Vamp Plugin SDK

h1. Output Sample Type and Sample Rate

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A Vamp plugin receives audio and produces a series of descriptive feature structures.

The audio input is provided as a series of fixed-length sample blocks, equally spaced in time, provided to successive calls to the plugin's @process@ function. The plugin may return any number of features from each @process@ call, and may also return any number of features from @getRemainingFeatures@ after all the audio has been received.

Features are each associated with a particular output of the plugin. The plugin declares that each output has certain properties, which constrain the sort of feature data the host can expect to see. (See diagram.)

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A feature may or may not have a timestamp (as well as, optionally, a duration). Whether a timestamp is needed -- and, if it is provided, what it means -- are determined by the @SampleType@ and @SampleRate@ properties of the plugin output on which the feature is returned.

h2. SampleType

A plugin output's @SampleType@ property may be either @OneSamplePerStep@, @FixedSampleRate@, or @VariableSampleRate@. Here's what they mean.

h3. OneSamplePerStep

This is the simplest option. If an output is declared as having a @SampleType@ of @OneSamplePerStep@, then any features returned from a @process@ call are assumed to match up with the audio block provided to that @process@ call.

h4. What this means

For any features returned through an output declared with @OneSamplePerStep@ type,

 * The plugin _should not_ set timestamps on these features;

 * The plugin _should not_ set durations on these features;

 * If the plugin does set timestamps or durations, the host _must_ ignore them;

 * The host _must_ treat all such features returned from a given @process@ call as if they had the same timestamp as it passed to that @process@ call;

 * The host _must_ treat all such features returned from @getRemainingFeatures@ as if they were immediately following the final @process@ block (i.e. with the same time as the next equally-spaced @process@ block would have had if the input had not ended);

 * The host _must_ treat all such features has having duration equal to the spacing between process blocks.

h4. Examples

@OneSamplePerStep@ is most often used for simple measurements and visualisations, in which some internal calculation is updated on each process call and a new result returned. For example: envelope trackers; power calculations; spectrograms. These outputs are typically visualised using line graphs or colour matrix plots.

@OneSamplePerStep@ is often used for intermediate results calculated during processing of a more sophisticated feature. For example, a beat tracker might have an auxiliary output with @OneSamplePerStep@ type returning its internal onset detection function value.

h3. VariableSampleRate

If the @OneSamplePerStep@ output type essentially means that the plugin leaves all time calculations up to the host, @VariableSampleRate@ is the opposite.

If an output is declared as having a @SampleType@ of @VariableSampleRate@, that means all of the features returned through it will have timestamps set by the plugin, and they won't necessarily have any relationship to the process block timestamps provided by the host.

h3. FixedSampleRate

If an output is declared as having a @SampleType@ of @FixedSampleRate@