2>
<figure>
<img src="/images/rhythm-ferret-demo-a.png" alt="The Rhythm Ferret: analysing">
<img src="/images/rhythm-ferret-demo-b.png" alt="The Rhythm Ferret: Splitting">
<img src="/images/rhythm-ferret-demo-c.png" alt="The Rhythm Ferret: Snapping to grid">
<figcaption>
The Rhythm Ferret: Analyzing, Splitting regions, and snapping to grid
</figcaption>
</figure>
<p>
Using the Rhythm Ferret consists usually in finding the right parameters to
split the audio, by adjusting them and clicking the
<kbd class="menu">Analyze</kbd> button. Each time an analysis is run, Ardour
erases the previous results, and creates grey markers on the region according
to the parameters. Those markers can be manually dragged with the
<kbd class="mouse">LEFT</kbd> mouse button to adjust their positions.
</p>
<p>
Once the markers are suitably placed, the second button in the down hand side
of the Rhythm Ferret window allows to <kbd class="menu">Apply</kbd> the operation.
At the moment of writing, only the <kdb class="menu">Split Region</kdb> is
available, which will split the region at the markers.
</p>
<p>
Those regions can then be manually aligned, or have their sync points set to
the closest grid (as per the <a href="@@grid-controls">Grid settings</a> in
effect), by selecting all the regions, and using the
<kbd class="mouse">RIGHT</kbd> click &gt; Selected Regions &gt; Position &gt;
Snap posiiton to grid.
</p>

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