用javascript播放PCM

Question

我在浏览器上播放PCM Audio时遇到了一些问题.PCM音频来自具有udp协议的Android设备,并以*.raw保存在服务器上

我在webaudioapi的帮助下试图播放这个保存的文件失败了.使用以下代码,播放一些带有白噪声的令人毛骨悚然的声音:

var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
audioCtx.sampleRate = 16000;


// Stereo
var channels = 1;
// Create an empty two second stereo buffer at the
// sample rate of the AudioContext
var frameCount = audioCtx.sampleRate * 10.0;

var myAudioBuffer = audioCtx.createBuffer(channels, frameCount, audioCtx.sampleRate);


var req = new XMLHttpRequest();
req.open('GET', "example.raw", false);
req.overrideMimeType('text\/plain; charset=x-user-defined');
req.send(null);

function play(){
    for (var channel = 0; channel < channels; channel++) {

        var nowBuffering = myAudioBuffer.getChannelData(channel,16,16000);
        for (var i = 0; i < frameCount; i++) {
            // audio needs to be in [-1.0; 1.0]
            // for this reason I also tried to divide it by 32767
            // as my pcm sample is in 16-Bit. It plays still the
            // same creepy sound less noisy.
            nowBuffering[i] = (req.responseText.charCodeAt(i) & 0xff;

        }
    }
    // Get an AudioBufferSourceNode.
    // This is the AudioNode to use when we want to play an AudioBuffer
    var source = audioCtx.createBufferSource();
    // set the buffer in the AudioBufferSourceNode
    source.buffer = myAudioBuffer;
    // connect the AudioBufferSourceNode to the
    // destination so we can hear the sound
    source.connect(audioCtx.destination);
    // start the source playing
    source.start();
}

它正在播放如此无法识别的声音,我不确定它是否正在播放我认为必须要播放的pcm文件.

我想它必须对pcm文件做一些事情.PCM文件的采样率为16 kHz,每个采样16位,只有一个通道或者更确切地说是单通道.

在这里遇到同样问题的人或有没有人有建议来解决我的问题？

我正在寻找一些解决方案,并感谢任何帮助.

Answer 1

首先:

audioCtx.sampleRate = 16000;不起作用.您无法修改audioCtx.sampleRate.相反,您需要执行以下操作:

var frameCount = req.responseText.length / 2;
var myAudioBuffer = audioCtx.createBuffer(channels, frameCount, 16000);

因为您的文件是16位,所以它的长度(以字节为单位)是您需要的帧数的两倍.

(req.responseText.charCodeAt(i) & 0xff)将产生0到255之间的值,表示单个8位字节.你需要16位.

您需要知道样本的字节顺序,并且每次处理两个字节

对于小端(LSB优先):

var word = (req.responseText.charCodeAt(i * 2) & 0xff) + ((req.responseText.charCodeAt(i * 2 + 1) & 0xff) << 8);

对于大端(MSB优先):

var unsignedWord = ((req.responseText.charCodeAt(i * 2) & 0xff) << 8) + (req.responseText.charCodeAt(i * 2 + 1) & 0xff);

这将产生0到65535之间的数字,表示无符号的16位整数.要转换为有符号整数,您需要执行以下操作(用上面的代码替换X)

var signedWord = (unsignedWord + 32768) % 65536 - 32768;

这将产生介于-32768和32767之间的数字,然后您可以除以32768.0以获得所需的结果.

nowBuffering[i] = signedWord / 32768.0;

编辑:工作示例https://o.lgm.cl/example.html(16位LSB)