改良HZRecorder.js的采样压缩

hz.js

(function (window) {
    //兼容
    window.URL = window.URL || window.webkitURL;
    navigator.getUserMedia = navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia;

    //**blob to dataURL**
    var blobToDataURL =  function (blob, callback) {
        var a = new FileReader();
        a.onload = function (e) { callback(e.target.result); }
        a.readAsDataURL(blob);
    }

    /** 降低采样率 @rate 新的采样率  @sampleRate 现在的采样率 **/
    function downsampleBuffer(buffer, rate, sampleRate) {
        if (rate == sampleRate) {
            return buffer;
        }
        if (rate > sampleRate) {
            throw "downsampling rate show be smaller than original sample rate";
        }
        var sampleRateRatio = sampleRate / rate;
        var newLength = Math.round(buffer.length / sampleRateRatio);
        var result = new Float32Array(newLength);
        var offsetResult = 0;
        var offsetBuffer = 0;
        while (offsetResult < result.length) {
            var nextOffsetBuffer = Math.round((offsetResult + 1) * sampleRateRatio);
            var accum = 0, count = 0;
            for (var i = offsetBuffer; i < nextOffsetBuffer && i < buffer.length; i++) {
                accum += buffer[i];
                count++;
            }
            result[offsetResult] = accum / count;
            offsetResult++;
            offsetBuffer = nextOffsetBuffer;
        }
        return result;
    }

    var HZRecorder = function (stream, config) {
        config = config || {};
        config.sampleBits = config.sampleBits || 16;      //采样数位 8, 16
        config.sampleRate = config.sampleRate || (16000);   //采样率(1/6 44100)

        var context = new AudioContext();
        var audioInput = context.createMediaStreamSource(stream);
        var recorder = context.createScriptProcessor(4096, 1, 1);

        var audioData = {
            size: 0          //录音文件长度
            , buffer: []     //录音缓存
            , inputSampleRate: context.sampleRate    //输入采样率
            , inputSampleBits: 16       //输入采样数位 8, 16
            , outputSampleRate: config.sampleRate    //输出采样率
            , oututSampleBits: config.sampleBits       //输出采样数位 8, 16
            , input: function (data) {
                this.buffer.push(new Float32Array(data));
                this.size += data.length;
            }
            , compress: function () { //合并压缩
                //合并
                var data = new Float32Array(this.size);
                var offset = 0;
                for (var i = 0; i < this.buffer.length; i++) {
                    data.set(this.buffer[i], offset);
                    offset += this.buffer[i].length;
                }
                //压缩
                return downsampleBuffer(data, this.outputSampleRate, this.inputSampleRate);
                /** 以下的压缩方法会导致声音变慢，使用downsampleBuffer来降低采样率 **/
                // var compression = parseInt(this.inputSampleRate / this.outputSampleRate);
                // var length = data.length / compression;
                // var result = new Float32Array(length);
                // var index = 0, j = 0;
                // while (index < length) {
                //     result[index] = data[j];
                //     j += compression;
                //     index++;
                // }
                // return result;
            }
            , encodeWAV: function () {
                var sampleRate = Math.min(this.inputSampleRate, this.outputSampleRate);
                var sampleBits = Math.min(this.inputSampleBits, this.oututSampleBits);
                var bytes = this.compress();
                var dataLength = bytes.length * (sampleBits / 8);
                var buffer = new ArrayBuffer(44 + dataLength);
                var data = new DataView(buffer);

                var channelCount = 1;//单声道
                var offset = 0;

                var writeString = function (str) {
                    for (var i = 0; i < str.length; i++) {
                        data.setUint8(offset + i, str.charCodeAt(i));
                    }
                }

                // 资源交换文件标识符
                writeString('RIFF'); offset += 4;
                // 下个地址开始到文件尾总字节数,即文件大小-8
                data.setUint32(offset, 36 + dataLength, true); offset += 4;
                // WAV文件标志
                writeString('WAVE'); offset += 4;
                // 波形格式标志
                writeString('fmt '); offset += 4;
                // 过滤字节,一般为 0x10 = 16
                data.setUint32(offset, 16, true); offset += 4;
                // 格式类别 (PCM形式采样数据)
                data.setUint16(offset, 1, true); offset += 2;
                // 通道数
                data.setUint16(offset, channelCount, true); offset += 2;
                // 采样率,每秒样本数,表示每个通道的播放速度
                data.setUint32(offset, sampleRate, true); offset += 4;
                // 波形数据传输率 (每秒平均字节数) 单声道×每秒数据位数×每样本数据位/8
                data.setUint32(offset, channelCount * sampleRate * (sampleBits / 8), true); offset += 4;
                // 快数据调整数 采样一次占用字节数 单声道×每样本的数据位数/8
                data.setUint16(offset, channelCount * (sampleBits / 8), true); offset += 2;
                // 每样本数据位数
                data.setUint16(offset, sampleBits, true); offset += 2;
                // 数据标识符
                writeString('data'); offset += 4;
                // 采样数据总数,即数据总大小-44
                data.setUint32(offset, dataLength, true); offset += 4;
                // 写入采样数据
                if (sampleBits === 8) {
                    for (var i = 0; i < bytes.length; i++, offset++) {
                        var s = Math.max(-1, Math.min(1, bytes[i]));
                        var val = s < 0 ? s * 0x8000 : s * 0x7FFF;
                        val = parseInt(255 / (65535 / (val + 32768)));
                        data.setInt8(offset, val, true);
                    }
                } else {
                    for (var i = 0; i < bytes.length; i++, offset += 2) {
                        var s = Math.max(-1, Math.min(1, bytes[i]));
                        data.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7FFF, true);
                    }
                }

                return new Blob([data], { type: 'audio/wav' });
            }
        };


        //开始录音
        this.start = function () {
            audioInput.connect(recorder);
            recorder.connect(context.destination);
        }


        //停止
        this.stop = function () {
            recorder.disconnect();
        }


        //获取音频文件
        this.getBlob = function () {
            this.stop();
            return audioData.encodeWAV();
        }


        //回放
        this.play = function (audio) {
            audio.src = window.URL.createObjectURL(this.getBlob());
        }


        //上传
        this.upload = function (url, callback) {

            blobToDataURL(this.getBlob(), function(dataUrlBase){
                let audioBase64 = dataUrlBase.split(',')[1];
                fetch('//your-server-domain.com/post-audio', {
                  method: 'POST', 
                  headers: {
                    'Content-Type': 'application/json'
                  },
                  body: JSON.stringify({audioBase64})
                });
            });
        }


        //音频采集
        recorder.onaudioprocess = function (e) {
            audioData.input(e.inputBuffer.getChannelData(0));
            //record(e.inputBuffer.getChannelData(0));
        }


    };
    //抛出异常
    HZRecorder.throwError = function (message) {
        alert(message);
        throw new function () { this.toString = function () { return message; } }
    }
    //是否支持录音
    HZRecorder.canRecording = (navigator.getUserMedia != null);
    //获取录音机
    HZRecorder.get = function (callback, config) {
        if (callback) {
            if (navigator.getUserMedia) {
                navigator.getUserMedia(
                    { audio: true } //只启用音频
                    , function (stream) {
                        var rec = new HZRecorder(stream, config);
                        callback(rec);
                    }
                    , function (error) {
                        switch (error.code || error.name) {
                            case 'PERMISSION_DENIED':
                            case 'PermissionDeniedError':
                                HZRecorder.throwError('用户拒绝提供信息。');
                                break;
                            case 'NOT_SUPPORTED_ERROR':
                            case 'NotSupportedError':
                                HZRecorder.throwError('浏览器不支持硬件设备。');
                                break;
                            case 'MANDATORY_UNSATISFIED_ERROR':
                            case 'MandatoryUnsatisfiedError':
                                HZRecorder.throwError('无法发现指定的硬件设备。');
                                break;
                            default:
                                console.log('无法打开麦克风。异常信息:', error);
                                HZRecorder.throwError('无法打开麦克风。异常信息:' + (error.code || error.name));
                                break;
                        }
                    });
            } else {
                HZRecorder.throwErr('当前浏览器不支持录音功能。'); return;
            }
        }
    }


    window.HZRecorder = HZRecorder;


})(window);

重新采样有许多方法只是简单的做插值计算就可以，
但对于像 44100 / 16000 = 2.75625 这样的浮点数factor，这种情况需要更复杂的算法。

解决方案来自于

https://stackoverflow.com/a/26245260/9286894

其他可能的解决方案

• Use OfflineAudioContext
  https://stackoverflow.com/a/27601521/9286894
• XAudioJS resampler
  https://stackoverflow.com/a/30032095/9286894
  https://github.com/taisel/XAudioJS/blob/master/resampler.js