added optional safe clipping

fritzo · Feb 15, 2012 · cf25732 · cf25732
1 parent 638033e
commit cf25732
Showing 1 changed file with 40 additions and 22 deletions.
diff --git a/wavencoder.js b/wavencoder.js
@@ -47,6 +47,7 @@ var WavEncoder = (function(){
     var bytesPerSample = options.bytesPerSample || defaults.bytesPerSample;
     var numChannels    = options.numChannels    || defaults.numChannels;
     var sampleRateHz   = options.sampleRateHz   || defaults.sampleRateHz;
+    this.clip          = 'clip' in options ? !!options.clip : defaults.clip;
 
     var PCM_FORMAT = 1;
     var bitsPerSample = bytesPerSample * 8;
@@ -63,14 +64,15 @@ var WavEncoder = (function(){
 
     switch (bytesPerSample) {
       case 1:
-        this.encode = this.encode8;
+        this.encode = this._encode8;
         break;
 
       case 2:
-        this.encode = this.encode16;
+        this.encode = this._encode16;
         break;
 
-      default: throw 'unsupported bytesPerSample: ' + bytesPerSample;
+      default: throw new WavEncoderException(
+            'unsupported bytesPerSample: ' + bytesPerSample);
     }
 
     // we encode using 16-bit words
@@ -98,14 +100,14 @@ var WavEncoder = (function(){
 
     var bytesPerWord = 2;
     var dataWords = dataBytes / bytesPerWord;
-    words.length = this.headerWords + dataWords;
+    words.length = this._headerWords + dataWords;
     while (words.length % 3) words.push(0);
   };
 
   WavEncoder.prototype = {
 
-    headerBytes: 44,
-    headerWords: 22,
+    _headerBytes: 44,
+    _headerWords: 22, // for 16-bit words
 
     /**
      * @param {string}
@@ -144,24 +146,33 @@ var WavEncoder = (function(){
      * @param {number[]}
      * @returns {string}
      */
-    encode8: function (samples) {
+    _encode8: function (samples) {
       // this is hard-coded for 8-bit mono
 
       assertEqual(samples.length, this.numSamples, 'Wrong number of samples');
 
+      var floor = Math.floor;
+      var sqrt = Math.sqrt;
+      var clip = this.clip;
+
       var words = this.words;
-      var h = this.headerWords;
+      var h = this._headerWords;
       for (var t = 0, T = this.numSamples - 1; t < T; t += 2) {
         var x1 = samples[t + 0];
         var x2 = samples[t + 1];
+        if (clip) {
+          x1 /= sqrt(1 + x1 * x1);
+          x2 /= sqrt(1 + x2 * x2);
+        }
         // 8-bit samples are unsigned
-        var sample1 = Math.floor(128 * (x1 + 1));
-        var sample2 = Math.floor(128 * (x2 + 1));
+        var sample1 = floor(128 * (x1 + 1));
+        var sample2 = floor(128 * (x2 + 1));
         words[h++] = (sample1 << 8) | sample2;
       }
       if (this.numSamples % 2) {
         var x1 = samples[t + 0];
-        var sample1 = Math.floor(128 * (x1 + 1));
+        if (clip) x1 /= sqrt(1 + x1 * x1);
+        var sample1 = floor(128 * (x1 + 1));
         words[h++] = sample1 << 8;
       }
 
@@ -172,17 +183,22 @@ var WavEncoder = (function(){
      * @param {number[]}
      * @returns {string}
      */
-    encode16: function (samples) {
+    _encode16: function (samples) {
       // this is hard-coded for 16-bit mono
 
       assertEqual(samples.length, this.numSamples, 'Wrong number of samples');
 
+      var floor = Math.floor;
+      var sqrt = Math.sqrt;
+      var clip = this.clip;
+
       var words = this.words;
-      var h = this.headerWords;
+      var h = this._headerWords;
       for (var t = 0, T = this.numSamples; t < T; ++t) {
         var x = samples[t];
+        if (clip) x /= sqrt(1 + x * x);
         // 16-bit samples are signed
-        var sample = Math.floor(32768 * x);
+        var sample = floor(32768 * x);
         if (sample < 0) sample += 65536; // 2's compliment
         words[h++] = ((sample >> 8) | (sample << 8)) & 65535;
       }
@@ -195,7 +211,7 @@ var WavEncoder = (function(){
      */
     _encodeWords: function () {
       var words = this.words;
-      var pairTable = WavEncoder.pairTable;
+      var pairTable = WavEncoder._pairTable;
 
       var result = new Array(1 + words.length * 4/3);
       var r = 0;
@@ -226,7 +242,8 @@ var WavEncoder = (function(){
   WavEncoder.defaults = {
     numChannels: 1,      // mono
     sampleRateHz: 22050, // 22050 Hz
-    bytesPerSample: 2    // 16 bit
+    bytesPerSample: 2,   // 16 bit
+    clip: true
   };
 
   (function(){
@@ -239,23 +256,24 @@ var WavEncoder = (function(){
       pairTable[ij] = charTable[ij >> 6] + charTable[ij & 63];
     }
 
-    WavEncoder.pairTable = pairTable;
+    WavEncoder._pairTable = pairTable;
   })();
 
+  WavEncoder.Exception = WavEncoderException;
+
   /**
    * WavEncoder is optimized to encode many data sequences of the same length,
-   * but we provide a one-off function for convenience.
+   * but we provide this one-off function for convenience.
    *
    * @param {number[]}
+   * @param {object?}
    * @returns {string}
    */
-  WavEncoder.encode = function (data) {
-    var encoder = new WavEncoder(data.length);
+  WavEncoder.encode = function (data, options) {
+    var encoder = new WavEncoder(data.length, options);
     return encoder.encode(data);
   };
 
-  WavEncoder.Exception = WavEncoderException;
-
   return WavEncoder;
 
 }());