SampleType » History » Version 34

Chris Cannam, 2014-02-10 03:17 PM

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h1. Output Sample Type and Sample Rate
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h2. Who should read this document
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This is a detailed document about the "sample type" and "sample rate" properties of a Vamp plugin's output descriptor.
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 * If you are new to the Vamp plugin API, read the "Programmer's Guide":http://vamp-plugins.org/guide.pdf first. The section "Sample Types and Timestamps" starting on page 9 introduces this subject.
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 * If you are writing a plugin, read the "Rules of Thumb" section (below) after the Programmer's Guide. You probably won't need to read the rest of this document. You should use the "Vamp Plugin Tester":/projects/vamp-plugin-tester to test your plugin.
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 * If you are writing a host, you should probably read the whole of this as well as the Guide. You should also use the "Vamp Test Plugin":/projects/vamp-test-plugin to test your host's interpretation of the feature structures.
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h2. Rules of Thumb for Plugin Developers
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The tl;dr summary:
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 * If your output returns things that are always regularly-spaced in time, and there is one such thing returned for every @process@ block, and the calculation is causal so that results are available immediately, and there is no latency added beyond the length of the processing block, then you probably want to use @OneSamplePerStep@ sample type and omit the feature timestamps.
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 * If your output returns things that are regularly-spaced in time but the other limitations above are not met, use @FixedSampleRate@ sample type, set the output sample rate to the (perhaps fractional) number of returned features per second, and use a timestamp for each feature.
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 * If your output returns anything else, use @VariableSampleRate@ sample type, set the output sample rate to zero unless you know better, and use a timestamp for each feature.
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h2. Introduction
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A Vamp plugin receives audio and produces a series of descriptive feature structures.
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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.
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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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!/attachments/download/980/feature-structures-20pc.png!
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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 output on which the feature is returned.
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An output's @sampleType@ property may be either @OneSamplePerStep@, @FixedSampleRate@, or @VariableSampleRate@. Here's what they mean.
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h2. OneSamplePerStep
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This is the simplest option.
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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.
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The @sampleRate@ and @hasDuration@ output properties are ignored for outputs of this type.
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For any features returned through an output declared with @OneSamplePerStep@ type,
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 * The plugin _should not_ set timestamps on these features and _should_ set their @hasTimestamp@ property to @false@;
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 * The plugin _should not_ set durations on these features and _should_ set their @hasDuration@ property to @false@;
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 * The host _must_ ignore any timestamps or durations that the plugin may set on these features;
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 * 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;
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 * 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);
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 * The host _must_ treat all such features has having duration equal to the spacing between process blocks.
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h3. Examples
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@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.
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@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.
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h2. VariableSampleRate
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If the @OneSamplePerStep@ output type essentially means that the plugin leaves all time calculations up to the host, @VariableSampleRate@ is the opposite.
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If an output is declared as having a @SampleType@ of @VariableSampleRate@, 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.
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h3. Timestamps
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For any features returned through an output declared with @VariableSampleRate@ type,
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 * The plugin _must_ set timestamps on these features and _must_ set their @hasTimestamp@ property to @true@;
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 * The host _must_ obtain the features' start times from their timestamps rather than calculating them itself.
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h3. Durations
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Features returned through @VariableSampleRate@ outputs may optionally have durations.
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If the output's @hasDuration@ property is @true@, then
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 * The plugin _may_ set the @hasDuration@ property of such features to @true@ and, if it does so, _must_ also set their @duration@ property;
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 * If a feature's @hasDuration@ property is true, then the host _must_ use the feature's @duration@ property as the feature duration; otherwise the host _must_ treat the feature as having "minimal" duration (see "Sample Rate" below).
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If the output's @hasDuration@ property is @false@, then
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 * The plugin _should not_ set the @duration@ property of that output's features;
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 * The host _must_ ignore the @hasDuration@ and @duration@ properties of the features and treat them as having "minimal" duration (see below).
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h3. Sample rate and "minimal" duration
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The plugin may optionally set a @sampleRate@ property for each @VariableSampleRate@ output. A @sampleRate@ of zero indicates no value.
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If a @sampleRate@ is set,
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 * The host _may_ optionally use the 1/@sampleRate@ seconds as indicating the resolution of the output feature timestamps, and _may_ round each output feature timestamp to a multiple of that resolution;
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 * The host _must_ use 1/@sampleRate@ seconds as the "minimal" duration assigned to features that have no duration supplied.
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If no @sampleRate@ is set,
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 * The host _must_ use the feature timestamps unmodified;
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 * The host must use zero as the "minimal" duration used for features with no duration supplied.
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h3. Examples
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h2. FixedSampleRate
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If an output is declared as having a @SampleType@ of @FixedSampleRate@
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h3. Examples