diff --git a/isom/isom.go b/isom/isom.go
index 7650eaf..951330f 100644
--- a/isom/isom.go
+++ b/isom/isom.go
@@ -181,6 +181,17 @@ func writeSampleRateIndex(w *bits.Writer, index uint) (err error) {
 	return
 }
 
+func (self MPEG4AudioConfig) Complete() (config MPEG4AudioConfig) {
+	config = self
+	if int(config.SampleRateIndex) < len(sampleRateTable) {
+		config.SampleRate = sampleRateTable[config.SampleRateIndex]
+	}
+	if int(config.ChannelConfig) < len(chanConfigTable) {
+		config.ChannelCount = chanConfigTable[config.ChannelConfig]
+	}
+	return
+}
+
 // copied from libavcodec/mpeg4audio.c avpriv_mpeg4audio_get_config()
 func ReadMPEG4AudioConfig(r io.Reader) (config MPEG4AudioConfig, err error) {
 	br := &bits.Reader{R: r}
@@ -191,15 +202,9 @@ func ReadMPEG4AudioConfig(r io.Reader) (config MPEG4AudioConfig, err error) {
 	if config.SampleRateIndex, err = readSampleRateIndex(br); err != nil {
 		return
 	}
-	if int(config.SampleRateIndex) < len(sampleRateTable) {
-		config.SampleRate = sampleRateTable[config.SampleRateIndex]
-	}
 	if config.ChannelConfig, err = br.ReadBits(4); err != nil {
 		return
 	}
-	if int(config.ChannelConfig) < len(chanConfigTable) {
-		config.ChannelCount = chanConfigTable[config.ChannelConfig]
-	}
 	return
 }
 
@@ -432,6 +437,17 @@ func ReadElemStreamDesc(r io.Reader) (decConfig []byte, err error) {
 	return
 }
 
+func ReadElemStreamDescAAC(r io.Reader) (config MPEG4AudioConfig, err error) {
+	var data []byte
+	if data, err = ReadElemStreamDesc(r); err != nil {
+		return
+	}
+	if config, err = ReadMPEG4AudioConfig(bytes.NewReader(data)); err != nil {
+		return
+	}
+	return
+}
+
 func WriteElemStreamDescAAC(w io.Writer, config MPEG4AudioConfig) (err error) {
 	// MP4ESDescrTag(ESDesc MP4DecConfigDescrTag(objectId streamType bufSize avgBitrate MP4DecSpecificDescrTag(decConfig)))
 
diff --git a/isom/isom_test.go b/isom/isom_test.go
index 00722fe..13a7558 100644
--- a/isom/isom_test.go
+++ b/isom/isom_test.go
@@ -1,10 +1,9 @@
-
 package isom
 
 import (
-	"testing"
-	"encoding/hex"
 	"bytes"
+	"encoding/hex"
+	"testing"
 )
 
 func TestReadElemStreamDesc(t *testing.T) {
@@ -23,6 +22,7 @@ func TestReadElemStreamDesc(t *testing.T) {
 	if aconfig, err = ReadMPEG4AudioConfig(bytes.NewReader(decConfig)); err != nil {
 		t.Error(err)
 	}
+	aconfig = aconfig.Complete()
 	t.Logf("aconfig=%v", aconfig)
 
 	bw := &bytes.Buffer{}
@@ -46,4 +46,3 @@ func TestReadElemStreamDesc(t *testing.T) {
 	objectType, sampleRateIndex, chanConfig, frameLength := ReadADTSHeader(data)
 	t.Logf("objectType=%d sampleRateIndex=%d chanConfig=%d frameLength=%d", objectType, sampleRateIndex, chanConfig, frameLength)
 }
-