Output Sample Type and Sample Rate¶

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.)

A feature may or may not have a timestamp. Whether a timestamp is provided -- 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.

SampleType¶

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

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.

What this means¶

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

• The plugin should not set timestamps on these features
• If the plugin does set timestamps, 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 having immediately followed the final process block (i.e. as having the same time as the next equally-spaced process block would have had if the input had not ended).

Examples¶

OneSamplePerStep type is most often used for simple measurements and visualisations, in which an output is updated on each process call and a new result returned. For example, envelope trackers, power calculations, spectrograms.

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.