diff --git a/aacparser/parser.go b/aacparser/parser.go
index 2bd4bf3..03733d4 100644
--- a/aacparser/parser.go
+++ b/aacparser/parser.go
@@ -3,6 +3,7 @@ package aacparser
 import (
 	"github.com/nareix/bits"
 	"github.com/nareix/av"
+	"time"
 	"fmt"
 	"bytes"
 	"io"
@@ -333,15 +334,7 @@ type CodecData struct {
 	Config MPEG4AudioConfig
 }
 
-func (self CodecData) IsVideo() bool {
-	return false
-}
-
-func (self CodecData) IsAudio() bool {
-	return true
-}
-
-func (self CodecData) Type() int {
+func (self CodecData) Type() av.CodecType {
 	return av.AAC
 }
 
@@ -361,6 +354,11 @@ func (self CodecData) SampleFormat() av.SampleFormat {
 	return av.FLTP
 }
 
+func (self CodecData) PacketDuration(data []byte) (dur time.Duration, err error) {
+	dur = time.Duration(1024) * time.Second / time.Duration(self.Config.SampleRate)
+	return
+}
+
 func (self CodecData) MakeADTSHeader(samples int, payloadLength int) []byte {
 	return MakeADTSHeader(self.Config, samples, payloadLength)
 }
diff --git a/codec.go b/codec.go
index e803d23..ab1ad46 100644
--- a/codec.go
+++ b/codec.go
@@ -2,54 +2,42 @@ package codec
 
 import (
 	"github.com/nareix/av"
+	"time"
 )
 
-type AudioCodecData struct {
-	CodecType int
-	CodecSampleRate int
-	CodecChannelLayout av.ChannelLayout
-	CodecSampleFormat av.SampleFormat
+type PCMUCodecData struct {
+	typ av.CodecType
 }
 
-func (self AudioCodecData) Type() int {
-	return self.CodecType
+func (self PCMUCodecData) Type() av.CodecType {
+	return self.typ
 }
 
-func (self AudioCodecData) IsAudio() bool {
-	return true
+func (self PCMUCodecData) SampleRate() int {
+	return 8000
 }
 
-func (self AudioCodecData) IsVideo() bool {
-	return false
+func (self PCMUCodecData) ChannelLayout() av.ChannelLayout {
+	return av.CH_MONO
 }
 
-func (self AudioCodecData) SampleRate() int {
-	return self.CodecSampleRate
+func (self PCMUCodecData) SampleFormat() av.SampleFormat {
+	return av.S16
 }
 
-func (self AudioCodecData) ChannelLayout() av.ChannelLayout {
-	return self.CodecChannelLayout
-}
-
-func (self AudioCodecData) SampleFormat() av.SampleFormat {
-	return self.CodecSampleFormat
+func (self PCMUCodecData) PacketDuration(data []byte) (time.Duration, error) {
+	return time.Duration(len(data)) * time.Second / time.Duration(8000), nil
 }
 
 func NewPCMMulawCodecData() av.AudioCodecData {
-	return AudioCodecData{
-		CodecType: av.PCM_MULAW,
-		CodecSampleFormat: av.S16,
-		CodecChannelLayout: av.CH_MONO,
-		CodecSampleRate: 8000,
+	return PCMUCodecData{
+		typ: av.PCM_MULAW,
 	}
 }
 
 func NewPCMAlawCodecData() av.AudioCodecData {
-	return AudioCodecData{
-		CodecType: av.PCM_ALAW,
-		CodecSampleFormat: av.S16,
-		CodecChannelLayout: av.CH_MONO,
-		CodecSampleRate: 8000,
+	return PCMUCodecData{
+		typ: av.PCM_ALAW,
 	}
 }
 
diff --git a/h264parser/parser.go b/h264parser/parser.go
index f3ae9b9..ea3506b 100644
--- a/h264parser/parser.go
+++ b/h264parser/parser.go
@@ -543,14 +543,10 @@ type CodecData struct {
 	SPSInfo SPSInfo
 }
 
-func (self CodecData) Type() int {
+func (self CodecData) Type() av.CodecType {
 	return av.H264
 }
 
-func (self CodecData) IsVideo() bool {
-	return true
-}
-
 func (self CodecData) AVCDecoderConfRecordBytes() []byte {
 	return self.Record
 }
@@ -563,10 +559,6 @@ func (self CodecData) PPS() []byte {
 	return self.RecordInfo.PPS[0]
 }
 
-func (self CodecData) IsAudio() bool {
-	return false
-}
-
 func (self CodecData) Width() int {
 	return int(self.SPSInfo.Width)
 }