diff --git a/util.go b/util.go
index a5f1713..44c5636 100644
--- a/util.go
+++ b/util.go
@@ -1,16 +1,158 @@
-
 package av
 
-const (
-	H264 = 1
-	AAC = 2
+import (
+	"fmt"
+	"github.com/nareix/codec/aacparser"
+	"github.com/nareix/codec/h264parser"
 )
 
-type Packet struct {
-	Codec int
-	Key bool
-	Pos float32
-	Data []byte
-	Idx int
+type CodecType int
+
+const (
+	H264 = CodecType(1)
+	AAC  = CodecType(2)
+)
+
+func (self CodecType) String() string {
+	switch self {
+	case H264:
+		return "H264"
+	case AAC:
+		return "AAC"
+	}
+	return "?"
+}
+
+func (self CodecType) IsAudio() bool {
+	return self == AAC
+}
+
+func (self CodecType) IsVideo() bool {
+	return self == H264
+}
+
+type Stream interface {
+	IsVideo() bool
+	IsAudio() bool
+	Type() CodecType
+	SetType(CodecType)
+	CodecData() []byte
+	SetCodecData([]byte) error
+	SampleRate() int
+	ChannelCount() int
+	Width() int
+	Height() int
+	String() string
+	FillParamsByStream(Stream) error
+}
+
+type StreamCommon struct {
+	codecType     CodecType
+	codecData     []byte
+	H264CodecInfo h264parser.CodecInfo
+	AACCodecInfo  aacparser.CodecInfo
+}
+
+type Muxer interface {
+	NewStream() Stream
+	WriteHeader() error
+	WriteTrailer() error
+	WritePacket(int, Packet) error
+	SetTime(float64)
+}
+
+type Demuxer interface {
+	ReadHeader() error
+	ReadPacket() (int, Packet, error)
+	Streams() []Stream
+	Time() float64
+}
+
+type SeekableDemuxer interface {
+	Demuxer
+	SeekToTime(float64) error
+}
+
+type Packet struct {
+	IsKeyFrame      bool
+	Data            []byte
+	Duration        float64
+	CompositionTime float64
+}
+
+func (self *StreamCommon) Type() CodecType {
+	return self.codecType
+}
+
+func (self *StreamCommon) SetType(CodecType CodecType) {
+	self.codecType = CodecType
+}
+
+func (self *StreamCommon) String() string {
+	str := self.codecType.String()
+	if self.IsAudio() {
+		str += fmt.Sprintf(" %dHz %dch", self.SampleRate(), self.ChannelCount())
+	} else if self.IsVideo() {
+		str += fmt.Sprintf(" %dx%d", self.Width(), self.Height())
+	}
+	return str
+}
+
+func (self *StreamCommon) SetCodecData(data []byte) (err error) {
+	if self.codecType == H264 {
+		if self.H264CodecInfo, err = h264parser.ParseCodecData(data); err != nil {
+			return
+		}
+	} else if self.codecType == AAC {
+		if self.AACCodecInfo, err = aacparser.ParseCodecData(data); err != nil {
+			return
+		}
+	} else {
+		err = fmt.Errorf("unknown codec type=%d", self.codecType)
+	}
+	self.codecData = data
+	return
+}
+
+func (self *StreamCommon) CodecData() (data []byte) {
+	return self.codecData
+}
+
+func (self *StreamCommon) ChannelCount() int {
+	return self.AACCodecInfo.ChannelCount
+}
+
+func (self *StreamCommon) SampleRate() int {
+	return self.AACCodecInfo.SampleRate
+}
+
+func (self *StreamCommon) Width() int {
+	return int(self.H264CodecInfo.SPSInfo.Width)
+}
+
+func (self *StreamCommon) Height() int {
+	return int(self.H264CodecInfo.SPSInfo.Height)
+}
+
+func (self *StreamCommon) IsVideo() bool {
+	if self.codecType == H264 {
+		return true
+	}
+	return false
+}
+
+func (self *StreamCommon) IsAudio() bool {
+	if self.codecType == AAC {
+		return true
+	}
+	return false
+}
+
+func (self *StreamCommon) FillParamsByStream(other Stream) (err error) {
+	self.codecType = other.Type()
+	if err = self.SetCodecData(other.CodecData()); err != nil {
+		return
+	}
+	return
 }