diff --git a/internal/hls/mp4writer.go b/internal/hls/mp4writer.go
deleted file mode 100644
index f964739b..00000000
--- a/internal/hls/mp4writer.go
+++ /dev/null
@@ -1,93 +0,0 @@
-package hls
-
-import (
-	"io"
-
-	"github.com/abema/go-mp4"
-	"github.com/orcaman/writerseeker"
-)
-
-type mp4Writer struct {
-	buf *writerseeker.WriterSeeker
-	w   *mp4.Writer
-}
-
-func newMP4Writer() *mp4Writer {
-	w := &mp4Writer{
-		buf: &writerseeker.WriterSeeker{},
-	}
-
-	w.w = mp4.NewWriter(w.buf)
-
-	return w
-}
-
-func (w *mp4Writer) writeBoxStart(box mp4.IImmutableBox) (int, error) {
-	bi := &mp4.BoxInfo{
-		Type: box.GetType(),
-	}
-	var err error
-	bi, err = w.w.StartBox(bi)
-	if err != nil {
-		return 0, err
-	}
-
-	_, err = mp4.Marshal(w.w, box, mp4.Context{})
-	if err != nil {
-		return 0, err
-	}
-
-	return int(bi.Offset), nil
-}
-
-func (w *mp4Writer) writeBoxEnd() error {
-	_, err := w.w.EndBox()
-	return err
-}
-
-func (w *mp4Writer) writeBox(box mp4.IImmutableBox) (int, error) {
-	off, err := w.writeBoxStart(box)
-	if err != nil {
-		return 0, err
-	}
-
-	err = w.writeBoxEnd()
-	if err != nil {
-		return 0, err
-	}
-
-	return off, nil
-}
-
-func (w *mp4Writer) rewriteBox(off int, box mp4.IImmutableBox) error {
-	prevOff, err := w.w.Seek(0, io.SeekCurrent)
-	if err != nil {
-		return err
-	}
-
-	_, err = w.w.Seek(int64(off), io.SeekStart)
-	if err != nil {
-		return err
-	}
-
-	_, err = w.writeBoxStart(box)
-	if err != nil {
-		return err
-	}
-
-	err = w.writeBoxEnd()
-	if err != nil {
-		return err
-	}
-
-	_, err = w.w.Seek(prevOff, io.SeekStart)
-	if err != nil {
-		return err
-	}
-
-	return nil
-}
-
-func (w *mp4Writer) bytes() []byte {
-	return w.buf.Bytes()
-}
diff --git a/internal/hls/muxer_variant_fmp4_init.go b/internal/hls/muxer_variant_fmp4_init.go
index c276ad01..b2c67705 100644
--- a/internal/hls/muxer_variant_fmp4_init.go
+++ b/internal/hls/muxer_variant_fmp4_init.go
@@ -1,26 +1,28 @@
 package hls
 
 import (
-	"github.com/abema/go-mp4"
+	gomp4 "github.com/abema/go-mp4"
 	"github.com/aler9/gortsplib"
 	"github.com/aler9/gortsplib/pkg/aac"
 	"github.com/aler9/gortsplib/pkg/h264"
+
+	"github.com/aler9/rtsp-simple-server/internal/mp4"
 )
 
 type myEsds struct {
-	mp4.FullBox `mp4:"0,extend"`
-	Data        []byte `mp4:"1,size=8"`
+	gomp4.FullBox `mp4:"0,extend"`
+	Data          []byte `mp4:"1,size=8"`
 }
 
-func (*myEsds) GetType() mp4.BoxType {
-	return mp4.StrToBoxType("esds")
+func (*myEsds) GetType() gomp4.BoxType {
+	return gomp4.StrToBoxType("esds")
 }
 
 func init() { //nolint:gochecknoinits
-	mp4.AddBoxDef(&myEsds{}, 0)
+	gomp4.AddBoxDef(&myEsds{}, 0)
 }
 
-func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.TrackH264) error {
+func mp4InitGenerateVideoTrack(w *mp4.Writer, trackID int, videoTrack *gortsplib.TrackH264) error {
 	/*
 		trak
 		- tkhd
@@ -44,7 +46,7 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 			  - stco
 	*/
 
-	_, err := w.writeBoxStart(&mp4.Trak{}) // <trak>
+	_, err := w.WriteBoxStart(&gomp4.Trak{}) // <trak>
 	if err != nil {
 		return err
 	}
@@ -61,8 +63,8 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 	width := spsp.Width()
 	height := spsp.Height()
 
-	_, err = w.writeBox(&mp4.Tkhd{ // <tkhd/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Tkhd{ // <tkhd/>
+		FullBox: gomp4.FullBox{
 			Flags: [3]byte{0, 0, 3},
 		},
 		TrackID: uint32(trackID),
@@ -74,12 +76,12 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Mdia{}) // <mdia>
+	_, err = w.WriteBoxStart(&gomp4.Mdia{}) // <mdia>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Mdhd{ // <mdhd/>
+	_, err = w.WriteBox(&gomp4.Mdhd{ // <mdhd/>
 		Timescale: fmp4VideoTimescale, // the number of time units that pass per second
 		Language:  [3]byte{'u', 'n', 'd'},
 	})
@@ -87,7 +89,7 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Hdlr{ // <hdlr/>
+	_, err = w.WriteBox(&gomp4.Hdlr{ // <hdlr/>
 		HandlerType: [4]byte{'v', 'i', 'd', 'e'},
 		Name:        "VideoHandler",
 	})
@@ -95,13 +97,13 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Minf{}) // <minf>
+	_, err = w.WriteBoxStart(&gomp4.Minf{}) // <minf>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Vmhd{ // <vmhd/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Vmhd{ // <vmhd/>
+		FullBox: gomp4.FullBox{
 			Flags: [3]byte{0, 0, 1},
 		},
 	})
@@ -109,20 +111,20 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Dinf{}) // <dinf>
+	_, err = w.WriteBoxStart(&gomp4.Dinf{}) // <dinf>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Dref{ // <dref>
+	_, err = w.WriteBoxStart(&gomp4.Dref{ // <dref>
 		EntryCount: 1,
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Url{ // <url/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Url{ // <url/>
+		FullBox: gomp4.FullBox{
 			Flags: [3]byte{0, 0, 1},
 		},
 	})
@@ -130,32 +132,32 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	err = w.writeBoxEnd() // </dref>
+	err = w.WriteBoxEnd() // </dref>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </dinf>
+	err = w.WriteBoxEnd() // </dinf>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Stbl{}) // <stbl>
+	_, err = w.WriteBoxStart(&gomp4.Stbl{}) // <stbl>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Stsd{ // <stsd>
+	_, err = w.WriteBoxStart(&gomp4.Stsd{ // <stsd>
 		EntryCount: 1,
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.VisualSampleEntry{ // <avc1>
-		SampleEntry: mp4.SampleEntry{
-			AnyTypeBox: mp4.AnyTypeBox{
-				Type: mp4.BoxTypeAvc1(),
+	_, err = w.WriteBoxStart(&gomp4.VisualSampleEntry{ // <avc1>
+		SampleEntry: gomp4.SampleEntry{
+			AnyTypeBox: gomp4.AnyTypeBox{
+				Type: gomp4.BoxTypeAvc1(),
 			},
 			DataReferenceIndex: 1,
 		},
@@ -171,9 +173,9 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.AVCDecoderConfiguration{ // <avcc/>
-		AnyTypeBox: mp4.AnyTypeBox{
-			Type: mp4.BoxTypeAvcC(),
+	_, err = w.WriteBox(&gomp4.AVCDecoderConfiguration{ // <avcc/>
+		AnyTypeBox: gomp4.AnyTypeBox{
+			Type: gomp4.BoxTypeAvcC(),
 		},
 		ConfigurationVersion:       1,
 		Profile:                    spsp.ProfileIdc,
@@ -181,14 +183,14 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		Level:                      spsp.LevelIdc,
 		LengthSizeMinusOne:         3,
 		NumOfSequenceParameterSets: 1,
-		SequenceParameterSets: []mp4.AVCParameterSet{
+		SequenceParameterSets: []gomp4.AVCParameterSet{
 			{
 				Length:  uint16(len(sps)),
 				NALUnit: sps,
 			},
 		},
 		NumOfPictureParameterSets: 1,
-		PictureParameterSets: []mp4.AVCParameterSet{
+		PictureParameterSets: []gomp4.AVCParameterSet{
 			{
 				Length:  uint16(len(pps)),
 				NALUnit: pps,
@@ -199,7 +201,7 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Btrt{ // <btrt/>
+	_, err = w.WriteBox(&gomp4.Btrt{ // <btrt/>
 		MaxBitrate: 1000000,
 		AvgBitrate: 1000000,
 	})
@@ -207,56 +209,56 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 		return err
 	}
 
-	err = w.writeBoxEnd() // </avc1>
+	err = w.WriteBoxEnd() // </avc1>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </stsd>
+	err = w.WriteBoxEnd() // </stsd>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stts{ // <stts>
+	_, err = w.WriteBox(&gomp4.Stts{ // <stts>
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stsc{ // <stsc>
+	_, err = w.WriteBox(&gomp4.Stsc{ // <stsc>
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stsz{ // <stsz>
+	_, err = w.WriteBox(&gomp4.Stsz{ // <stsz>
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stco{ // <stco>
+	_, err = w.WriteBox(&gomp4.Stco{ // <stco>
 	})
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </stbl>
+	err = w.WriteBoxEnd() // </stbl>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </minf>
+	err = w.WriteBoxEnd() // </minf>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </mdia>
+	err = w.WriteBoxEnd() // </mdia>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </trak>
+	err = w.WriteBoxEnd() // </trak>
 	if err != nil {
 		return err
 	}
@@ -264,7 +266,7 @@ func mp4InitGenerateVideoTrack(w *mp4Writer, trackID int, videoTrack *gortsplib.
 	return nil
 }
 
-func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.TrackAAC) error {
+func mp4InitGenerateAudioTrack(w *mp4.Writer, trackID int, audioTrack *gortsplib.TrackAAC) error {
 	/*
 		trak
 		- tkhd
@@ -287,13 +289,13 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 			  - stco
 	*/
 
-	_, err := w.writeBoxStart(&mp4.Trak{}) // <trak>
+	_, err := w.WriteBoxStart(&gomp4.Trak{}) // <trak>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Tkhd{ // <tkhd/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Tkhd{ // <tkhd/>
+		FullBox: gomp4.FullBox{
 			Flags: [3]byte{0, 0, 3},
 		},
 		TrackID:        uint32(trackID),
@@ -305,12 +307,12 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Mdia{}) // <mdia>
+	_, err = w.WriteBoxStart(&gomp4.Mdia{}) // <mdia>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Mdhd{ // <mdhd/>
+	_, err = w.WriteBox(&gomp4.Mdhd{ // <mdhd/>
 		Timescale: uint32(audioTrack.ClockRate()),
 		Language:  [3]byte{'u', 'n', 'd'},
 	})
@@ -318,7 +320,7 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Hdlr{ // <hdlr/>
+	_, err = w.WriteBox(&gomp4.Hdlr{ // <hdlr/>
 		HandlerType: [4]byte{'s', 'o', 'u', 'n'},
 		Name:        "SoundHandler",
 	})
@@ -326,31 +328,31 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Minf{}) // <minf>
+	_, err = w.WriteBoxStart(&gomp4.Minf{}) // <minf>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Smhd{ // <smhd/>
+	_, err = w.WriteBox(&gomp4.Smhd{ // <smhd/>
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Dinf{}) // <dinf>
+	_, err = w.WriteBoxStart(&gomp4.Dinf{}) // <dinf>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Dref{ // <dref>
+	_, err = w.WriteBoxStart(&gomp4.Dref{ // <dref>
 		EntryCount: 1,
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Url{ // <url/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Url{ // <url/>
+		FullBox: gomp4.FullBox{
 			Flags: [3]byte{0, 0, 1},
 		},
 	})
@@ -358,32 +360,32 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 		return err
 	}
 
-	err = w.writeBoxEnd() // </dref>
+	err = w.WriteBoxEnd() // </dref>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </dinf>
+	err = w.WriteBoxEnd() // </dinf>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Stbl{}) // <stbl>
+	_, err = w.WriteBoxStart(&gomp4.Stbl{}) // <stbl>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Stsd{ // <stsd>
+	_, err = w.WriteBoxStart(&gomp4.Stsd{ // <stsd>
 		EntryCount: 1,
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBoxStart(&mp4.AudioSampleEntry{ // <mp4a>
-		SampleEntry: mp4.SampleEntry{
-			AnyTypeBox: mp4.AnyTypeBox{
-				Type: mp4.BoxTypeMp4a(),
+	_, err = w.WriteBoxStart(&gomp4.AudioSampleEntry{ // <mp4a>
+		SampleEntry: gomp4.SampleEntry{
+			AnyTypeBox: gomp4.AnyTypeBox{
+				Type: gomp4.BoxTypeMp4a(),
 			},
 			DataReferenceIndex: 1,
 		},
@@ -406,14 +408,14 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 	decSpecificInfoTagSize := uint8(len(conf))
 	decSpecificInfoTag := append(
 		[]byte{
-			mp4.DecSpecificInfoTag,
+			gomp4.DecSpecificInfoTag,
 			0x80, 0x80, 0x80, decSpecificInfoTagSize, // size
 		},
 		conf...,
 	)
 
 	esDescrTag := []byte{
-		mp4.ESDescrTag,
+		gomp4.ESDescrTag,
 		0x80, 0x80, 0x80, 32 + decSpecificInfoTagSize, // size
 		0x00,
 		byte(trackID), // ES_ID
@@ -421,7 +423,7 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 	}
 
 	decoderConfigDescrTag := []byte{
-		mp4.DecoderConfigDescrTag,
+		gomp4.DecoderConfigDescrTag,
 		0x80, 0x80, 0x80, 18 + decSpecificInfoTagSize, // size
 		0x40, // object type indicator (MPEG-4 Audio)
 		0x15, 0x00,
@@ -431,7 +433,7 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 	}
 
 	slConfigDescrTag := []byte{
-		mp4.SLConfigDescrTag,
+		gomp4.SLConfigDescrTag,
 		0x80, 0x80, 0x80, 0x01, // size (1)
 		0x02,
 	}
@@ -444,14 +446,14 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 	pos += copy(data[pos:], decSpecificInfoTag)
 	copy(data[pos:], slConfigDescrTag)
 
-	_, err = w.writeBox(&myEsds{ // <esds/>
+	_, err = w.WriteBox(&myEsds{ // <esds/>
 		Data: data,
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Btrt{ // <btrt/>
+	_, err = w.WriteBox(&gomp4.Btrt{ // <btrt/>
 		MaxBitrate: 128825,
 		AvgBitrate: 128825,
 	})
@@ -459,56 +461,56 @@ func mp4InitGenerateAudioTrack(w *mp4Writer, trackID int, audioTrack *gortsplib.
 		return err
 	}
 
-	err = w.writeBoxEnd() // </mp4a>
+	err = w.WriteBoxEnd() // </mp4a>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </stsd>
+	err = w.WriteBoxEnd() // </stsd>
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stts{ // <stts>
+	_, err = w.WriteBox(&gomp4.Stts{ // <stts>
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stsc{ // <stsc>
+	_, err = w.WriteBox(&gomp4.Stsc{ // <stsc>
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stsz{ // <stsz>
+	_, err = w.WriteBox(&gomp4.Stsz{ // <stsz>
 	})
 	if err != nil {
 		return err
 	}
 
-	_, err = w.writeBox(&mp4.Stco{ // <stco>
+	_, err = w.WriteBox(&gomp4.Stco{ // <stco>
 	})
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </stbl>
+	err = w.WriteBoxEnd() // </stbl>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </minf>
+	err = w.WriteBoxEnd() // </minf>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </mdia>
+	err = w.WriteBoxEnd() // </mdia>
 	if err != nil {
 		return err
 	}
 
-	err = w.writeBoxEnd() // </trak>
+	err = w.WriteBoxEnd() // </trak>
 	if err != nil {
 		return err
 	}
@@ -528,12 +530,12 @@ func mp4InitGenerate(videoTrack *gortsplib.TrackH264, audioTrack *gortsplib.Trac
 		  - trex (audio)
 	*/
 
-	w := newMP4Writer()
+	w := mp4.NewWriter()
 
-	_, err := w.writeBox(&mp4.Ftyp{ // <ftyp/>
+	_, err := w.WriteBox(&gomp4.Ftyp{ // <ftyp/>
 		MajorBrand:   [4]byte{'m', 'p', '4', '2'},
 		MinorVersion: 1,
-		CompatibleBrands: []mp4.CompatibleBrandElem{
+		CompatibleBrands: []gomp4.CompatibleBrandElem{
 			{CompatibleBrand: [4]byte{'m', 'p', '4', '1'}},
 			{CompatibleBrand: [4]byte{'m', 'p', '4', '2'}},
 			{CompatibleBrand: [4]byte{'i', 's', 'o', 'm'}},
@@ -544,12 +546,12 @@ func mp4InitGenerate(videoTrack *gortsplib.TrackH264, audioTrack *gortsplib.Trac
 		return nil, err
 	}
 
-	_, err = w.writeBoxStart(&mp4.Moov{}) // <moov>
+	_, err = w.WriteBoxStart(&gomp4.Moov{}) // <moov>
 	if err != nil {
 		return nil, err
 	}
 
-	_, err = w.writeBox(&mp4.Mvhd{ // <mvhd/>
+	_, err = w.WriteBox(&gomp4.Mvhd{ // <mvhd/>
 		Timescale:   1000,
 		Rate:        65536,
 		Volume:      256,
@@ -578,7 +580,7 @@ func mp4InitGenerate(videoTrack *gortsplib.TrackH264, audioTrack *gortsplib.Trac
 		}
 	}
 
-	_, err = w.writeBoxStart(&mp4.Mvex{}) // <mvex>
+	_, err = w.WriteBoxStart(&gomp4.Mvex{}) // <mvex>
 	if err != nil {
 		return nil, err
 	}
@@ -586,7 +588,7 @@ func mp4InitGenerate(videoTrack *gortsplib.TrackH264, audioTrack *gortsplib.Trac
 	trackID = 1
 
 	if videoTrack != nil {
-		_, err = w.writeBox(&mp4.Trex{ // <trex/>
+		_, err = w.WriteBox(&gomp4.Trex{ // <trex/>
 			TrackID:                       uint32(trackID),
 			DefaultSampleDescriptionIndex: 1,
 		})
@@ -598,7 +600,7 @@ func mp4InitGenerate(videoTrack *gortsplib.TrackH264, audioTrack *gortsplib.Trac
 	}
 
 	if audioTrack != nil {
-		_, err = w.writeBox(&mp4.Trex{ // <trex/>
+		_, err = w.WriteBox(&gomp4.Trex{ // <trex/>
 			TrackID:                       uint32(trackID),
 			DefaultSampleDescriptionIndex: 1,
 		})
@@ -607,15 +609,15 @@ func mp4InitGenerate(videoTrack *gortsplib.TrackH264, audioTrack *gortsplib.Trac
 		}
 	}
 
-	err = w.writeBoxEnd() // </mvex>
+	err = w.WriteBoxEnd() // </mvex>
 	if err != nil {
 		return nil, err
 	}
 
-	err = w.writeBoxEnd() // </moov>
+	err = w.WriteBoxEnd() // </moov>
 	if err != nil {
 		return nil, err
 	}
 
-	return w.bytes(), nil
+	return w.Bytes(), nil
 }
diff --git a/internal/hls/muxer_variant_fmp4_part.go b/internal/hls/muxer_variant_fmp4_part.go
index 950c807d..d5c25cee 100644
--- a/internal/hls/muxer_variant_fmp4_part.go
+++ b/internal/hls/muxer_variant_fmp4_part.go
@@ -7,9 +7,11 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/abema/go-mp4"
+	gomp4 "github.com/abema/go-mp4"
 	"github.com/aler9/gortsplib"
 	"github.com/aler9/gortsplib/pkg/aac"
+
+	"github.com/aler9/rtsp-simple-server/internal/mp4"
 )
 
 func durationGoToMp4(v time.Duration, timescale time.Duration) int64 {
@@ -17,10 +19,10 @@ func durationGoToMp4(v time.Duration, timescale time.Duration) int64 {
 }
 
 func mp4PartGenerateVideoTraf(
-	w *mp4Writer,
+	w *mp4.Writer,
 	trackID int,
 	videoSamples []*fmp4VideoSample,
-) (*mp4.Trun, int, error) {
+) (*gomp4.Trun, int, error) {
 	/*
 		traf
 		- tfhd
@@ -28,15 +30,15 @@ func mp4PartGenerateVideoTraf(
 		- trun
 	*/
 
-	_, err := w.writeBoxStart(&mp4.Traf{}) // <traf>
+	_, err := w.WriteBoxStart(&gomp4.Traf{}) // <traf>
 	if err != nil {
 		return nil, 0, err
 	}
 
 	flags := 0
 
-	_, err = w.writeBox(&mp4.Tfhd{ // <tfhd/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Tfhd{ // <tfhd/>
+		FullBox: gomp4.FullBox{
 			Flags: [3]byte{2, byte(flags >> 8), byte(flags)},
 		},
 		TrackID: uint32(trackID),
@@ -45,8 +47,8 @@ func mp4PartGenerateVideoTraf(
 		return nil, 0, err
 	}
 
-	_, err = w.writeBox(&mp4.Tfdt{ // <tfdt/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Tfdt{ // <tfdt/>
+		FullBox: gomp4.FullBox{
 			Version: 1,
 		},
 		// sum of decode durations of all earlier samples
@@ -63,8 +65,8 @@ func mp4PartGenerateVideoTraf(
 	flags |= 0x400 // sample flags present
 	flags |= 0x800 // sample composition time offset present or v1
 
-	trun := &mp4.Trun{ // <trun/>
-		FullBox: mp4.FullBox{
+	trun := &gomp4.Trun{ // <trun/>
+		FullBox: gomp4.FullBox{
 			Version: 1,
 			Flags:   [3]byte{0, byte(flags >> 8), byte(flags)},
 		},
@@ -79,7 +81,7 @@ func mp4PartGenerateVideoTraf(
 			flags |= 1 << 16 // sample_is_non_sync_sample
 		}
 
-		trun.Entries = append(trun.Entries, mp4.TrunEntry{
+		trun.Entries = append(trun.Entries, gomp4.TrunEntry{
 			SampleDuration:                uint32(durationGoToMp4(e.duration(), fmp4VideoTimescale)),
 			SampleSize:                    uint32(len(e.avcc)),
 			SampleFlags:                   flags,
@@ -87,12 +89,12 @@ func mp4PartGenerateVideoTraf(
 		})
 	}
 
-	trunOffset, err := w.writeBox(trun)
+	trunOffset, err := w.WriteBox(trun)
 	if err != nil {
 		return nil, 0, err
 	}
 
-	err = w.writeBoxEnd() // </traf>
+	err = w.WriteBoxEnd() // </traf>
 	if err != nil {
 		return nil, 0, err
 	}
@@ -101,11 +103,11 @@ func mp4PartGenerateVideoTraf(
 }
 
 func mp4PartGenerateAudioTraf(
-	w *mp4Writer,
+	w *mp4.Writer,
 	trackID int,
 	audioTrack *gortsplib.TrackAAC,
 	audioSamples []*fmp4AudioSample,
-) (*mp4.Trun, int, error) {
+) (*gomp4.Trun, int, error) {
 	/*
 		traf
 		- tfhd
@@ -117,15 +119,15 @@ func mp4PartGenerateAudioTraf(
 		return nil, 0, nil
 	}
 
-	_, err := w.writeBoxStart(&mp4.Traf{}) // <traf>
+	_, err := w.WriteBoxStart(&gomp4.Traf{}) // <traf>
 	if err != nil {
 		return nil, 0, err
 	}
 
 	flags := 0
 
-	_, err = w.writeBox(&mp4.Tfhd{ // <tfhd/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Tfhd{ // <tfhd/>
+		FullBox: gomp4.FullBox{
 			Flags: [3]byte{2, byte(flags >> 8), byte(flags)},
 		},
 		TrackID: uint32(trackID),
@@ -134,8 +136,8 @@ func mp4PartGenerateAudioTraf(
 		return nil, 0, err
 	}
 
-	_, err = w.writeBox(&mp4.Tfdt{ // <tfdt/>
-		FullBox: mp4.FullBox{
+	_, err = w.WriteBox(&gomp4.Tfdt{ // <tfdt/>
+		FullBox: gomp4.FullBox{
 			Version: 1,
 		},
 		// sum of decode durations of all earlier samples
@@ -150,8 +152,8 @@ func mp4PartGenerateAudioTraf(
 	flags |= 0x100 // sample duration present
 	flags |= 0x200 // sample size present
 
-	trun := &mp4.Trun{ // <trun/>
-		FullBox: mp4.FullBox{
+	trun := &gomp4.Trun{ // <trun/>
+		FullBox: gomp4.FullBox{
 			Version: 0,
 			Flags:   [3]byte{0, byte(flags >> 8), byte(flags)},
 		},
@@ -159,18 +161,18 @@ func mp4PartGenerateAudioTraf(
 	}
 
 	for _, e := range audioSamples {
-		trun.Entries = append(trun.Entries, mp4.TrunEntry{
+		trun.Entries = append(trun.Entries, gomp4.TrunEntry{
 			SampleDuration: uint32(durationGoToMp4(e.duration(), time.Duration(audioTrack.ClockRate()))),
 			SampleSize:     uint32(len(e.au)),
 		})
 	}
 
-	trunOffset, err := w.writeBox(trun)
+	trunOffset, err := w.WriteBox(trun)
 	if err != nil {
 		return nil, 0, err
 	}
 
-	err = w.writeBoxEnd() // </traf>
+	err = w.WriteBoxEnd() // </traf>
 	if err != nil {
 		return nil, 0, err
 	}
@@ -192,14 +194,14 @@ func mp4PartGenerate(
 		mdat
 	*/
 
-	w := newMP4Writer()
+	w := mp4.NewWriter()
 
-	moofOffset, err := w.writeBoxStart(&mp4.Moof{}) // <moof>
+	moofOffset, err := w.WriteBoxStart(&gomp4.Moof{}) // <moof>
 	if err != nil {
 		return nil, err
 	}
 
-	_, err = w.writeBox(&mp4.Mfhd{ // <mfhd/>
+	_, err = w.WriteBox(&gomp4.Mfhd{ // <mfhd/>
 		SequenceNumber: 0,
 	})
 	if err != nil {
@@ -208,7 +210,7 @@ func mp4PartGenerate(
 
 	trackID := 1
 
-	var videoTrun *mp4.Trun
+	var videoTrun *gomp4.Trun
 	var videoTrunOffset int
 	if videoTrack != nil {
 		var err error
@@ -221,7 +223,7 @@ func mp4PartGenerate(
 		trackID++
 	}
 
-	var audioTrun *mp4.Trun
+	var audioTrun *gomp4.Trun
 	var audioTrunOffset int
 	if audioTrack != nil {
 		var err error
@@ -231,12 +233,12 @@ func mp4PartGenerate(
 		}
 	}
 
-	err = w.writeBoxEnd() // </moof>
+	err = w.WriteBoxEnd() // </moof>
 	if err != nil {
 		return nil, err
 	}
 
-	mdat := &mp4.Mdat{} // <mdat/>
+	mdat := &gomp4.Mdat{} // <mdat/>
 
 	dataSize := 0
 	videoDataSize := 0
@@ -269,14 +271,14 @@ func mp4PartGenerate(
 		}
 	}
 
-	mdatOffset, err := w.writeBox(mdat)
+	mdatOffset, err := w.WriteBox(mdat)
 	if err != nil {
 		return nil, err
 	}
 
 	if videoTrack != nil {
 		videoTrun.DataOffset = int32(mdatOffset - moofOffset + 8)
-		err = w.rewriteBox(videoTrunOffset, videoTrun)
+		err = w.RewriteBox(videoTrunOffset, videoTrun)
 		if err != nil {
 			return nil, err
 		}
@@ -284,13 +286,13 @@ func mp4PartGenerate(
 
 	if audioTrack != nil && audioTrun != nil {
 		audioTrun.DataOffset = int32(videoDataSize + mdatOffset - moofOffset + 8)
-		err = w.rewriteBox(audioTrunOffset, audioTrun)
+		err = w.RewriteBox(audioTrunOffset, audioTrun)
 		if err != nil {
 			return nil, err
 		}
 	}
 
-	return w.bytes(), nil
+	return w.Bytes(), nil
 }
 
 func fmp4PartName(id uint64) string {
diff --git a/internal/mp4/writer.go b/internal/mp4/writer.go
new file mode 100644
index 00000000..906d78d1
--- /dev/null
+++ b/internal/mp4/writer.go
@@ -0,0 +1,100 @@
+package mp4
+
+import (
+	"io"
+
+	gomp4 "github.com/abema/go-mp4"
+	"github.com/orcaman/writerseeker"
+)
+
+// Writer is a MP4 writer.
+type Writer struct {
+	buf *writerseeker.WriterSeeker
+	w   *gomp4.Writer
+}
+
+// NewWriter allocates a Writer.
+func NewWriter() *Writer {
+	w := &Writer{
+		buf: &writerseeker.WriterSeeker{},
+	}
+
+	w.w = gomp4.NewWriter(w.buf)
+
+	return w
+}
+
+// WriteBoxStart writes a box start.
+func (w *Writer) WriteBoxStart(box gomp4.IImmutableBox) (int, error) {
+	bi := &gomp4.BoxInfo{
+		Type: box.GetType(),
+	}
+	var err error
+	bi, err = w.w.StartBox(bi)
+	if err != nil {
+		return 0, err
+	}
+
+	_, err = gomp4.Marshal(w.w, box, gomp4.Context{})
+	if err != nil {
+		return 0, err
+	}
+
+	return int(bi.Offset), nil
+}
+
+// WriteBoxEnd writes a box end.
+func (w *Writer) WriteBoxEnd() error {
+	_, err := w.w.EndBox()
+	return err
+}
+
+// WriteBox writes a self-closing box.
+func (w *Writer) WriteBox(box gomp4.IImmutableBox) (int, error) {
+	off, err := w.WriteBoxStart(box)
+	if err != nil {
+		return 0, err
+	}
+
+	err = w.WriteBoxEnd()
+	if err != nil {
+		return 0, err
+	}
+
+	return off, nil
+}
+
+// RewriteBox rewrites a box.
+func (w *Writer) RewriteBox(off int, box gomp4.IImmutableBox) error {
+	prevOff, err := w.w.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return err
+	}
+
+	_, err = w.w.Seek(int64(off), io.SeekStart)
+	if err != nil {
+		return err
+	}
+
+	_, err = w.WriteBoxStart(box)
+	if err != nil {
+		return err
+	}
+
+	err = w.WriteBoxEnd()
+	if err != nil {
+		return err
+	}
+
+	_, err = w.w.Seek(prevOff, io.SeekStart)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Bytes returns the MP4 content.
+func (w *Writer) Bytes() []byte {
+	return w.buf.Bytes()
+}