The Web Audio API's AudioParam
interface represents an audio-related parameter, usually a parameter of an AudioNode
(such as GainNode.gain
).
An AudioParam
can be set to a specific value or a change in value, and can be scheduled to happen at a specific time and following a specific pattern.
Each AudioParam
has a list of events, initially empty, that define when and how values change. When this list is not empty, changes using the AudioParam.value
attributes are ignored. This list of events allows us to schedule changes that have to happen at very precise times, using arbitrary timeline-based automation curves. The time used is the one defined in AudioContext.currentTime
.
There are two kinds of AudioParam
: a-rate and k-rate parameters. Each AudioNode
defines which of its parameters are a-rate or k-rate in the spec.
An a-rate AudioParam
takes the current audio parameter value for each sample frame of the audio signal.
A k-rate AudioParam
uses the same initial audio parameter value for the whole block processed; that is, 128 sample frames. In other words, the same value applies to every frame in the audio as it's processed by the node.
First, a basic example showing a GainNode
having its gain
value set. gain
is an example of an a-rate AudioParam, as the value can potentially be set differently for each sample frame of the audio.
var AudioContext = window.AudioContext || window.webkitAudioContext;
var audioCtx = new AudioContext();
var gainNode = audioCtx.createGain();
gainNode.gain.value = 0;
Next, an example showing a DynamicsCompressorNode
having some param values manipulated. These are examples of k-rate AudioParam's, as the values are set for the entire audio block at once.
var compressor = audioCtx.createDynamicsCompressor();
compressor.threshold.setValueAtTime(-50, audioCtx.currentTime);
compressor.knee.setValueAtTime(40, audioCtx.currentTime);
compressor.ratio.setValueAtTime(12, audioCtx.currentTime);
compressor.attack.setValueAtTime(0, audioCtx.currentTime);
compressor.release.setValueAtTime(0.25, audioCtx.currentTime);