该失真只发生在webkitaudiocontext.decodeAudioData()生成的音频上,然后通过webkitaudiocontext.createBufferSource()播放. webkitaudiocontext.createMediaElementSource()的音频播放不会变形.
我缺少一些初始化步骤吗？以下是我向Apple提交的代码和HTML,作为错误报告(但没有收到回复)：
<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
var buffer = null;
var context = null;
var voice = null;
function load_music(file) {
context = new webkitaudiocontext();
voice = context.createBufferSource();
var request = new XMLHttpRequest();
request.onload = function() {
context.decodeAudioData(request.response,function(result) {
buffer = result;
document.getElementById("start").value = "Start";
});
};
var base = window.location.pathname;
base = base.substring(0,base.lastIndexOf("/") + 1);
request.open("GET",base + file,true);
request.responseType = "arraybuffer";
request.send(null);
}
function start_music() {
if (!buffer) {
alert("Not ready yet");
return;
}
voice.buffer = buffer;
voice.connect(context.destination);
voice.noteOn(0);
document.getElementById("compare").style.display = "block";
}
</script>
</head>
<body onload="load_music('music.mp3')">
<p>This is a simple demo page to reproduce a <strong>webkitAudio</strong>
problem occurring in Safari on iOS 6.1.4. This is a stripped down demo
of a phenomenon discovered in our HTML5 game under development,using different assets.</p>
<p><u>Steps to reproduce:</u></p>
<ol>
<li>Power cycle <strong>iPhone 5 with iOS 6.1.4</strong>.</li>
<li>Launch Safari immediately,and visit this page.</li>
<li>Wait for "Loading..." below to change to
"Start".</li>
<li>Tap "Start".</li>
</ol>
<p><u>Issue:</u></p>
<p>Audio will be excessively distorted and play at wrong pitch. If
another audio-enabled web site is visited before this one,or this
site is reloaded,the audio will fix. The distortion only happens on
the first visit after cold boot. <strong>To reproduce the bug,it is
critical to power cycle before testing.</strong></p>
<p>This bug has not been observed on any other iOS version (e.g. does
not occur on iPad Mini or iPod 5 using iOS 6.1.3).</p>
<input id="start" type="button" value="Loading..." onmousedown="start_music()" />
<span id="compare" style="display:none;"><p><a href="music.mp3">Direct link</a> to audio file,for
comparison.</p></span>
</body>
</html>
注意：身体文本表明这仅发生在iOS 6.1.4上,但我的意思是说,问题只发生在这种情况下的电源循环.在6.1.3的iPad Mini下,我也遇到过这个问题,但是没有电力循环.
编辑：我尝试过的一些事情…推迟缓冲源的创建没有什么区别.使用不同的代码转换器来生成它播放的.mp3文件没有什么区别.播放一次性的沉默作为第一个声音没有区别,因为每个decodeAudioData声音的失真持续到页面重新加载.如果createMediaElementSource和createBufferSource源在同一页面中混合,则只有createBufferSource音频(使用decodeAudioData)才会失真.当我在失败案例和非故障案例中检查request.response.byteLength时,它们是一样的,建议XMLHttpRequest不返回不正确的数据,尽管我认为数据损坏会损坏MP3头并渲染文件无法播放.
故障条件与非故障条件之间存在一个可观察的差异.只读值context.sampleRate在失败状态下将为48000,在非故障状态下为44100. (但是失败状态听起来比非故障状态的音调更低).唯一发生在我身上的是一个黑客,其中如果在浏览器上检测到48000,应该是报告44100,而通过JavaScript刷新页面,但这是严重的userAgent筛选并不是非常未来的证明,这让我很紧张.
解决方法
即使没有< video>标签后,首次在冷启动后在页面上播放音频时播放失真.重新加载后,它工作正常.
最初的AudioContext似乎默认为48 kHz,这是发生失真的地方(即使是采用48 kHz采样率的音频).当播放正常工作时,AudioContext的采样率为44.1 kHz.
我找到了一个解决方法：可以在播放初始声音后重新创建AudioContext.新创建的AudioContext似乎具有正确的采样率.去做这个：
// inside the click/touch handler
var playInitSound = function playInitSound() {
var source = context.createBufferSource();
source.buffer = context.createBuffer(1,1,48000);
source.connect(context.destination);
if (source.start) {
source.start(0);
} else {
source.noteOn(0);
}
};
playInit();
if (context.sampleRate === 48000) {
context = new AudioContext();
playInit();
}