diff --git a/go.mod b/go.mod
index 438d886..a1251c5 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module git.netflux.io/rob/avframes
 
 go 1.15
+
+require github.com/stretchr/testify v1.7.0
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..7b4dc41
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,11 @@
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
+github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/main.go b/main.go
index ed4d18d..75e0c42 100644
--- a/main.go
+++ b/main.go
@@ -2,16 +2,32 @@ package main
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 	"log"
 	"os"
 )
 
-const fname = "/home/rob/Desktop/example.mp3"
+const fname = "/home/rob/home-office/example.mp3"
 
-const syncHeader1 = 0xFF
-const syncHeader2 = 0xFE
+const (
+	maskSync        = 0b11111111111000000000000000000000
+	maskVersion     = 0b00000000000110000000000000000000
+	maskLayer       = 0b00000000000001100000000000000000
+	maskProtection  = 0b00000000000000010000000000000000
+	maskBitrate     = 0b00000000000000001111000000000000
+	maskSamplerate  = 0b00000000000000000000110000000000
+	maskPadding     = 0b00000000000000000000001000000000
+	maskPrivate     = 0b00000000000000000000000100000000
+	maskChannelMode = 0b00000000000000000000000011000000
+	maskExtension   = 0b00000000000000000000000000110000
+	maskCopyright   = 0b00000000000000000000000000001000
+	maskOriginal    = 0b00000000000000000000000000000100
+	maskEmphasis    = 0b00000000000000000000000000000011
+)
+
+type header uint32
 
 func main() {
 	f, err := os.Open(fname)
@@ -26,18 +42,34 @@ func main() {
 	for {
 		_, err := io.ReadFull(bufreader, buf)
 		if err != nil {
+			if err == io.EOF {
+				break
+			}
 			log.Fatal(err)
 		}
 
-		if parseBytes(buf) {
-			fmt.Printf("Got possible header %X at %d\n", buf, i)
-		}
+		header, err := newHeader(buf)
 
+		if err == nil {
+			fmt.Printf("Got header: %08X\n", header)
+		}
 		i++
-		bufreader.UnreadByte()
+
+		bufreader.Discard(3)
 	}
 }
 
-func parseBytes(buf []byte) bool {
-	return buf[0]&syncHeader1 == syncHeader1 || buf[0]&syncHeader2 == syncHeader2
+// TODO use unsafe here? https://codereview.stackexchange.com/a/60161
+func newHeader(buf []byte) (header, error) {
+	if len(buf) < 4 {
+		return 0, errors.New("insufficient bytes")
+	}
+	if buf[0] != 0xFF || buf[1]&0xE0 != 0xE0 {
+		return 0, errors.New("no header")
+	}
+
+	// always little-endian for now
+	n := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+
+	return header(n), nil
 }
diff --git a/main_test.go b/main_test.go
new file mode 100644
index 0000000..11f2218
--- /dev/null
+++ b/main_test.go
@@ -0,0 +1,35 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMain(t *testing.T) {
+	testCases := []struct {
+		input         []byte
+		expectedError string
+	}{
+		{
+			input:         []byte{0, 0, 0, 0},
+			expectedError: "no header",
+		},
+		{
+			input:         []byte{0, 0, 0, 1},
+			expectedError: "no header",
+		},
+		{
+			input:         []byte{0xff, 0xfe, 0, 0}, // wrong
+			expectedError: "",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("Header %08b", tc.input), func(t *testing.T) {
+			_, err := newHeader(tc.input)
+			assert.EqualError(t, err, tc.expectedError)
+		})
+	}
+}