make Opus SDP always return 48khz and 2 channels (#204)

RFC7587 mandates 48khz as sample rate and 2 channels inside the SDP.
These values can be dynamically adjusted by the stream, but they must
not be touched inside the SDP.

examples/client-publish-format-opus/main.go

@@ -40,8 +40,7 @@ func main() {
 		Type: media.TypeAudio,
 		Formats: []format.Format{&format.Opus{
 			PayloadTyp: 96,
-			SampleRate:   48000,
-			ChannelCount: 2,
+			IsStereo:   false,
 		}},
 	}
 

pkg/format/format_test.go

@@ -297,8 +297,7 @@ func TestNewFromMediaDescription(t *testing.T) {
 			},
 			&Opus{
 				PayloadTyp: 96,
-				SampleRate:   48000,
-				ChannelCount: 2,
+				IsStereo:   true,
 			},
 		},
 		{

pkg/format/opus.go

@@ -13,8 +13,7 @@ import (
 // Opus is a format that uses the Opus codec.
 type Opus struct {
 	PayloadTyp uint8
-	SampleRate   int
-	ChannelCount int
+	IsStereo   bool
 }
 
 // String implements Format.
@@ -24,7 +23,9 @@ func (t *Opus) String() string {
 
 // ClockRate implements Format.
 func (t *Opus) ClockRate() int {
-	return t.SampleRate
+	// RFC7587: the RTP timestamp is incremented with a 48000 Hz
+	// clock rate for all modes of Opus and all sampling rates.
+	return 48000
 }
 
 // PayloadType implements Format.
@@ -44,13 +45,36 @@ func (t *Opus) unmarshal(payloadType uint8, clock string, codec string, rtpmap s
 	if err != nil {
 		return err
 	}
-	t.SampleRate = int(sampleRate)
+	if sampleRate != 48000 {
+		return fmt.Errorf("invalid sample rate: %d", sampleRate)
+	}
 
 	channelCount, err := strconv.ParseInt(tmp[1], 10, 64)
 	if err != nil {
 		return err
 	}
-	t.ChannelCount = int(channelCount)
+	if channelCount != 2 {
+		return fmt.Errorf("invalid channel count: %d", channelCount)
+	}
+
+	if fmtp != "" {
+		for _, kv := range strings.Split(fmtp, ";") {
+			kv = strings.Trim(kv, " ")
+
+			if len(kv) == 0 {
+				continue
+			}
+
+			tmp := strings.SplitN(kv, "=", 2)
+			if len(tmp) != 2 {
+				return fmt.Errorf("invalid fmtp (%v)", fmtp)
+			}
+
+			if strings.ToLower(tmp[0]) == "sprop-stereo" {
+				t.IsStereo = (tmp[1] == "1")
+			}
+		}
+	}
 
 	return nil
 }
@@ -58,14 +82,15 @@ func (t *Opus) unmarshal(payloadType uint8, clock string, codec string, rtpmap s
 // Marshal implements Format.
 func (t *Opus) Marshal() (string, string) {
 	fmtp := "sprop-stereo=" + func() string {
-		if t.ChannelCount == 2 {
+		if t.IsStereo {
 			return "1"
 		}
 		return "0"
 	}()
 
-	return "opus/" + strconv.FormatInt(int64(t.SampleRate), 10) +
-		"/" + strconv.FormatInt(int64(t.ChannelCount), 10), fmtp
+	// RFC7587: The RTP clock rate in "a=rtpmap" MUST be 48000, and the
+	// number of channels MUST be 2.
+	return "opus/48000/2", fmtp
 }
 
 // PTSEqualsDTS implements Format.
@@ -76,7 +101,7 @@ func (t *Opus) PTSEqualsDTS(*rtp.Packet) bool {
 // CreateDecoder creates a decoder able to decode the content of the format.
 func (t *Opus) CreateDecoder() *rtpsimpleaudio.Decoder {
 	d := &rtpsimpleaudio.Decoder{
-		SampleRate: t.SampleRate,
+		SampleRate: 48000,
 	}
 	d.Init()
 	return d
@@ -86,7 +111,7 @@ func (t *Opus) CreateDecoder() *rtpsimpleaudio.Decoder {
 func (t *Opus) CreateEncoder() *rtpsimpleaudio.Encoder {
 	e := &rtpsimpleaudio.Encoder{
 		PayloadType: t.PayloadTyp,
-		SampleRate:  8000,
+		SampleRate:  48000,
 	}
 	e.Init()
 	return e

pkg/format/opus_test.go

@@ -10,8 +10,7 @@ import (
 func TestOpusAttributes(t *testing.T) {
 	format := &Opus{
 		PayloadTyp: 96,
-		SampleRate:   48000,
-		ChannelCount: 2,
+		IsStereo:   true,
 	}
 	require.Equal(t, "Opus", format.String())
 	require.Equal(t, 48000, format.ClockRate())
@@ -22,8 +21,7 @@ func TestOpusAttributes(t *testing.T) {
 func TestOpusMediaDescription(t *testing.T) {
 	format := &Opus{
 		PayloadTyp: 96,
-		SampleRate:   48000,
-		ChannelCount: 2,
+		IsStereo:   true,
 	}
 
 	rtpmap, fmtp := format.Marshal()

pkg/media/medias_test.go

@@ -194,7 +194,7 @@ var casesMedias = []struct {
 			"a=control\r\n" +
 			"a=sendonly\r\n" +
 			"a=rtpmap:111 opus/48000/2\r\n" +
-			"a=fmtp:111 sprop-stereo=1\r\n" +
+			"a=fmtp:111 sprop-stereo=0\r\n" +
 			"a=rtpmap:103 ISAC/16000\r\n" +
 			"a=rtpmap:104 ISAC/32000\r\n" +
 			"a=rtpmap:9 G722/8000\r\n" +
@@ -231,8 +231,7 @@ var casesMedias = []struct {
 				Formats: []format.Format{
 					&format.Opus{
 						PayloadTyp: 111,
-						SampleRate:   48000,
-						ChannelCount: 2,
+						IsStereo:   false,
 					},
 					&format.Generic{
 						PayloadTyp: 103,
@@ -535,8 +534,7 @@ func TestMediasFindFormat(t *testing.T) {
 			Formats: []format.Format{
 				&format.Opus{
 					PayloadTyp: 111,
-					SampleRate:   48000,
-					ChannelCount: 2,
+					IsStereo:   true,
 				},
 			},
 		},