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• 句法
• 描述
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

setTargetAtTime() 方法在 AudioParam interface schedules the start of a gradual change to the AudioParam value. This is useful for decay or release portions of ADSR envelopes.

句法

var paramRef = param.setTargetAtTime(target, startTime, timeConstant);
					

参数

target

The value the parameter will start to transition towards at the given start time.

startTime

The time that the exponential transition will begin, in the same time coordinate system as AudioContext.currentTime . If it is less than or equal to AudioContext.currentTime , the parameter will start changing immediately.

timeConstant

The time-constant value, given in seconds, of an exponential approach to the target value. The larger this value is, the slower the transition will be.

返回

A reference to this AudioParam object. Some older browser implementations of this interface return void.

描述

The change starts at the time specified in startTime and exponentially moves towards the value given by the target parameter. The decay rate as defined by the timeConstant parameter is exponential; therefore the value will never reach target completely, but after each timestep of length timeConstant , the value will have approached target by another $1 - e^{-1} \approx 63.2$ . For the complete formula (which uses a first-order linear continuous time-invariant system), check the Web Audio specification .

If you absolutely need to reach the target value by a specific time, you can use AudioParam.exponentialRampToValueAtTime() . However, for mathematical reasons, that method does not work if the current value or the target value is 0 .

Choosing a good timeConstant

As mentioned above, the value changes exponentially, with each timeConstant bringing you another 63.2% toward the target value. You don't have to worry about reaching the target value; once you are close enough, any further changes will be imperceptible to a human listener.

Depending on your use case, getting 95% toward the target value may already be enough; in that case, you could set timeConstant to one third of the desired duration.

For more details, check the following table on how the value changes from 0% to 100% as the time progresses.

Time since startTime	值
0 * timeConstant	0%
0.5 * timeConstant	39.3%
1 * timeConstant	63.2%
2 * timeConstant	86.5%
3 * timeConstant	95.0%
4 * timeConstant	98.2%
5 * timeConstant	99.3%
n * timeConstant	$1 - e^{-n}$

范例

In this example, we have a media source with two control buttons (see the webaudio-examples repo for the source code, or view the example live .) When these buttons are pressed, setTargetAtTime() is used to fade the gain value up to 1.0, and down to 0, respectively, with the effect starting after 1 second, and the length of time the effect lasts being controlled by the timeConstant.

// create audio context
var AudioContext = window.AudioContext || window.webkitAudioContext;
var audioCtx = new AudioContext();
// set basic variables for example
var myAudio = document.querySelector('audio');
var pre = document.querySelector('pre');
var myScript = document.querySelector('script');
pre.innerHTML = myScript.innerHTML;
var atTimePlus = document.querySelector('.at-time-plus');
var atTimeMinus = document.querySelector('.at-time-minus');
// Create a MediaElementAudioSourceNode
// Feed the HTMLMediaElement into it
var source = audioCtx.createMediaElementSource(myAudio);
// Create a gain node and set it's gain value to 0.5
var gainNode = audioCtx.createGain();
gainNode.gain.value = 0.5;
var currGain = gainNode.gain.value;
// connect the AudioBufferSourceNode to the gainNode
// and the gainNode to the destination
source.connect(gainNode);
gainNode.connect(audioCtx.destination);
// set buttons to do something onclick
atTimePlus.onclick = function() {
  currGain = 1.0;
  gainNode.gain.setTargetAtTime(1.0, audioCtx.currentTime + 1, 0.5);
}
atTimeMinus.onclick = function() {
  currGain = 0;
  gainNode.gain.setTargetAtTime(0, audioCtx.currentTime + 1, 0.5);
}
					

规范

规范	状态	注释
Web 音频 API The definition of 'setTargetAtTime' in that specification.	工作草案

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
setTargetAtTime	Chrome 14	Edge 12	Firefox 25	IE 不支持 No	Opera 15	Safari 6	WebView Android Yes	Chrome Android 18	Firefox Android 26	Opera Android 14	Safari iOS Yes	Samsung Internet Android 1.0

图例

完整支持

完整支持

不支持

不支持

另请参阅

• 使用 Web 音频 API

元数据

• 最后修改： 2019 年 3 月 23 日

相关话题

1. Web 音频 API
2. AudioParam
3. 特性

   1. defaultValue
   2. maxValue
   3. minValue
   4. value
4. 方法

   1. cancelAndHoldAtTime()
   2. cancelScheduledValues()
   3. exponentialRampToValueAtTime()
   4. linearRampToValueAtTime()
   5. setTargetAtTime()
   6. setValueAtTime()
   7. setValueCurveAtTime()
5. Related pages for Web Audio API

   1. AnalyserNode
   2. AudioBuffer
   3. AudioBufferSourceNode
   4. AudioContext
   5. AudioContextOptions
   6. AudioDestinationNode
   7. AudioListener
   8. AudioNode
   9. AudioNodeOptions
   10. AudioProcessingEvent
   11. AudioScheduledSourceNode
   12. AudioWorklet
   13. AudioWorkletGlobalScope
   14. AudioWorkletNode
   15. AudioWorkletProcessor
   16. BaseAudioContext
   17. BiquadFilterNode
   18. ChannelMergerNode
   19. ChannelSplitterNode
   20. ConstantSourceNode
   21. ConvolverNode
   22. DelayNode
   23. DynamicsCompressorNode
   24. GainNode
   25. IIRFilterNode
   26. MediaElementAudioSourceNode
   27. MediaStreamAudioDestinationNode
   28. MediaStreamAudioSourceNode
   29. OfflineAudioCompletionEvent
   30. OfflineAudioContext
   31. OscillatorNode
   32. PannerNode
   33. PeriodicWave
   34. StereoPannerNode
   35. WaveShaperNode