signalsmith-stretch/web/release/README.md

Signalsmith Stretch Web

This is an official release of the Signalsmith Stretch library for Web Audio, using WASM/AudioWorklet. It includes both plain .js (UMD), and ES6 .mjs versions.

How to use it

Call SignalsmithStretch(audioContext, ?channelOptions) from the main thread. This returns a Promise which resolves to an AudioNode, with extra methods attached to it. The optional channelOptions object can specify the number of inputs/outputs and channels.

It can operate either on live/streaming input (if you configure it to have an input), or on a sample buffer you load into it.

stretch.inputTime

The current input time, within the sample buffer. You can change how often this is updated, with an optional callback function, using stretch.setUpdateInterval(seconds, ?callback).

stretch.schedule({...})

This adds a scheduled change, removing any scheduled changes occuring after this one. The object properties are:

• output (seconds): audio context time for this change. The node compensates for its own latency, but this means you might want to schedule some things ahead of time, otherwise you'll have a softer transition as it catches up.
• active (bool): processing audio
• input (seconds): position in input buffer
• rate (number): playback rate, e.g. 0.5 == half speed
• semitones (number): pitch shift
• loopStart / loopEnd: sets a section of the input to auto-loop. Disabled if both are set to the same value.

stretch.start(?when) / stretch.stop(?when)

Starts/stops playback or processing, immediately or at some future time. These are convenience methods which call .schedule(...) under the hood.

.start() actually has more parameters, presenting a similar interface to AudioBufferSourceNode.

stretch.addBuffers([...])

This adds buffers to the end of the current input. Buffers should be typed arrays of equal length, one per channel.

It can be called multiple times, and the new buffers are inserted after the existing ones, which lets you start playback before the entire audio is loaded. It returns (as a Promise) the new end time for the stored input, in seconds.

stretch.dropBuffers()

This drops all input buffers, and resets the input buffer start time to 0.

stretch.dropBuffers(toSeconds)

This drops all input buffers before the given time. It returns (as a Promise) the an object with the current input buffer extent: {start: ..., end: ...}.

This can be useful when processing streams or very long audio files, letting the Stretch node release old buffers once that section of the input will no longer be played back.

stretch.latency()

Returns the latency when used in "live" mode. This is also how far ahead you might want to schedule things (output in .schedule()) to give the node enough time to fully compensate for its own latency.

stretch.configure({...})

Optionally reconfigure, with the following fields:

• blockMs: block length in ms (default 120ms)
• intervalMs: interval (default is 30ms)
• splitComputation: spread computation more evenly across time (default false, but worth trying if you're getting dropouts)