OfflineAudioContext使用chrome进行FFT分析 [英] OfflineAudioContext FFT analysis with chrome

问题描述

我试图建立一个波形发生器，获取audiofile振幅值，并在javascript中尽可能快地（快于实时）将它们显示在画布上。所以我使用OfflineAudioContext / webkitOfflineAudioContext，加载文件并开始分析。
波形填充宽阔的画布。

我在processor.onaudioprocess函数中分析缓冲区。 （我猜这是它的工作方式？）

它在Firefox中正常工作，但我在Chrome中遇到了一个问题：它似乎跳过了很多分析完成它尽快工作，只返回一些坐标（如16）。

这里是jsfiddle：
http://jsfiddle.net/bestiole/95EBf/

 <$ c $如果（！window.OfflineAudioContext）{
 if（！window.webkitOfflineAudioContext）{
 $（'＃output'）。append（'failed：no）创建音频上下文
 audiocontext发现，改变浏览器'）; 
} 
 window.OfflineAudioContext = window.webkitOfflineAudioContext; 
} 
 // var context = new AudioContext（）; 
 var length = 15241583; 
 var samplerate = 44100; 
 var fftSamples = 2048; 
 var waveformWidth = 1920; 
 
 var context = new OfflineAudioContext（1，length，samplerate）; 
 var source; 
 var splitter; 
 var analyzer; 
 var处理器; 
 var i = 0; 
 var average; 
 var max; 
 var coord; 
 
 processor = context.createScriptProcessor（fftSamples，1，1）; 
 processor.connect（context.destination）; 
 
 analyzer = context.createAnalyser（）; 
 analyser.smoothingTimeConstant = 0; 
 analyser.fftSize = fftSamples; 
 
 source = context.createBufferSource（）; 
 splitter = context.createChannelSplitter（）; 
 source.connect（splitter）; 
 splitter.connect（analyzer，0,0）; 
 
 analyser.connect（处理器）; 
 
 context.oncomplete = function（）{
 $（'＃output'）。append（'< br /> complete'）; 
} 
 
 var request = new XMLHttpRequest（）; 
 request.open（'GET'，http://www.mindthepressure.org/bounce.ogg，true）; 
 request.responseType ='arraybuffer'; 
 request.onload = function（）{
 $（'＃output'）。append（'loaded！'）; 
 context.decodeAudioData（request.response，function（buffer）{
 $（'＃output'）。append（'starting analysis< br />>'）; 
 processor.onaudioprocess =函数（e）{
 var data = new Uint8Array（analyser.frequencyBinCount）; 
 analyser.getByteFrequencyData（data）; 
 average = getAverageVolume（data）; 
 max = Math。 （数学，数据）; 
 coord = Math.min（average * 2,255）; 
 coord = Math.round（（max + coord）/ 2）; 
 ctx.fillStyle = gradient; 
 ctx.fillRect（i，255-coord，1,255）; 
 console.log（i +' - >'+ coord）; 
 i ++; 
} 
 source.buffer = buffer; 
 source.start（0）; 
 context.startRendering（）; 
}，onError）; 
} 
 request.send（）; 
 
函数onError（e）{
 $（'＃output'）。append（'error，check the console'）; 
 console.log（e）; 
} 
 
函数getAverageVolume（array）{
 var values = 0; 
 var average; 
 var length = array.length; （var k = 0; k  values + = array [k]; 
} 
 average =值/长度; 
平均回报; 
 
 
 
 $ b $ p 
 $ b 
（这里是另一个版本，强制波形适合1920px宽，并生成为谁感兴趣的波形数据： http://jsfiddle.net/bestiole/E3rSx/ ）
 
 
 
我真的没有明白，chrome如何处理音频文件的每一部分？
 
 
 
感谢您的帮助！ p> 
 
解决方案
 Chrome在离线模式下的脚本处理器中存在一个bug。
 
i'm trying to build a waveform generator that get audiofile amplitudes values and display them to a canvas as quick as possible (faster than realtime) in javascript. so i use the OfflineAudioContext / webkitOfflineAudioContext , load the file and start the analyse. 
the waveform is to fill a wide canvas.

i analyse buffer in a processor.onaudioprocess function. (i guess it's the way it works ?)

it works fine in firefox but i've got an issue in chrome : it seems it "jumps" over much analyse to finish it works as soon as possible and only returns a few coordinates (something like 16).

here is the jsfiddle :
http://jsfiddle.net/bestiole/95EBf/
// create the audio context
if (! window.OfflineAudioContext) {
    if (! window.webkitOfflineAudioContext) {
        $('#output').append('failed : no audiocontext found, change browser');
    }
    window.OfflineAudioContext = window.webkitOfflineAudioContext;
}
//var context = new AudioContext();
var length = 15241583;
var samplerate = 44100;
var fftSamples = 2048;
var waveformWidth = 1920;

var context = new OfflineAudioContext(1,length,samplerate);
var source;
var splitter;
var analyser;
var processor;
var i=0;
var average;
var max;
var coord;

processor = context.createScriptProcessor(fftSamples, 1, 1);
processor.connect(context.destination);

analyser = context.createAnalyser();
analyser.smoothingTimeConstant = 0;
analyser.fftSize = fftSamples;

source = context.createBufferSource();
splitter = context.createChannelSplitter();
source.connect(splitter);
splitter.connect(analyser,0,0);

analyser.connect(processor);

context.oncomplete = function(){
    $('#output').append('<br />complete');
}

var request = new XMLHttpRequest();
request.open('GET', "http://www.mindthepressure.org/bounce.ogg", true);
request.responseType = 'arraybuffer';
request.onload = function(){
    $('#output').append('loaded ! ');
    context.decodeAudioData(request.response, function(buffer) {
        $('#output').append('starting analysis<br />');
        processor.onaudioprocess = function(e){
            var data =  new Uint8Array(analyser.frequencyBinCount);
            analyser.getByteFrequencyData(data);
            average = getAverageVolume(data);
            max = Math.max.apply(Math, data);
            coord = Math.min(average*2,255);
            coord = Math.round((max+coord)/2);
            ctx.fillStyle=gradient;
            ctx.fillRect(i,255-coord,1,255);
            console.log(i+' -> '+coord);
            i++;
        }
        source.buffer = buffer;
        source.start(0);
        context.startRendering();
    }, onError);
}
request.send();

function onError(e) {
    $('#output').append('error, check the console');
    console.log(e);
}

function getAverageVolume(array) {
    var values = 0;
    var average;
    var length = array.length;
    for (var k = 0; k < length; k++) {
        values += array[k];
    }
    average = values / length;
    return average;
}
(here is another version that forces waveform to fit in 1920px wide and generate a waveform data for who is interested : http://jsfiddle.net/bestiole/E3rSx/ )

i really dont get the point, how doesn't chrome treat every part of the audio file ?

thanks for any help !
解决方案
Chrome has a bug in script processors in offline mode.

                        
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