Convert blob to WAV file without loosing data or compressing

问题

I am working on recording speeches and converting them to downloadable WAV files. I am using Angular6 and MediaRecorder. Firstly I obtain the blob and from the blob, I get the .wav file. The problem is that the WAV file (which can be played and sounds good) loss much of its properties during the process and is not a valid WAV. It keeps being a WebM file. For further processing, I need really valid and high-quality WAV files. In the end, I get files of ~20KB, instead of bigger files of ~300KB.

My code looks like this:

//convert Blob to File. Pass the blob and the file title as arguments
var blobToFile = (theBlob: Blob, fileName:string): File => {
  var b: any = theBlob;
  //Add properties to the blob
  b.lastModifiedDate = new Date();
  b.name = fileName;
  return <File>theBlob;
}

var browser = <any>navigator;  
var headers = new Headers();  
var audioCtx = new AudioContext();
var chunks =[];
var constraints = { audio: true, video: false }; 

var promisifiedOldGUM = function(constraints, successCallback, errorCallback) {
  var getUserMedia = (browser.getUserMedia || browser.webkitGetUserMedia || browser.mozGetUserMedia || browser.msGetUserMedia);

  // Some browsers just don't implement it - return a rejected promise with an error to keep a consistent interface
  if(!getUserMedia) {
    return Promise.reject(new Error('getUserMedia is not implemented in this browser'));
  }

  // Otherwise, wrap the call to the old navigator.getUserMedia with a Promise
  return new Promise(function(successCallback, errorCallback) {
    getUserMedia.call(browser, constraints, successCallback, errorCallback);
  });

}
if (browser.mediaDevices.getUserMedia) {
  browser.mediaDevices.getUserMedia(constraints).then((stream) => { 
  this.mediaRecorder = new MediaRecorder(stream);        

this.mediaRecorder.onstop  = function(){
        var last_bit= chunks[chunks.length-1];
        var blob = new Blob([last_bit], { 'type' : 'audio/wav' });
        var audioURL = window.URL.createObjectURL(blob);
        //convert Blob to file 
        var file = blobToFile(blob, "my-recording.wav"); 

        var link = document.createElement("a");
        link.href = audioURL;
        link.download = 'audio_recording_' + new Date().getTime() + '.wav';
        link.innerHTML = "download file";
        document.body.appendChild(link);  
};     

And the MediaRecorder setup in typings.d.ts file looks like this:

declare class MediaRecorder extends EventTarget {

// readonly mimeType: string;
readonly MimeType: 'audio/x-wav';  // 'audio/vnd.wav';
readonly state: 'inactive' | 'recording' | 'paused';
readonly stream: MediaStream;
ignoreMutedMedia: boolean;
videoBitsPerSecond: number;
audioBitsPerSecond: 16000//number;

ondataavailable: (event : MediaRecorderDataAvailableEvent) => void;
onerror: (event: MediaRecorderErrorEvent) => void;
onpause: () => void;
onresume: () => void;
onstart: () => void;
onstop: () => void;

constructor(stream: MediaStream);

start();

stop();

resume();

pause();

isTypeSupported(type: string): boolean;

requestData();


addEventListener<K extends keyof MediaRecorderEventMap>(type: K, listener: (this: MediaStream, ev: MediaRecorderEventMap[K]) => any, options?: boolean | AddEventListenerOptions): void;

addEventListener(type: string, listener: EventListenerOrEventListenerObject, options?: boolean | AddEventListenerOptions): void;

removeEventListener<K extends keyof MediaRecorderEventMap>(type: K, listener: (this: MediaStream, ev: MediaRecorderEventMap[K]) => any, options?: boolean | EventListenerOptions): void;

removeEventListener(type: string, listener: EventListenerOrEventListenerObject, options?: boolean | EventListenerOptions): void;

} 

I don't know how can change the blobToFile() function in order to preserve the recording quality during conversion. If you can help me with this, I would appreciate very much.

回答1:

As I can see in the metadatas of the file you've linked in the comment. You use a codec (here Opus) that compresses the file.

I see two solutions:

• Reduce the compressing ratio of the codec if you can.
• Use WAV container with an uncompressed configuration (for example PCM)

as you described me the problem in the chat I think it's more of the second solution. I don't use mediarecorder myself but I've found this method to check if a mime type works with it.

MediaRecorder.isTypeSupported("audio/wav;codecs=MS_PCM")

then you I suggest you change the mime type when creating the Blob to

new Blob(chunks, { 'type' : 'audio/wav; codecs=MS_PCM' });

or

new Blob(chunks, { 'type' : 'audio/wav; codecs=0' });

回答2:

You can just convert the results of your mediaRecorder instance to wav by creating a new Blob. Easy but not that obvious, and only available with Chrome (I tested this with v77.0.3865.90).

Even if MediaRecorder.isTypeSupported('audio/wav') returns false on your browser's console you can do the following:

    navigator.mediaDevices.getUserMedia({ audio: true })
      .then(stream => {
          const mediaRecorder = new MediaRecorder(stream);
            this.mediaRecorder.ondataavailable = (e) => {
                const blobDataInWebaFormat = e.data; // .weba = webaudio; subset of webm
                const blobDataInWavFormat: Blob = new Blob([blobDataInWebaFormat], { type : 'audio/wav; codecs=0' });
                const dataUrl = URL.createObjectURL(blobDataInWavFormat);
                console.log(dataUrl); // There you can listen to your recording in a wav format
            });
            };
      });

Hope it's useful for someone, even one year later!

来源：https://stackoverflow.com/questions/52021331/convert-blob-to-wav-file-without-loosing-data-or-compressing