diff --git a/demuxer.go b/demuxer.go
index 2780b7c..3304bd2 100644
--- a/demuxer.go
+++ b/demuxer.go
@@ -229,6 +229,7 @@ func (self *Stream) incSampleIndex() (duration int64) {
 		fmt.Printf("incSampleIndex sampleIndex=%d sampleOffsetInChunk=%d sampleIndexInChunk=%d chunkGroupIndex=%d chunkIndex=%d\n",
 			self.sampleIndex, self.sampleOffsetInChunk, self.sampleIndexInChunk, self.chunkGroupIndex, self.chunkIndex)
 	}
+
 	self.sampleIndexInChunk++
 	if self.sampleIndexInChunk == self.sample.SampleToChunk.Entries[self.chunkGroupIndex].SamplesPerChunk {
 		self.chunkIndex++
diff --git a/muxer.go b/muxer.go
index f60436a..82a55a9 100644
--- a/muxer.go
+++ b/muxer.go
@@ -91,7 +91,6 @@ func (self *Stream) fillTrackAtom() (err error) {
 			SubType: "vide",
 			Name:    "Video Media Handler",
 		}
-		self.sample.CompositionOffset = &atom.CompositionOffset{}
 		self.trackAtom.Media.Info.Video = &atom.VideoMediaInfo{
 			Flags: 0x000001,
 		}
@@ -131,6 +130,11 @@ func (self *Muxer) WriteHeader() (err error) {
 	if self.mdatWriter, err = atom.WriteAtomHeader(self.W, "mdat"); err != nil {
 		return
 	}
+	for _, stream := range self.streams {
+		if stream.Type().IsVideo() {
+			stream.sample.CompositionOffset = &atom.CompositionOffset{}
+		}
+	}
 	return
 }
 
diff --git a/stream.go b/stream.go
index 5252423..550b398 100644
--- a/stream.go
+++ b/stream.go
@@ -37,7 +37,6 @@ type Stream struct {
 
 	sttsEntry *atom.TimeToSampleEntry
 	cttsEntry *atom.CompositionOffsetEntry
-	writeMdat func([]byte) (int64, error)
 }
 
 func (self *Stream) timeToTs(time float64) int64 {