Basic support for FLAC-in-MP4 audio tracks

Allows for creating an MP4 stream with FLAC contents using
the MP4 box generation functions or AudioSegmentStream.

Use codec string 'fLaC' and specify a 'metadata' field in the
track info with the FLAC METADATA_BLOCK_HEADER fields to go
into the dfLa box.

See https://github.com/xiph/flac/blob/master/doc/isoflac.txt
for details of the MP4 embedding of FLAC audio.

Also allows variable-length audio packets in AudioSegmentStream

FLAC may have variable-length packets, so allow passing a
duration in samples on incoming frames.

This gets FLAC with, say, 960 samples per frame working
consistently in Firefox and Chrome.

lib/mp4/audio-frame-utils.js

@@ -121,7 +121,7 @@ var generateSampleTable = function(frames) {
     currentFrame = frames[i];
     samples.push({
       size: currentFrame.data.byteLength,
-      duration: 1024 // For AAC audio, all samples contain 1024 samples
+      duration: currentFrame.duration || 1024 // For AAC audio, all samples contain 1024 samples
     });
   }
   return samples;

lib/mp4/mp4-generator.js

@@ -11,7 +11,7 @@
 
 var UINT32_MAX = Math.pow(2, 32) - 1;
 
-var box, dinf, esds, ftyp, mdat, mfhd, minf, moof, moov, mvex, mvhd,
+var box, dfLa, dinf, esds, ftyp, mdat, mfhd, minf, moof, moov, mvex, mvhd,
     trak, tkhd, mdia, mdhd, hdlr, sdtp, stbl, stsd, traf, trex,
     trun, types, MAJOR_BRAND, MINOR_VERSION, AVC1_BRAND, VIDEO_HDLR,
     AUDIO_HDLR, HDLR_TYPES, VMHD, SMHD, DREF, STCO, STSC, STSZ, STTS;
@@ -23,10 +23,12 @@ var box, dinf, esds, ftyp, mdat, mfhd, minf, moof, moov, mvex, mvhd,
     avc1: [], // codingname
     avcC: [],
     btrt: [],
+    dfLa: [],
     dinf: [],
     dref: [],
     esds: [],
     ftyp: [],
+    fLaC: [],
     hdlr: [],
     mdat: [],
     mdhd: [],
@@ -184,6 +186,13 @@ box = function(type) {
   return result;
 };
 
+dfLa = function(track) {
+  return box(types.dfLa, new Uint8Array([
+    0x00, // version
+    0x00, 0x00, 0x00 // flags
+  ]), track.metadata || []);
+};
+
 dinf = function() {
   return box(types.dinf, box(types.dref, DREF));
 };
@@ -465,7 +474,10 @@ stbl = function(track) {
   };
 
   audioSample = function(track) {
-    return box(types.mp4a, new Uint8Array([
+    var audioSampleBox = new Uint8Array([
+
+      // FLACSampleEntry extends AudioSampleEntry
+      // https://github.com/xiph/flac/blob/master/doc/isoflac.txt
 
       // SampleEntry, ISO/IEC 14496-12
       0x00, 0x00, 0x00,
@@ -488,7 +500,11 @@ stbl = function(track) {
       0x00, 0x00 // samplerate, 16.16
 
       // MP4AudioSampleEntry, ISO/IEC 14496-14
-    ]), esds(track));
+    ]);
+    if (track.codec === 'fLaC') {
+      return box(types.fLaC, audioSampleBox, dfLa(track));
+    }
+    return box(types.mp4a, audioSampleBox, esds(track));
   };
 }());