feat(tci): the voice keyer can send its messages through the radio

The radio now appears as a device in both audio lists — 'Radio (TCI
network audio)' — so the receive audio and the voice keyer can both take
the CAT link instead of a sound card. No virtual cable, no second card, no
Windows mixer between the recording and the air.

The transmit side reuses the exchange the tone probe established, with a
WAV in place of the sine: the radio asks, we answer with the next slice,
and it sets the pace. The message is converted once, up front, rather than
per frame — a voice message is a few hundred kilobytes, and resampling
inside a callback that has 21 ms to answer would put arithmetic on the
path where a late frame is a gap on the air.

The PTT is untouched by all this: the keyer keys before and unkeys after
exactly as it does with a sound card, so a transmission is bracketed by
the same code whichever way the audio travels. And the playback runs OFF
the CAT goroutine, since everything else about the rig goes through that
one place and a ten-second message would otherwise freeze the frequency
display and the antenna following for ten seconds.

Two refusals rather than silent failure. A radio that has gone away stops
being offered as a device at all, and a radio whose transmit audio source
is still the microphone is caught within a fifth of a second — a voice
keyer that transmits silence is worse than one that says why it will not.

app.go

@@ -781,8 +781,8 @@ type App struct {
 	// so closing the QSL Manager — or starting another download — stops the previous
 	// one instead of leaving it running against the app-lifetime context (which made
 	// a still-running QRZ sync bleed its log into a freshly started LoTW download).
-	confDLMu       sync.Mutex
+	confDLMu     sync.Mutex
-	confDLCancel   context.CancelFunc
+	confDLCancel context.CancelFunc
 
 	// hamlogUnmatched holds the confirmations the last HAMLOG.online import
 	// could not place onto a QSO, kept so they can be exported and worked
@@ -790,31 +790,31 @@ type App struct {
 	// accumulated: it describes one run, not a history.
 	hamlogUnmatchedMu sync.Mutex
 	hamlogUnmatched   []qso.QSO
-	udpLogMu       sync.Mutex             // serialises UDP auto-log so concurrent packets can't both pass the dedup check
+	udpLogMu          sync.Mutex             // serialises UDP auto-log so concurrent packets can't both pass the dedup check
-	adifMonMu      sync.Mutex             // guards the ADIF-monitor config (file list + per-file read offsets)
+	adifMonMu         sync.Mutex             // guards the ADIF-monitor config (file list + per-file read offsets)
-	syncMu         sync.Mutex             // serialises folder synchronisation: config, the seq counter, and the append to our own file
+	syncMu            sync.Mutex             // serialises folder synchronisation: config, the seq counter, and the append to our own file
-	syncSent       int64                  // changes written to the folder this session
+	syncSent          int64                  // changes written to the folder this session
-	syncReceived   int64                  // changes taken from the other machines this session
+	syncReceived      int64                  // changes taken from the other machines this session
-	syncLast       time.Time              // last completed pass, for the status panel
+	syncLast          time.Time              // last completed pass, for the status panel
-	syncErr        string                 // last folder error, shown in settings — a share that dropped is otherwise invisible
+	syncErr           string                 // last folder error, shown in settings — a share that dropped is otherwise invisible
-	relayAutoMu    sync.Mutex             // serialises relay auto-control evaluation
+	relayAutoMu       sync.Mutex             // serialises relay auto-control evaluation
-	relayAutoLast  map[string]bool        // deviceID|relay → last applied on/off, so we only switch on a real change
+	relayAutoLast     map[string]bool        // deviceID|relay → last applied on/off, so we only switch on a real change
-	relayAutoOn    atomic.Bool            // cached "auto-control enabled" so the CAT hot path skips work when off
+	relayAutoOn       atomic.Bool            // cached "auto-control enabled" so the CAT hot path skips work when off
-	relayDrvMu     sync.Mutex             // guards the cached relay drivers below
+	relayDrvMu        sync.Mutex             // guards the cached relay drivers below
-	relayDrv       map[string]cachedRelay // deviceID → live driver (reused across polls; stateful boards can't be reopened per call)
+	relayDrv          map[string]cachedRelay // deviceID → live driver (reused across polls; stateful boards can't be reopened per call)
-	pttPort        serial.Port            // open serial port while PTT (RTS/DTR) is asserted
+	pttPort           serial.Port            // open serial port while PTT (RTS/DTR) is asserted
-	pttKeyedMethod string                 // "cat" | "rts" | "dtr" while keyed; "" when idle
+	pttKeyedMethod    string                 // "cat" | "rts" | "dtr" while keyed; "" when idle
-	pttGen         int64                  // bumped on every key; a delayed unkey only fires if unchanged (guards against a stale release cutting a new transmission)
+	pttGen            int64                  // bumped on every key; a delayed unkey only fires if unchanged (guards against a stale release cutting a new transmission)
-	startupErr     string                 // captured for surfacing to the frontend
+	startupErr        string                 // captured for surfacing to the frontend
-	settingsScoped atomic.Bool            // true once a.settings is scoped to the active profile — GetUIPref/SetUIPref (per-profile) must wait for it, else an early call reads the wrong scope and e.g. resets the theme
+	settingsScoped    atomic.Bool            // true once a.settings is scoped to the active profile — GetUIPref/SetUIPref (per-profile) must wait for it, else an early call reads the wrong scope and e.g. resets the theme
-	dbPath         string                 // settings/config database file (settings + profiles); may be a user-chosen location
+	dbPath            string                 // settings/config database file (settings + profiles); may be a user-chosen location
-	logbookPath    string                 // default SQLite logbook file (QSOs), next to the settings db — used when a profile doesn't point elsewhere
+	logbookPath       string                 // default SQLite logbook file (QSOs), next to the settings db — used when a profile doesn't point elsewhere
-	logDbPath      string                 // resolved SQLite file actually backing logDb ("" on MySQL, or when the settings db serves as the logbook) — the file the backup snapshots
+	logDbPath         string                 // resolved SQLite file actually backing logDb ("" on MySQL, or when the settings db serves as the logbook) — the file the backup snapshots
-	logDb          *sql.DB                // QSO logbook connection — MySQL, a per-profile SQLite file, or the default logbook.db (never the settings db, except on fallback)
+	logDb             *sql.DB                // QSO logbook connection — MySQL, a per-profile SQLite file, or the default logbook.db (never the settings db, except on fallback)
-	dbBackend      string                 // "sqlite" | "mysql" — the logbook backend actually opened at startup
+	dbBackend         string                 // "sqlite" | "mysql" — the logbook backend actually opened at startup
-	dbBackendErr   string                 // non-empty when a configured MySQL backend failed and we fell back to SQLite
+	dbBackendErr      string                 // non-empty when a configured MySQL backend failed and we fell back to SQLite
-	offlineQ       *offlineq.Queue        // ADIF outbox: QSOs logged while the DB was unreachable
+	offlineQ          *offlineq.Queue        // ADIF outbox: QSOs logged while the DB was unreachable
-	offlineMode    bool                   // last write failed because the DB was unreachable
+	offlineMode       bool                   // last write failed because the DB was unreachable
 
 	catFlexSpots       bool // push cluster spots to the FlexRadio panadapter
 	catFlexDVKDax      bool // raise the Flex transmit DAX button around a voice message
@@ -8313,8 +8313,49 @@ type AudioSettings struct {
 
 // ListAudioInputDevices / ListAudioOutputDevices enumerate WASAPI endpoints
 // for the device dropdowns.
-func (a *App) ListAudioInputDevices() ([]audio.Device, error)  { return audio.ListInputDevices() }
+// ListAudioInputDevices lists the microphones and line inputs, plus THE RADIO
-func (a *App) ListAudioOutputDevices() ([]audio.Device, error) { return audio.ListOutputDevices() }
+// when the CAT link carries its receive audio.
+//
+// Same reasoning as the output list: over TCI there is no sound device for
+// Windows to show, so without this the one correct answer to "where does the
+// received audio come from" could not be chosen at all.
+func (a *App) ListAudioInputDevices() ([]audio.Device, error) {
+	devs, err := audio.ListInputDevices()
+	if err != nil {
+		return devs, err
+	}
+	if a.tciAudioAvailable() {
+		devs = append([]audio.Device{{ID: audio.NetworkDeviceID, Name: "Radio (TCI network audio)"}}, devs...)
+	}
+	return devs, nil
+}
+
+// tciAudioAvailable says whether the active CAT backend is a radio that streams
+// its audio over the CAT link.
+func (a *App) tciAudioAvailable() bool {
+	if a.cat == nil {
+		return false
+	}
+	_, ok := a.cat.TCIAudioState()
+	return ok
+}
+
+// ListAudioOutputDevices lists the sound cards, plus THE RADIO ITSELF when the
+// CAT link can carry transmit audio.
+//
+// Offered only while it is actually available, and named as a radio rather than
+// as a protocol: an operator choosing where their voice goes is picking between
+// "my sound card" and "the radio", not between WASAPI and TCI.
+func (a *App) ListAudioOutputDevices() ([]audio.Device, error) {
+	devs, err := audio.ListOutputDevices()
+	if err != nil {
+		return devs, err
+	}
+	if audio.NetworkPlayerReady() {
+		devs = append([]audio.Device{{ID: audio.NetworkDeviceID, Name: "Radio (TCI network audio)"}}, devs...)
+	}
+	return devs, nil
+}
 
 // GetAudioSettings returns the stored audio config (preroll defaults to 8s).
 func (a *App) GetAudioSettings() (AudioSettings, error) {
@@ -8416,6 +8457,15 @@ func (a *App) SaveAudioSettings(s AudioSettings) error {
 			return err
 		}
 	}
+	// Choosing the radio as the receive device opens its stream, and choosing
+	// anything else closes it. Done HERE rather than left to the next restart:
+	// a device chosen in a dropdown that only takes effect after a relaunch
+	// reads as a device that does not work.
+	if s.FromRadio == audio.NetworkDeviceID {
+		a.startTCIRecording()
+	} else if a.tciAudioAvailable() && a.settingOr(keyTCIRecAudio, "") != "1" {
+		_ = a.cat.TCIAudioDo(func(t cat.TCIAudioController) error { return t.StopTCIAudio() })
+	}
 	// Apply device/preroll/enable changes to the running recorder.
 	a.startQSORecorderIfEnabled()
 	// And to a monitor ALREADY RUNNING: the operator is listening while they
@@ -8460,7 +8510,7 @@ func (a *App) startQSORecorderIfEnabled() {
 	// nothing right to point at. The stream is pushed into the recorder instead
 	// — same samples, no sound card in the middle, and no virtual cable to set up.
 	from := cfg.FromRadio
-	a.qsoRecPushed = a.icomNetAudioActive()
+	a.qsoRecPushed = a.icomNetAudioActive() || cfg.FromRadio == audio.NetworkDeviceID
 	if a.qsoRecPushed {
 		from = audio.PushedSource
 	}
@@ -15141,6 +15191,12 @@ func (a *App) reloadCAT() {
 	} else {
 		a.catSig = sig
 	}
+	// Withdraw the radio as an audio output before deciding anything else. The
+	// TCI case below puts it back; every other backend, and a CAT link turned
+	// off entirely, leaves it withdrawn — a voice keyer that still lists a radio
+	// it can no longer reach would play a message to nowhere, and the operator
+	// hears their own PTT click and assumes it went out.
+	a.installTCITXPlayer(false)
 	if !s.Enabled {
 		a.cat.Stop()
 		return
@@ -15267,6 +15323,9 @@ func (a *App) reloadCAT() {
 		// app_tci_rec.go. Armed after the backend is up, since it is the
 		// backend that carries the stream.
 		defer a.startTCIRecording()
+		// And the other direction: the voice keyer can send its messages over
+		// the same link — see app_tci_dvk.go.
+		defer a.installTCITXPlayer(true)
 		tb := cat.NewTCI(s.TCIHost, s.TCIPort, s.DigitalDefault, s.TCISpots)
 		// Clicking one of our spots on the ExpertSDR panorama fills the entry form.
 		tb.OnSpotClick = func(call string, hz int64) {
@@ -20341,8 +20400,6 @@ func clampSpotMax(n int) int {
 	return n
 }
 
-
-
 // GetSpotTTLMinutes returns how long a spot stays in the list, in minutes.
 // 0 means spots are kept until the count cap pushes them out, which is what
 // OpsLog always did.

app_tci_dvk.go

@@ -0,0 +1,60 @@
+package main
+
+// The voice keyer, through the radio's own link.
+//
+// Selecting the radio as the "To radio" output makes the voice keyer hand its
+// messages to the CAT backend instead of a sound card. Everything around it is
+// unchanged — the same PTT before and after, the same gain, the same files —
+// which is the point: the audio takes a different road, not a different route.
+
+import (
+	"fmt"
+
+	"hamlog/internal/applog"
+	"hamlog/internal/audio"
+	"hamlog/internal/cat"
+)
+
+// tciTXPlayer hands one message to the radio.
+//
+// The controller is fetched on the CAT goroutine and the message is then played
+// OFF it. Playing on it would hold that goroutine for the length of the
+// message, and everything else about the rig — frequency, mode, PTT state —
+// goes through the same place: a ten-second call would freeze the display and
+// the antenna following for ten seconds. The TCI backend serialises its own
+// writes, so this is safe to call from here.
+func (a *App) tciTXPlayer(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	if a.cat == nil {
+		return fmt.Errorf("CAT not initialized")
+	}
+	type txPlayer interface {
+		PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error
+	}
+	var player txPlayer
+	err := a.cat.TCIAudioDo(func(t cat.TCIAudioController) error {
+		p, ok := t.(txPlayer)
+		if !ok {
+			return fmt.Errorf("this radio cannot take transmit audio over its CAT link")
+		}
+		player = p
+		return nil
+	})
+	if err != nil {
+		return err
+	}
+	return player.PlayTXAudio(pcm, rate, ch, bits, stop)
+}
+
+// installTCITXPlayer offers the radio as an audio output, or withdraws it.
+//
+// Withdrawing matters as much as offering: a radio that has gone away must stop
+// being a device the voice keyer will happily "play" to, or a message goes
+// nowhere and the operator hears their own PTT click and assumes it worked.
+func (a *App) installTCITXPlayer(on bool) {
+	if !on {
+		audio.SetNetworkPlayer(nil)
+		return
+	}
+	audio.SetNetworkPlayer(a.tciTXPlayer)
+	applog.Printf("tci: the radio is available as an audio output — no virtual cable needed for the voice keyer")
+}

app_tci_rec.go

@@ -71,7 +71,15 @@ func tciToRecorderPCM(rate int, samples []float32) []byte {
 // for it: opening a 384 kB/s stream on a station that records nothing is work
 // the radio does for nobody.
 func (a *App) startTCIRecording() {
-	if a.cat == nil || a.settingOr(keyTCIRecAudio, "") != "1" {
+	if a.cat == nil {
+		return
+	}
+	// Two ways to ask for the same thing: the option in the TCI section, or
+	// simply choosing the radio as the "From radio" device. The second is where
+	// an operator looks first — it is the question they are already answering —
+	// so it has to work as well as the tick box.
+	cfg, _ := a.GetAudioSettings()
+	if a.settingOr(keyTCIRecAudio, "") != "1" && cfg.FromRadio != audio.NetworkDeviceID {
 		return
 	}
 	err := a.cat.TCIAudioDo(func(t cat.TCIAudioController) error {

internal/audio/manager.go

@@ -157,7 +157,21 @@ func (m *Manager) Play(deviceID, path string, gainPct int) error {
 		// instantly, the PTT is released 120 ms later, and NOTHING says why —
 		// which is exactly what a station heard as "it plays once, then never
 		// again": the call succeeded, the sound did not.
-		if err := playPCM(deviceID, pcm, rate, ch, bits, stop); err != nil {
+		play := func() error { return playPCM(deviceID, pcm, rate, ch, bits, stop) }
+		if deviceID == NetworkDeviceID {
+			// Straight to the radio over its own link. Decided HERE rather than
+			// inside playPCM because there is no Windows endpoint to open: asked
+			// for one, the system complains about a missing device instead of
+			// saying the true thing, which is that no radio is connected.
+			fn := networkPlayer()
+			play = func() error {
+				if fn == nil {
+					return errNoNetworkRadio
+				}
+				return fn(pcm, rate, ch, bits, stop)
+			}
+		}
+		if err := play(); err != nil {
 			LogSink("audio: playback on %q failed: %v", DeviceName(deviceID), err)
 		}
 		m.mu.Lock()

internal/audio/network.go

@@ -0,0 +1,67 @@
+package audio
+
+// Playing a message through the RADIO instead of a sound card.
+//
+// A SunSDR takes its transmit audio over TCI, on the same socket as the
+// commands, so the voice keyer can hand it the message directly: no virtual
+// cable, no second sound card, no Windows mixer between the recording and the
+// air. To everything above, that radio is simply another output device.
+//
+// The device it presents itself as is a name rather than a WASAPI endpoint id,
+// which is why Play checks for it before opening anything: there is no endpoint
+// to open, and asking Windows for one produces a confusing error about a device
+// that does not exist rather than the truth, which is that nothing is connected
+// to the radio.
+
+import (
+	"errors"
+	"sync"
+)
+
+// NetworkDeviceID is the id the radio-over-network output carries in the
+// settings and in the device lists. A fixed string, not a Windows endpoint id:
+// it is chosen by us and must survive a radio being switched off and on.
+const NetworkDeviceID = "net:radio"
+
+// NetworkPlayer sends already-decoded PCM to the radio, returning when the
+// message has been played or when stop is closed.
+//
+// It carries the same arguments as the sound-card path so that Play can hand
+// over whatever it read, and the radio can decide what converting it needs —
+// the sample rate a WAV was recorded at is not the radio's business until the
+// moment it has to be resampled.
+type NetworkPlayer func(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error
+
+var (
+	netMu     sync.RWMutex
+	netPlayer NetworkPlayer
+)
+
+// SetNetworkPlayer installs (or clears, with nil) the radio's transmit path.
+//
+// Package-level rather than per-Manager: there is one radio, the CAT backend
+// owns it, and a Manager that happened to be built before the radio connected
+// would otherwise be permanently unable to reach it.
+func SetNetworkPlayer(fn NetworkPlayer) {
+	netMu.Lock()
+	netPlayer = fn
+	netMu.Unlock()
+}
+
+// networkPlayer returns the installed player, or nil.
+func networkPlayer() NetworkPlayer {
+	netMu.RLock()
+	defer netMu.RUnlock()
+	return netPlayer
+}
+
+// NetworkPlayerReady says whether a radio is currently able to take transmit
+// audio, so the settings panel can offer the option honestly rather than
+// listing a device that would fail when used.
+func NetworkPlayerReady() bool { return networkPlayer() != nil }
+
+// errNoNetworkRadio is what a message played to a radio that is not there
+// 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)")

internal/cat/tci_tx_play.go

@@ -0,0 +1,189 @@
+package cat
+
+// Playing a recorded message to the radio over TCI — the voice keyer's path.
+//
+// The same exchange the tone probe established, with a WAV in place of the
+// sine: the radio asks for a frame, we answer with the next slice of the
+// message, and it sets the pace. What is added here is the conversion, because
+// a recording is whatever the microphone gave it — 16-bit, often mono, often
+// not 48 kHz — and the radio wants interleaved float32 at the stream's rate.
+//
+// The message is converted ONCE, up front, rather than per frame. A voice
+// message is a few hundred kilobytes; resampling it inside the callback would
+// put arithmetic on the path that has 21 ms to answer, and a late frame is a
+// gap in what goes out.
+
+import (
+	"encoding/binary"
+	"fmt"
+	"math"
+	"time"
+)
+
+// tciTXFirstAskTimeout is how long to wait for the radio to ask for the first
+// frame before giving up.
+//
+// It answers within a frame or two when it is going to answer at all, so this
+// is generous. When it stays quiet the cause is always the same — the transmit
+// audio source is the microphone rather than TCI — and a fifth of a second of
+// carrier is a cheap way to find that out.
+const tciTXFirstAskTimeout = 200 * time.Millisecond
+
+// PlayTXAudio sends one message and returns when it has all been handed over,
+// or when stop is closed.
+//
+// The PTT is NOT touched here. The voice keyer keys before calling and unkeys
+// after, exactly as it does with a sound card, so the transmission is bracketed
+// by the same code whichever way the audio travels.
+func (t *TCI) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	t.mu.Lock()
+	connected := t.conn != nil
+	t.mu.Unlock()
+	if !connected {
+		return fmt.Errorf("not connected to the radio")
+	}
+
+	t.audio.mu.Lock()
+	outRate := t.audio.rate
+	t.audio.mu.Unlock()
+	if outRate <= 0 {
+		outRate = 48000
+	}
+
+	mono := decodeToMono(pcm, ch, bits)
+	if len(mono) == 0 {
+		return fmt.Errorf("the message is empty")
+	}
+	if rate > 0 && rate != outRate {
+		mono = resampleLinear(mono, rate, outRate)
+	}
+
+	// Served from here on. The callback does nothing but copy and interleave,
+	// which is what keeps it inside the frame interval.
+	pos := 0
+	done := make(chan struct{})
+	var closed bool
+	t.setTXFeed(func(samples int) []byte {
+		if samples <= 0 {
+			samples = 2048
+		}
+		pairs := samples / 2
+		if pos >= len(mono) {
+			if !closed {
+				closed = true
+				close(done)
+			}
+			return nil
+		}
+		payload := make([]byte, samples*4)
+		le := binary.LittleEndian
+		for i := 0; i < pairs; i++ {
+			var v float32
+			if pos < len(mono) {
+				v = mono[pos]
+				pos++
+			}
+			bits := math.Float32bits(v)
+			le.PutUint32(payload[(i*2)*4:], bits)   // left
+			le.PutUint32(payload[(i*2+1)*4:], bits) // right
+		}
+		return payload
+	})
+	defer t.setTXFeed(nil)
+
+	// Nothing asked for in a fifth of a second means nothing is listening.
+	// Reported plainly: the message would otherwise go out as silence, and a
+	// voice keyer that transmits silence is worse than one that refuses.
+	deadline := time.Now().Add(tciTXFirstAskTimeout)
+	for time.Now().Before(deadline) {
+		t.audio.mu.Lock()
+		asked := t.audio.txSent > 0
+		t.audio.mu.Unlock()
+		if asked {
+			break
+		}
+		select {
+		case <-stop:
+			return nil
+		case <-time.After(10 * time.Millisecond):
+		}
+	}
+	t.audio.mu.Lock()
+	asked := t.audio.txSent
+	t.audio.mu.Unlock()
+	if asked == 0 {
+		return fmt.Errorf("the radio did not ask for any audio — set its transmit audio source to TCI instead of the microphone")
+	}
+
+	// The radio drains the message at real time, so this waits for the feed to
+	// run out. The cap is the message's own length with a second to spare: a
+	// radio that stops asking mid-message must not hold the transmitter up.
+	limit := time.Duration(float64(len(mono))/float64(outRate)*float64(time.Second)) + time.Second
+	select {
+	case <-done:
+	case <-stop:
+	case <-time.After(limit):
+		debugLog.Printf("TCI: the radio stopped asking for audio before the message ended")
+	}
+	return nil
+}
+
+// decodeToMono turns interleaved PCM into one channel of -1…1 floats.
+func decodeToMono(pcm []byte, ch, bits int) []float32 {
+	if ch <= 0 {
+		ch = 1
+	}
+	switch bits {
+	case 16:
+		frame := ch * 2
+		out := make([]float32, 0, len(pcm)/frame+1)
+		for i := 0; i+frame <= len(pcm); i += frame {
+			var sum float32
+			for c := 0; c < ch; c++ {
+				v := int16(uint16(pcm[i+c*2]) | uint16(pcm[i+c*2+1])<<8)
+				sum += float32(v) / 32768
+			}
+			out = append(out, sum/float32(ch))
+		}
+		return out
+	case 8:
+		// Unsigned, centred on 128 — the one format where silence is not zero.
+		out := make([]float32, 0, len(pcm)/ch+1)
+		for i := 0; i+ch <= len(pcm); i += ch {
+			var sum float32
+			for c := 0; c < ch; c++ {
+				sum += (float32(pcm[i+c]) - 128) / 128
+			}
+			out = append(out, sum/float32(ch))
+		}
+		return out
+	}
+	return nil
+}
+
+// resampleLinear moves samples from one rate to another.
+//
+// Linear interpolation, which is crude and entirely adequate here: a voice
+// recording at 16 kHz going to 48 kHz is being INTERPOLATED, and interpolation
+// invents no frequencies to alias. Going the other way would want a filter
+// first, but a message recorded above the radio's stream rate is not a case
+// that arises — the recorder works at 16 kHz and radios stream at 48.
+func resampleLinear(in []float32, from, to int) []float32 {
+	if from <= 0 || to <= 0 || from == to || len(in) == 0 {
+		return in
+	}
+	ratio := float64(from) / float64(to)
+	n := int(float64(len(in)) / ratio)
+	out := make([]float32, n)
+	for i := 0; i < n; i++ {
+		src := float64(i) * ratio
+		j := int(src)
+		frac := float32(src - float64(j))
+		if j+1 < len(in) {
+			out[i] = in[j]*(1-frac) + in[j+1]*frac
+		} else {
+			out[i] = in[len(in)-1]
+		}
+	}
+	return out
+}