AudioWorkletProcessor.parameterDescriptors (static getter) - Web APIs 编辑

Experimental

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The read-only parameterDescriptors property of an AudioWorkletProcessor-derived class is a static getter, which returns an iterable of AudioParamDescriptor-based objects.

The property is not a part of the AudioWorkletProcessor interface, but, if defined, it is called internally by the AudioWorkletProcessor constructor to create a list of custom AudioParam objects in the parameters property of the associated AudioWorkletNode.

Defining the getter is optional.

Syntax

AudioWorkletProcessorSubclass.parameterDescriptors;

Value

An iterable of AudioParamDescriptor-based objects. The properties of these objects are as follows:

name

The DOMString which represents the name of the AudioParam. Under this name the AudioParam will be available in the parameters property of the node, and under this name the AudioWorkletProcessor.process method will acquire the calculated values of this AudioParam.

automationRate Optional

Either "a-rate", or "k-rate" string which represents an automation rate of this AudioParam. Defaults to "a-rate".

minValue Optional

A float which represents minimum value of the AudioParam. Defaults to -3.4028235e38.

maxValue Optional

A float which represents maximum value of the AudioParam. Defaults to 3.4028235e38.

defaultValue Optional

A float which represents initial value of the AudioParam. Defaults to 0.

Examples

To demonstrate creation and usage of custom AudioParams, we'll expand the example from AudioWorkletNode page. There we've created a simple node which outputs white noise. Here, additionally, we'll create a custom gain parameter, so we can directly change volume of the output (although you could use GainNode to achieve this as well).

First, we need to define a custom AudioWorkletProcessor, and register it. Note that this should be done in a separate file.

We expand the processor by adding a static parameterDescriptors getter. It will be used internally by the AudioWorkletNode constructor to populate its parameters with instantiated AudioParam objects.

// white-noise-processor.js
class WhiteNoiseProcessor extends AudioWorkletProcessor {
  static get parameterDescriptors () {
    return [{
      name: 'customGain',
      defaultValue: 1,
      minValue: 0,
      maxValue: 1,
      automationRate: 'a-rate'
    }]
  }

  process (inputs, outputs, parameters) {
    const output = outputs[0]
    output.forEach(channel => {
      for (let i = 0; i < channel.length; i++) {
        channel[i] = (Math.random() * 2 - 1) *
          (parameters['customGain'].length > 1 ? parameters['customGain'][i] : parameters['customGain'][0])
        // note: a parameter contains an array of 128 values (one value for each of 128 samples),
        // however it may contain a single value which is to be used for all 128 samples
        // if no automation is scheduled for the moment.
      }
    })
    return true
  }
}

registerProcessor('white-noise-processor', WhiteNoiseProcessor)

Next, in our main scripts file we'll load the processor, create an instance of AudioWorkletNode passing it the name of the processor, and connect the node to an audio graph.

const audioContext = new AudioContext()
await audioContext.audioWorklet.addModule('white-noise-processor.js')
const whiteNoiseNode = new AudioWorkletNode(audioContext, 'white-noise-processor')
whiteNoiseNode.connect(audioContext.destination)

Now we can change the gain on the node like this:

const gainParam = whiteNoiseNode.parameters.get('customGain')
gainParam.setValueAtTime(0, audioContext.currentTime)
gainParam.linearRampToValueAtTime(0.5, audioContext.currentTime + 0.5)

Specifications

See also

• Web Audio API
• Using the Web Audio API