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<h1 id="header"><span>Vamp Plugins</span></h1>

<h2>Vamp Example Plugins</h2>

<p>The &ldquo;vamp-example-plugins&rdquo; library contains a number of
 <a href="http://www.vamp-plugins.org/">Vamp audio analysis
 plugins</a> provided as part of the Vamp plugin SDK.

</p>
<p>These are simple, but sometimes useful, plugins whose source code you
 are free to study and reuse in any proprietary or non-proprietary
 plugins of your own without any licensing obligation.
</p>
<p>User documentation for the individual plugins in this library follows.
</p>
<div class="toc2">1. &nbsp;<a href="#amplitudefollower">Amplitude Follower</a></div>
<div class="toc2">2. &nbsp;<a href="#fixedtempo">Simple Fixed Tempo Estimator</a></div>
<div class="toc2">3. &nbsp;<a href="#percussiononsets">Simple Percussion Onset Detector</a></div>
<div class="toc2">4. &nbsp;<a href="#powerspectrum">Simple Power Spectrum</a></div>
<div class="toc2">5. &nbsp;<a href="#spectralcentroid">Spectral Centroid</a></div>
<div class="toc2">6. &nbsp;<a href="#zerocrossing">Zero Crossings</a></div>

<div class="oddcontent"><a name="amplitudefollower"></a><h2>1. &nbsp;Amplitude Follower</h2>

<p><b>System identifier</b> &ndash; <code>vamp-example-plugins:amplitudefollower</code><br>
 <b>RDF URI</b> &ndash; <a href="http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#amplitudefollower">http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#amplitudefollower</a>
<br><b>Links</b> &ndash; <a href="#">Back to top of library documentation</a> &ndash; <a href="http://sourceforge.net/project/showfiles.php?group_id=192001&package_id=225765">Download location</a> &ndash; <a href="http://vamp.svn.sourceforge.net/viewvc/vamp/vamp-plugin-sdk/trunk/examples/AmplitudeFollower.cpp?view=markup">Source code</a>
</p>
<p>Amplitude Follower tracks and returns the amplitude of the audio
 signal sample by sample, returning peak values block by block.
</p>
</div><div class="evencontent"><a name="toc2"></a><h3>1.1. &nbsp;Parameters</h3>

<p><b>Attack time</b> (seconds) &ndash; The 60dB convergence time for an increase in amplitude.<br>
 <b>Release time</b> (seconds) &ndash; The 60dB convergence time for a decrease in amplitude.
</p>
<p>For example, if you feed the plugin with a simple step function that
 jumps from level A to level B, then the output will start off as A,
 then at the moment of stepping it will start to converge exponentially
 to B, reaching with 60dB of the actual value within the time specified
 by the Attack time parameter.
</p>
<p>Similarly, if the plugin's input then steps down from B to A, the
 output will start converging at the moment of stepping, reaching
 within 60dB of the new value within the time specified by the Release
 time parameter.
</p>
</div><div class="oddcontent"><a name="toc3"></a><h3>1.2. &nbsp;Outputs</h3>

</div><div class="evencontent"><a name="toc4"></a><h4>1.2.1. &nbsp;Amplitude</h4>

<p>The peak tracked amplitude (in volts) for the current processing block.
</p>
</div><div class="oddcontent"><a name="toc5"></a><h3>1.3. &nbsp;References and Credits</h3>

<p>Amplitude Follower uses a method from the SuperCollider audio
 processing language.  It was implemented as a Vamp plugin by Dan
 Stowell.
</p>
</div><div class="evencontent"><a name="fixedtempo"></a><h2>2. &nbsp;Simple Fixed Tempo Estimator</h2>

<p><b>System identifier</b> &ndash; <code>vamp-example-plugins:fixedtempo</code><br>
 <b>RDF URI</b> &ndash; <a href="http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#fixedtempo">http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#fixedtempo</a>
<br><b>Links</b> &ndash; <a href="#">Back to top of library documentation</a> &ndash; <a href="http://sourceforge.net/project/showfiles.php?group_id=192001&package_id=225765">Download location</a> &ndash; <a href="http://vamp.svn.sourceforge.net/viewvc/vamp/vamp-plugin-sdk/trunk/examples/FixedTempoEstimator.cpp?view=markup">Source code</a>
</p>
<p>Simple Fixed Tempo Estimator analyses a fragment of audio and
 estimates its tempo.  It assumes that its input is of fixed tempo, and
 it analyses only the first (small but configurable number of) seconds
 before returning a result, discarding all subsequent input.
</p>
<p>The plugin calculates an overall energy rise function across a series
 of short frequency-domain input frames, takes the autocorrelation of
 this function, filters it to stress possible metrical patterns,
 locates peaks, and converts from autocorrelation lag to the
 corresponding tempo.
</p>
<p>The filtering process involves searching for peaks at simple
 metrically related intervals (at a given autocorrelation lag as well
 as at 0.5, 2, and 4 times that lag), boosting each peak that shows
 strong related peaks.  A simplistic perceptual curve is also applied
 in order to increase the probability of detecting a "likely" tempo.
 For improved tempo precision, each tempo with strong related peaks is
 averaged with the tempi calculated from those peaks.
</p>
<p>The method is best suited for 4/4 pop and dance rhythms.
</p>
<p>This plugin returns many of its intermediate calculations as
 additional outputs, as well as the most favoured tempo.  Although as a
 tempo estimator it's still fairly primitive, it is intended to provide
 a useful example of a slightly more complex feature extraction plugin
 than the other examples, as well as one that returns several different
 types of output at a time.
</p>
</div><div class="oddcontent"><a name="toc7"></a><h3>2.1. &nbsp;Parameters</h3>

<p><b>Minimum estimated tempo</b>, <b>Maximum estimated tempo</b> (bpm) &ndash; These
 parameters control the range of values within which the tempo
 estimator will return its estimate.
</p>
<p><b>Input duration to study</b> (seconds) &ndash; The tempo estimator uses only the
 first part of its input, discarding any that follows.  This parameter
 controls how much input it will use.  There is no value in increasing
 this beyond 8x the duration of the slowest returned beat.  The default
 of 10 seconds is likely to be appropriate for most purposes.
</p>
</div><div class="evencontent"><a name="toc8"></a><h3>2.2. &nbsp;Outputs</h3>

</div><div class="oddcontent"><a name="toc9"></a><h4>2.2.1. &nbsp;Tempo</h4>

<p>The tempo estimator's best guess at the tempo of its input, in beats
 per minute.
</p>
<p>This is returned as a feature whose timestamp and duration cover the
 range of the input which was used in estimating the tempo, with a
 single value containing the tempo.
</p>
</div><div class="evencontent"><a name="toc10"></a><h4>2.2.2. &nbsp;Tempo candidates</h4>

<p>Several guesses at the possible tempo.  This output is returned as a
 single feature whose timestamp and duration cover the range of the
 input which was used in estimating the tempo, with up to 10 bins
 containing one tempo value in each bin, with the "best guess" tempo in
 bin 0.
</p>
</div><div class="oddcontent"><a name="toc11"></a><h4>2.2.3. &nbsp;Detection function</h4>

<p>The basic onset detection function used in tempo estimation.
</p>
</div><div class="evencontent"><a name="toc12"></a><h4>2.2.4. &nbsp;Autocorrelation function</h4>

<p>The autocorrelation of the onset detection function.
</p>
</div><div class="oddcontent"><a name="toc13"></a><h4>2.2.5. &nbsp;Filtered Autocorrelation</h4>

<p>The autocorrelation after filtering to boost values with possible
 metrically related peaks and to apply perceptual weighting.  The peak
 value of this function is the one that will be used as the "best
 guess".
</p>
</div><div class="evencontent"><a name="toc14"></a><h3>2.3. &nbsp;References and Credits</h3>

<p>Simple Fixed Tempo Estimator uses a method derived from work by
 Matthew Davies: see for example M. E. P. Davies and M. D. Plumbley,
 <i>Beat Tracking With A Two State Model</i>, in Proceedings of the IEEE
 International Conference on Acoustics, Speech and Signal Processing
 2005.  This plugin, made by Chris Cannam, is only an unsubtle
 simplification of a small part of the published method.
</p>
<p>The Queen Mary plugin set
 (<a href="http://www.elec.qmul.ac.uk/digitalmusic/downloads/index.html#qm-vamp-plugins">http://www.elec.qmul.ac.uk/digitalmusic/downloads/index.html#qm-vamp-plugins</a>)
 contains a Tempo and Beat Tracker plugin by Matthew Davies providing a
 more realistic implementation.
</p>
</div><div class="oddcontent"><a name="percussiononsets"></a><h2>3. &nbsp;Simple Percussion Onset Detector</h2>

<p><b>System identifier</b> &ndash; <code>vamp-example-plugins:percussiononsets</code><br>
 <b>RDF URI</b> &ndash; <a href="http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#percussiononsets">http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#percussiononsets</a>
<br><b>Links</b> &ndash; <a href="#">Back to top of library documentation</a> &ndash; <a href="http://sourceforge.net/project/showfiles.php?group_id=192001&package_id=225765">Download location</a> &ndash; <a href="http://vamp.svn.sourceforge.net/viewvc/vamp/vamp-plugin-sdk/trunk/examples/PercussionOnsetDetector.cpp?view=markup">Source code</a>
</p>
<p>Simple Percussion Onset Detector estimates the locations of percussive
 onsets in the audio signal.
</p>
<p>The principle is to exploit the broadband nature of noisy percussive
 onsets by identifying only those frames in which the energy rise shows
 a broadband profile.
</p>
<p>The plugin takes a series of frequency domain frames, and examines
 each frame to count the number of bins whose energy content has
 increased by more than a certain threshold since the prior frame.
 Frames in which this number is at a peak relative to prior and
 following frames and also exceeds another threshold value are
 classified as percussive onsets.
</p>
</div><div class="evencontent"><a name="toc16"></a><h3>3.1. &nbsp;Parameters</h3>

<p><b>Energy rise threshold</b> (dB) &ndash; The rise in energy within a bin from one
 frame to the next that is required for a bin to be counted toward the
 detection function's bin count.  This roughly corresponds to how
 "loud" a percussive sound must be in order to be detected.
</p>
<p><b>Sensitivity</b> (%) &ndash; The proportion of bins 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.
</p>
</div><div class="oddcontent"><a name="toc17"></a><h3>3.2. &nbsp;Outputs</h3>

</div><div class="evencontent"><a name="toc18"></a><h4>3.2.1. &nbsp;Onsets</h4>

<p>The estimated onset locations.
</p>
</div><div class="oddcontent"><a name="toc19"></a><h4>3.2.2. &nbsp;Detection Function</h4>

<p>The energy rise detection function whose peaks were used to estimate
 onset locations.
</p>
</div><div class="evencontent"><a name="toc20"></a><h3>3.3. &nbsp;References and Credits</h3>

<p>The method used in Simple Percussion Onset Detector was described in
Dan Barry, Derry Fitzgerald, Eugene Coyle and
 Bob Lawlor, <i>Drum Source Separation using Percussive Feature Detection and
 Spectral Modulation</i>, ISSC 2005.  The plugin was made by Chris Cannam.
</p>
</div><div class="oddcontent"><a name="powerspectrum"></a><h2>4. &nbsp;Simple Power Spectrum</h2>

<p><b>System identifier</b> &ndash; <code>vamp-example-plugins:powerspectrum</code><br>
 <b>RDF URI</b> &ndash; <a href="http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#powerspectrum">http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#powerspectrum</a>
<br><b>Links</b> &ndash; <a href="#">Back to top of library documentation</a> &ndash; <a href="http://sourceforge.net/project/showfiles.php?group_id=192001&package_id=225765">Download location</a> &ndash; <a href="http://vamp.svn.sourceforge.net/viewvc/vamp/vamp-plugin-sdk/trunk/examples/PowerSpectrum.cpp?view=markup">Source code</a>
</p>
<p>Simple Power Spectrum returns a power spectrum calculated from
 windowed short-time Fourier transforms of the input audio.  (The power
 spectrum for a frame consists of a sequence of the squares of the
 magnitudes of the complex values for each frequency bin in the result
 of the Fourier transform.)
</p>
<p>This very simple plugin is an illustration of the fact that if a
 plugin requests frequency-domain input, its input will already be in
 the form needed for a spectrum such as this.  The plugin has no work
 left to do except to calculate the squared magnitude from the
 cartesian complex representation.
</p>
<p>This plugin also illustrates how to return "grid-type" visualisation
 data from a Vamp plugin.
</p>
</div><div class="evencontent"><a name="toc22"></a><h3>4.1. &nbsp;Parameters</h3>

<p>None.
</p>
</div><div class="oddcontent"><a name="toc23"></a><h3>4.2. &nbsp;Outputs</h3>

</div><div class="evencontent"><a name="toc24"></a><h4>4.2.1. &nbsp;Power Spectrum</h4>

<p>The power spectrum calculated from the input frame.  This output
 returns a single feature per processing block, containing
 blocksize/2+1 power values corresponding to the FFT bins from DC to
 Nyquist inclusive.  The DC bin is always returned.
</p>
</div><div class="oddcontent"><a name="spectralcentroid"></a><h2>5. &nbsp;Spectral Centroid</h2>

<p><b>System identifier</b> &ndash; <code>vamp-example-plugins:spectralcentroid</code><br>
 <b>RDF URI</b> &ndash; <a href="http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#spectralcentroid">http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#spectralcentroid</a>
<br><b>Links</b> &ndash; <a href="#">Back to top of library documentation</a> &ndash; <a href="http://sourceforge.net/project/showfiles.php?group_id=192001&package_id=225765">Download location</a> &ndash; <a href="http://vamp.svn.sourceforge.net/viewvc/vamp/vamp-plugin-sdk/trunk/examples/SpectralCentroid.cpp?view=markup">Source code</a>
</p>
<p>Spectral Centroid calculates the "centre of gravity" of the frequency
 spectrum for each input frame.
</p>
</div><div class="evencontent"><a name="toc26"></a><h3>5.1. &nbsp;Parameters</h3>

<p>None.
</p>
</div><div class="oddcontent"><a name="toc27"></a><h3>5.2. &nbsp;Outputs</h3>

</div><div class="evencontent"><a name="toc28"></a><h4>5.2.1. &nbsp;Log Frequency Centroid</h4>

<p>The centroid of the log-weighted frequency spectrum.  That is, the sum
 across Fourier transform output bins of the logarithm of the bin
 frequency multiplied by the bin magnitude, divided by the sum of the
 bin magnitudes, and the inverse logarithm taken so as to give the
 result as a frequency in Hz.
</p>
</div><div class="oddcontent"><a name="toc29"></a><h4>5.2.2. &nbsp;Linear Frequency Centroid</h4>

<p>The centroid of the linear-weighted frequency spectrum.  That is, the
 sum across Fourier transform output bins of the bin frequency
 multiplied by the bin magnitude, divided by the sum of the bin
 magnitudes.  The result is a frequency in Hz.
</p>
</div><div class="evencontent"><a name="zerocrossing"></a><h2>6. &nbsp;Zero Crossings</h2>

<p><b>System identifier</b> &ndash; <code>vamp-example-plugins:zerocrossing</code><br>
 <b>RDF URI</b> &ndash; <a href="http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#zerocrossing">http://vamp-plugins.org/rdf/plugins/vamp-example-plugins#zerocrossing</a>
<br><b>Links</b> &ndash; <a href="#">Back to top of library documentation</a> &ndash; <a href="http://sourceforge.net/project/showfiles.php?group_id=192001&package_id=225765">Download location</a> &ndash; <a href="http://vamp.svn.sourceforge.net/viewvc/vamp/vamp-plugin-sdk/trunk/examples/ZeroCrossing.cpp?view=markup">Source code</a>
</p>
<p>Zero Crossings calculates the positions and density of "zero-crossing"
 points in an audio waveform.  For the purposes of this plugin, that
 means those positions at which the sampled value switches from
 zero-or-less to greater-than-zero, or vice versa.
</p>
</div><div class="oddcontent"><a name="toc31"></a><h3>6.1. &nbsp;Parameters</h3>

<p>None.
</p>
</div><div class="evencontent"><a name="toc32"></a><h3>6.2. &nbsp;Outputs</h3>

</div><div class="oddcontent"><a name="toc33"></a><h4>6.2.1. &nbsp;Zero Crossing Counts</h4>

<p>The number of zero-crossing points found in the current block of
 samples, as a single-valued feature returned per processing block.
</p>
</div><div class="evencontent"><a name="toc34"></a><h4>6.2.2. &nbsp;Zero Crossings</h4>

<p>The locations of zero-crossing points, returning one feature
 timestamped to the zero-crossing location, without values, for each
 crossing point.
</p>
<p></p>

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