feat(icom): phase 5 — the voice keyer plays to the radio over the LAN

The audio session opens its TX side alongside RX (txenable in the conninfo,
16 kHz both ways), the stream gains a paced sender — 320 samples every 20 ms,
the frame mirroring what the rig itself sends on this socket, IDs swapped —
and the network Icom takes the same NetworkPlayer slot a TCI radio does: pick
'Radio (network audio)' as the To-radio device and the voice keyer needs no
cable and no virtual sound card. PTT brackets the message through the same
code as every other audio path.

internal/audio/network.go

@@ -64,4 +64,4 @@ func NetworkPlayerReady() bool { return networkPlayer() != nil }
 // comes back with. Named, because "the device could not be opened" would send
 // an operator hunting through Windows sound settings for a device that never
 // existed.
-var errNoNetworkRadio = errors.New("no radio is connected to take the audio — check the CAT link (the radio output only works with a TCI radio)")
+var errNoNetworkRadio = errors.New("no radio is connected to take the audio — check the CAT link (a TCI radio, or a network Icom with RX audio enabled)")

internal/cat/icomaudio.go

@@ -22,6 +22,7 @@ package cat
 // audio stream is opt-in and entirely separate from control/CI-V.
 
 import (
+	"fmt"
 	"net"
 	"sync"
 	"sync/atomic"
@@ -236,3 +237,58 @@ func (a *icomAudio) sendRetransmitReq() {
 		_, _ = a.conn.Write(b)
 	}
 }
+
+// PlayTX sends one already-decoded message out over the audio session, paced
+// at the stream's own 20 ms / 320-sample cadence, and returns when the last
+// packet has gone or stop is closed.
+//
+// The frame mirrors what the rig itself sends on this socket byte for byte —
+// ident 0x81 0x01 (LPCM mono 16-bit), a big-endian send sequence at 0x12, the
+// payload length at 0x14, PCM from 0x18 — with the IDs swapped for direction.
+// PTT is the caller's business, exactly as on the TCI and sound-card paths.
+func (a *icomAudio) PlayTX(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	mono := decodeToMono(pcm, ch, bits)
+	if len(mono) == 0 {
+		return fmt.Errorf("the message is empty")
+	}
+	if rate > 0 && rate != 16000 {
+		mono = resampleLinear(mono, rate, 16000)
+	}
+	const frame = 320 // samples per packet: 20 ms at 16 kHz, the rig's own cadence
+	tick := time.NewTicker(20 * time.Millisecond)
+	defer tick.Stop()
+	var outerSeq uint16 = 1
+	var sendSeq uint16
+	for pos := 0; pos < len(mono); pos += frame {
+		select {
+		case <-stop:
+			return nil
+		case <-a.done:
+			return fmt.Errorf("the audio stream closed mid-message")
+		case <-tick.C:
+		}
+		pkt := make([]byte, 0x18+frame*2)
+		icnLE.PutUint32(pkt[0:], uint32(len(pkt)))
+		icnLE.PutUint16(pkt[6:], outerSeq)
+		icnLE.PutUint32(pkt[8:], a.aID)
+		icnLE.PutUint32(pkt[12:], a.aRemote)
+		pkt[0x10], pkt[0x11] = 0x81, 0x01
+		icnBE.PutUint16(pkt[0x12:], sendSeq)
+		icnBE.PutUint32(pkt[0x14:], frame*2)
+		for i := 0; i < frame && pos+i < len(mono); i++ {
+			v := mono[pos+i] * 32767
+			if v > 32767 {
+				v = 32767
+			} else if v < -32768 {
+				v = -32768
+			}
+			icnLE.PutUint16(pkt[0x18+i*2:], uint16(int16(v)))
+		}
+		if _, err := a.conn.Write(pkt); err != nil {
+			return fmt.Errorf("sending audio to the rig: %w", err)
+		}
+		outerSeq++
+		sendSeq++
+	}
+	return nil
+}

internal/cat/icomnet.go

@@ -940,7 +940,11 @@ func icnConnInfo(seq, innerSeq, tokReq uint16, sentid, rcvdid, token uint32, use
 	copy(b[0x40:0x60], []byte("IC-7610"))
 	copy(b[0x60:0x70], icnPasscode(user))
 	b[0x70] = rxEnable // rxenable: 1 opens the 50003 RX audio stream, 0 = CI-V only
-	b[0x71] = 0x00     // txenable (Phase 5)
+	// TX rides the same audio session: whenever the operator wants RX audio the
+	// TX side is opened with it, so the voice keyer can play to the radio with
+	// no cable. Costs nothing when unused — no packets flow until a message is
+	// actually played.
+	b[0x71] = rxEnable // txenable
 	// rxcodec 0x04 = LPCM, ONE channel, 16-BIT — settled by experiment on a
 	// real IC-7760, one wrong guess at a time: 0x10 produced 1280-byte payloads
 	// of interleaved 16-bit stereo, 0x02 produced 320-byte payloads of 8-bit
@@ -949,7 +953,7 @@ func icnConnInfo(seq, innerSeq, tokReq uint16, sentid, rcvdid, token uint32, use
 	b[0x72] = 0x04 // rxcodec
 	b[0x73] = 0x04 // txcodec
 	icnBE.PutUint32(b[0x74:], 16000)
-	icnBE.PutUint32(b[0x78:], 8000)
+	icnBE.PutUint32(b[0x78:], 16000) // TX sample rate — the same 16 kHz everything else here runs at
 	icnBE.PutUint32(b[0x7c:], uint32(civPort))
 	icnBE.PutUint32(b[0x80:], uint32(audioPort))
 	icnBE.PutUint32(b[0x84:], 100)
@@ -957,6 +961,17 @@ func icnConnInfo(seq, innerSeq, tokReq uint16, sentid, rcvdid, token uint32, use
 	return b
 }
 
+// PlayTXAudio plays one voice-keyer message through the 50003 audio session.
+// On the transport because the transport owns the session; the IcomSerial
+// controller forwards here after a type assertion, exactly as the TCI path
+// reaches its radio.
+func (n *icomNet) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	if n.audio == nil {
+		return fmt.Errorf("the radio's audio stream is not open — enable RX audio in Settings → CAT first")
+	}
+	return n.audio.PlayTX(pcm, rate, ch, bits, stop)
+}
+
 func icnOpenClose(seq uint16, sentid, rcvdid uint32, civSeq uint16, magic byte) []byte {
 	b := make([]byte, 0x16)
 	icnLE.PutUint32(b[0:], 0x16)

internal/cat/icomserial.go

@@ -2047,3 +2047,20 @@ func agcValue(name string) byte {
 	}
 	return 0
 }
+
+// PlayTXAudio hands a voice-keyer message to the network audio session, when
+// this rig is reached over one. A USB-connected Icom takes its audio through
+// its own sound card instead, and says so.
+func (b *IcomSerial) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	// port is written once per Connect, on the CAT goroutine this is fetched
+	// from (IcomDo); at worst a reconnect leaves this one connection stale, and
+	// a stale transport fails fast on its closed done channel.
+	port := b.port
+	type txPlayer interface {
+		PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error
+	}
+	if p, ok := port.(txPlayer); ok {
+		return p.PlayTXAudio(pcm, rate, ch, bits, stop)
+	}
+	return fmt.Errorf("this rig takes transmit audio through its USB sound card, not the CAT link")
+}