AudioParam - Web APIs 编辑

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.

There are two kinds of AudioParam, a-rate and k-rate parameters:

• 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.

Each AudioNode defines which of its parameters are a-rate or k-rate in the spec.

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 timelime-based automation curves. The time used is the one defined in AudioContext.currentTime.

Properties

AudioParam.defaultValue Read only

Represents the initial volume of the attribute as defined by the specific AudioNode creating the AudioParam.

AudioParam.maxValue Read only

Represents the maximum possible value for the parameter's nominal (effective) range. 

AudioParam.minValue Read only

Represents the minimum possible value for the parameter's nominal (effective) range. 

AudioParam.value

Represents the parameter's current value as of the current time; initially set to the value of defaultValue.

Methods

AudioParam.setValueAtTime()

Schedules an instant change to the value of the AudioParam at a precise time, as measured against AudioContext.currentTime. The new value is given by the value parameter.

AudioParam.linearRampToValueAtTime()

Schedules a gradual linear change in the value of the AudioParam. The change starts at the time specified for the previous event, follows a linear ramp to the new value given in the value parameter, and reaches the new value at the time given in the endTime parameter.

AudioParam.exponentialRampToValueAtTime()

Schedules a gradual exponential change in the value of the AudioParam. The change starts at the time specified for the previous event, follows an exponential ramp to the new value given in the value parameter, and reaches the new value at the time given in the endTime parameter.

AudioParam.setTargetAtTime()

Schedules the start of a change to the value of the AudioParam. The change starts at the time specified in startTime and exponentially moves towards the value given by the target parameter. The exponential decay rate is defined by the timeConstant parameter, which is a time measured in seconds.

AudioParam.setValueCurveAtTime()

Schedules the values of the AudioParam to follow a set of values, defined by an array of floating-point numbers scaled to fit into the given interval, starting at a given start time and spanning a given duration of time.

AudioParam.cancelScheduledValues()

Cancels all scheduled future changes to the AudioParam.

AudioParam.cancelAndHoldAtTime()

Cancels all scheduled future changes to the AudioParam but holds its value at a given time until further changes are made using other methods.

Examples

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 maniuplated. 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);

Specifications

Browser compatibility

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See also

• Using the Web Audio API