Add support alsa source

pkg/alsa/capture_linux.go

@@ -0,0 +1,89 @@
+package alsa
+
+import (
+	"github.com/AlexxIT/go2rtc/pkg/alsa/device"
+	"github.com/AlexxIT/go2rtc/pkg/core"
+	"github.com/pion/rtp"
+)
+
+type Capture struct {
+	core.Connection
+	dev    *device.Device
+	closed core.Waiter
+}
+
+func newCapture(dev *device.Device) (*Capture, error) {
+	medias := []*core.Media{
+		{
+			Kind:      core.KindAudio,
+			Direction: core.DirectionRecvonly,
+			Codecs: []*core.Codec{
+				{Name: core.CodecPCML, ClockRate: 16000},
+			},
+		},
+	}
+	return &Capture{
+		Connection: core.Connection{
+			ID:         core.NewID(),
+			FormatName: "alsa",
+			Medias:     medias,
+			Transport:  dev,
+		},
+		dev: dev,
+	}, nil
+}
+
+// readBufferSize - 20ms * 2 bytes per sample * 16000 frames per second * 2 channels / 1000ms per second
+const readBufferSize = 20 * 2 * 16000 * 2 / 1000
+
+// bytesPerFrame - 2 bytes per sample * 2 channels
+const bytesPerFrame = 2 * 2
+
+func (c *Capture) Start() error {
+	if err := c.dev.SetHWParams(device.SNDRV_PCM_FORMAT_S16_LE, 16000, 2); err != nil {
+		return err
+	}
+
+	var ts uint32
+
+	b := make([]byte, readBufferSize)
+	for {
+		n, err := c.dev.Read(b)
+		if err != nil {
+			return err
+		}
+
+		c.Recv += n
+
+		if len(c.Receivers) == 0 {
+			continue
+		}
+
+		pkt := &rtp.Packet{
+			Header: rtp.Header{
+				Version:   2,
+				Marker:    true,
+				Timestamp: ts,
+			},
+			Payload: stereoToMono(b[:n]),
+		}
+		c.Receivers[0].WriteRTP(pkt)
+
+		ts += uint32(n / bytesPerFrame)
+	}
+}
+
+func stereoToMono(stereo []byte) (mono []byte) {
+	n := len(stereo)
+	mono = make([]byte, n/2)
+	var i, j int
+	for i < n {
+		mono[j] = stereo[i]
+		j++
+		i++
+		mono[j] = stereo[i]
+		j++
+		i += 3
+	}
+	return
+}