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