diff --git a/stream/mts/psi/psi.go b/stream/mts/psi/psi.go
index 31393c00..5eb9ce6e 100644
--- a/stream/mts/psi/psi.go
+++ b/stream/mts/psi/psi.go
@@ -1,9 +1,14 @@
 package psi
 
+const (
+	ESSDHeadLen       = 5
+	DescriptorHeadLen = 2
+)
+
 // Program specific information
 type PSI struct {
 	pf  byte   // Point field
-	pfb []byte // pointer filler bytes
+	pfb []byte // Pointer filler bytes
 	tid byte   // Table ID
 	ssi bool   // Sectiopn syntax indicator (1 for PAT, PMT, CAT)
 	pb  bool   // Private bit (0 for PAT, PMT, CAT)
@@ -18,7 +23,7 @@ type TSS struct {
 	cni bool   // Current/next indicator
 	sn  byte   // Section number
 	lsn byte   // Last section number
-	sd  SD     // specific data PAT/PMT
+	sd  SD     // Specific data PAT/PMT
 	crc []byte // crc32 of entire table excluding pointer field, pointer filler bytes and the trailing CRC32
 }
 
@@ -30,34 +35,72 @@ type SD interface {
 // Program association table, implements SD
 type PAT struct {
 	pn    uint16 // Program Number
-	pmpid uint16 // program map PID
+	pmpid uint16 // Program map PID
 }
 
-type Descriptor struct {
+// Program mapping table, implements SD
+type PMT struct {
+	pcrpid uint16       // Program clock reference pid
+	pil    uint16       // Program info length
+	pd     []Descriptor // Program descriptors
+	essd   []ESSD       // Elementary stream specific data
+}
+
+// Elementary stream specific data
+type ESSD struct {
+	st   byte         // Stream type
+	epid byte         // Elementary pid
+	esil uint16       // Elementary stream
+	esd  []Descriptor // Elementary stream desriptors
+}
+
+// Descriptor
+type Desc struct {
 	dt byte   // Descriptor tag
 	dl byte   // Descriptor length
 	dd []byte // Descriptor data
 }
 
-// Program mapping table, implements SD
-type PMT struct {
-	pcrpid uint16       // program clock reference pid
-	PIL    uint16       // program info length
-	PD     []Descriptor // program descriptors
-}
-
-func ReadPSI(data []byte) *PAT {
+func ReadPSI(data []byte) *PSI {
 	return nil
 }
 
-func (p *PSI) Bytes() (output []byte) {
+func (p *PSI) Bytes() (out []byte) {
 	return nil
 }
 
-func (p *PAT) Bytes() (output []byte) {
+func (t *TSS) Fill(space []byte) error {
 	return nil
 }
 
-func (p *PMT) Bytes() (output []byte) {
+func (p *PAT) Fill(space []byte) error {
 	return nil
 }
+
+func (p *PMT) Fill(space []byte) error {
+	checkSpace(space)
+	return nil
+}
+
+func (d *Descriptor) Fill(space []byte) error {
+	checkSpace(space, DescriptorHeadLen+int(d.dl))
+	space[0] = d.dt
+	space[1] = d.dl
+	copy(space[2:], d.dd)
+	return out
+}
+
+func (e *ESSD) Fill(space []byte) error {
+	checkSpace(space, ESSDHeadLen+e.esil)
+	return nil
+}
+
+func checkSpace(s []byte, c uint) {
+	if s == nil {
+		panic("Slice provided is nil")
+	} else if len(s) != 0 {
+		panic("Slice provided already has something in it")
+	} else if cap(s) != c {
+		panic("Slice provided has wrong capacity")
+	}
+}