MIDI clock sync + note output over USB/BLE (v0.18.0)

Add a MIDI Sync settings card (before the binding cards) with Send and Receive
MIDI clock toggles, both off by default and persisted.

Send: a dedicated thread emits clock (0xF8) at 24 PPQN plus Start/Stop on the
transport edges to every attached MIDI device (USB + Bluetooth), and — because a
sequencer should drive gear, not just its tempo — also forwards the tracks'
note-on/off on each cell's MIDI channel. Notes are queued from the audio thread
through a lock-free ring and drained by the send thread, mirroring the mono-per-
track cell lifecycle; stop/pause/panic release everything so external notes never
hang. (Per-channel Arpeggiator/Transposer are internal FX and aren't reflected.)

Receive: MidiInput now forwards real-time bytes to the clock module, which follows
an external clock — Start/Continue starts the transport, Stop pauses it, and the
pulse rate drives the tempo (a tempo + transport follow, not a sample-locked slave).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/build.gradle.kts

@@ -22,8 +22,8 @@ android {
         // (android.media.midi) and AAudio low-latency audio we rely on.
         minSdk = 26
         targetSdk = 34
-        versionCode = 42
-        versionName = "0.16.1"
+        versionCode = 44
+        versionName = "0.18.0"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"

app/src/main/java/space/rcmd/android/sizzle/MainActivity.kt

@@ -52,7 +52,7 @@ class MainActivity : ComponentActivity() {
                         app.project, app.audioEngine, app.inputRouter,
                         app.gamepadInput, app.midiInput, app.keyboardInput,
                         app.bindingStore, app.presetLibrary, app.songLibrary,
-                        app.settingsStore, app.themeLibrary, app.applicationContext,
+                        app.settingsStore, app.themeLibrary, app.midiClock, app.applicationContext,
                     )
                 }
             },
@@ -80,6 +80,7 @@ class MainActivity : ComponentActivity() {
     override fun onStart() {
         super.onStart()
         midi.start()
+        app.midiClock.start()
     }
 
     override fun onResume() {
@@ -95,6 +96,7 @@ class MainActivity : ComponentActivity() {
         // system-kill (or crash) resumes from here on next launch.
         app.projectStore.save(app.project)
         midi.stop()
+        app.midiClock.stop()
         super.onStop()
     }
 

app/src/main/java/space/rcmd/android/sizzle/SizzleApp.kt

@@ -45,6 +45,11 @@ class SizzleApp : Application() {
     val gamepadInput: GamepadInput by lazy { GamepadInput(inputRouter) }
     val midiInput: MidiInput by lazy { MidiInput(this, inputRouter) }
 
+    /** MIDI clock sync (send/receive) over USB + Bluetooth MIDI. */
+    val midiClock: space.rcmd.android.sizzle.input.MidiClock by lazy {
+        space.rcmd.android.sizzle.input.MidiClock(this)
+    }
+
     /** Persists the input binding maps across restarts (DataStore-backed). */
     val bindingStore: BindingStore by lazy { BindingStore(this) }
 
@@ -80,6 +85,8 @@ class SizzleApp : Application() {
         // single small read; blocking briefly at cold start keeps the input maps
         // authoritative from the first keypress rather than racing an async load.
         runBlocking { bindingStore.loadInto(keyboardInput, gamepadInput, midiInput) }
+        // Feed incoming MIDI real-time (clock / start / stop) to the clock sync module.
+        midiInput.onRealtime = midiClock::handleRealtime
         installCrashAutosave()
     }
 

app/src/main/java/space/rcmd/android/sizzle/audio/AudioEngine.kt

@@ -88,6 +88,19 @@ class AudioEngine(
     // track last started, on the bus(es) that channel routed to. -1 = nothing yet.
     private val trackLastChannel = IntArray(Arrangement.LANE_COUNT * Pattern.TRACK_COUNT) { -1 }
 
+    // ---- outgoing MIDI (send the sequenced notes to external gear) ----
+    // The sequencer runs on the audio thread, which must not do MIDI I/O; note events
+    // are packed into a lock-free single-producer/single-consumer ring that the MIDI
+    // clock thread drains via [pollMidiOut]. Only filled when [midiOutEnabled]. Each
+    // slot packs (status<<16)|(data1<<8)|data2. Note-on/off mirror the tracks' cells
+    // (mono per track, matching the internal note lifecycle) on the cell's MIDI channel.
+    @Volatile var midiOutEnabled = false
+    private val moutRing = IntArray(256)
+    @Volatile private var moutHead = 0 // producer (audio thread)
+    @Volatile private var moutTail = 0 // consumer (MIDI clock thread)
+    private val moutNote = IntArray(Arrangement.LANE_COUNT * Pattern.TRACK_COUNT) { -1 }
+    private val moutCh = IntArray(Arrangement.LANE_COUNT * Pattern.TRACK_COUNT)
+
     // Cached subtractive-synth patch per bus, reparsed only when the slot changes
     // (version) or tempo changes — so note triggers don't allocate a patch each time,
     // and active voices can be refreshed per block for live parameter feedback.
@@ -520,6 +533,7 @@ class AudioEngine(
         busSampleVoices.forEach { it.kill() }
         arps.forEach { it.stop() }
         trackLastChannel.fill(-1)
+        allMidiNotesOff() // release any notes sent to external gear
     }
 
     /** Stop playback. If already stopped, rewind to the very beginning. */
@@ -870,6 +884,8 @@ class AudioEngine(
     private fun triggerCell(proj: Project, lane: Int, track: Int, cell: space.rcmd.android.sizzle.model.Cell) {
         val trackKey = lane * Pattern.TRACK_COUNT + track
         if (cell.isNoteOff) {
+            // Release this track's outgoing MIDI note first (external gear).
+            if (moutNote[trackKey] >= 0) { emitNoteOff(moutCh[trackKey], moutNote[trackKey]); moutNote[trackKey] = -1 }
             val ch = trackLastChannel[trackKey]
             if (ch < 0) return // this track hasn't played anything yet
             for (bus in 0 until Pattern.TRACK_COUNT) {
@@ -882,6 +898,13 @@ class AudioEngine(
         if (!cell.isPlayable) return
         // Remember the channel so a later note-off on this track releases it.
         trackLastChannel[trackKey] = cell.channel
+        // Send this track's note out to external gear (mono per track): off the note it
+        // was holding, then on the new one, on the cell's MIDI channel. Mirrors the
+        // tracks' cells — a channel's Arpeggiator/Transposer are internal FX and aren't
+        // reflected here.
+        if (moutNote[trackKey] >= 0) emitNoteOff(moutCh[trackKey], moutNote[trackKey])
+        emitNoteOn(cell.channel, cell.note, cell.velocity)
+        moutNote[trackKey] = cell.note; moutCh[trackKey] = cell.channel
         // Mono-per-track: a new note releases whatever this track was still holding,
         // so a held note is stopped when the next one appears under the playhead — no
         // explicit note-off cell needed. Only this track's own voices are released, so
@@ -1382,6 +1405,45 @@ class AudioEngine(
         seqVoices.forEach { it.noteOff() }
         sampleVoices.forEach { it.noteOff() }
         arps.forEach { it.stop() }
+        allMidiNotesOff() // release any notes we sent to external gear
+    }
+
+    // ------------------------------------------------------------- outgoing MIDI
+    /** Enqueue one MIDI message for the send thread (audio-thread producer, no alloc). */
+    private fun emitMidi(status: Int, d1: Int, d2: Int) {
+        if (!midiOutEnabled) return
+        val h = moutHead
+        val next = (h + 1) and (moutRing.size - 1)
+        if (next == moutTail) return // ring full — drop rather than block the audio thread
+        moutRing[h] = (status shl 16) or ((d1 and 0x7F) shl 8) or (d2 and 0x7F)
+        moutHead = next
+    }
+
+    /** Send a note-on for [note] on app channel [ch] (1..16 → MIDI wire 0..15). */
+    private fun emitNoteOn(ch: Int, note: Int, vel: Int) {
+        if (ch < 1) return
+        emitMidi(0x90 or ((ch - 1) and 0x0F), note.coerceIn(0, 127), vel.coerceIn(1, 127))
+    }
+
+    private fun emitNoteOff(ch: Int, note: Int) {
+        if (ch < 1) return
+        emitMidi(0x80 or ((ch - 1) and 0x0F), note.coerceIn(0, 127), 0)
+    }
+
+    /** Release every note we've sent out (on transport stop / pause / panic) so an
+     *  external instrument never hangs after Sizzle stops. */
+    private fun allMidiNotesOff() {
+        for (k in moutNote.indices) if (moutNote[k] >= 0) { emitNoteOff(moutCh[k], moutNote[k]); moutNote[k] = -1 }
+    }
+
+    /** Drain one queued outgoing MIDI message, or -1 if the queue is empty. Called by
+     *  the MIDI clock thread (single consumer). */
+    fun pollMidiOut(): Int {
+        val t = moutTail
+        if (t == moutHead) return -1
+        val v = moutRing[t]
+        moutTail = (t + 1) and (moutRing.size - 1)
+        return v
     }
 
     private fun publish() {

app/src/main/java/space/rcmd/android/sizzle/input/MidiClock.kt

@@ -0,0 +1,169 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package space.rcmd.android.sizzle.input
+
+import android.content.Context
+import android.media.midi.MidiDeviceInfo
+import android.media.midi.MidiInputPort
+import android.media.midi.MidiManager
+import android.os.Handler
+import android.os.Looper
+import java.util.concurrent.CopyOnWriteArrayList
+import java.util.concurrent.locks.LockSupport
+
+/**
+ * MIDI clock sync over every attached MIDI device (USB *and* Bluetooth — Android
+ * surfaces both through the same [MidiManager]).
+ *
+ * SEND (drive external gear): while [sendEnabled] and the transport is playing, a
+ * dedicated thread emits the MIDI real-time clock (0xF8) at 24 PPQN from the current
+ * tempo, plus Start (0xFA) / Stop (0xFC) on the play/stop edges, to every device we
+ * can write to. RECEIVE (be driven): [handleRealtime] — fed the real-time status
+ * bytes by [MidiInput] — follows an external clock, calling back Start / Stop and a
+ * tempo derived from the incoming pulse rate. Receive is a tempo + transport follow,
+ * not a sample-locked slave.
+ *
+ * Both directions are off until enabled; nothing is sent or acted on otherwise.
+ */
+class MidiClock(context: Context) {
+    private val midiManager = context.getSystemService(Context.MIDI_SERVICE) as? MidiManager
+    private val handler = Handler(Looper.getMainLooper())
+    private val outPorts = CopyOnWriteArrayList<MidiInputPort>() // device input ports we WRITE to
+
+    @Volatile var sendEnabled = false
+    @Volatile var receiveEnabled = false
+
+    /** Live transport state for the send thread, supplied by the ViewModel. */
+    var playingProvider: () -> Boolean = { false }
+    var bpmProvider: () -> Float = { 120f }
+
+    /** Source of queued outgoing note messages (packed (status<<16)|(d1<<8)|d2), or -1
+     *  when empty — drained by the send thread while [sendEnabled]. Set by the ViewModel
+     *  to the audio engine's queue, so Sizzle's sequenced notes reach external gear. */
+    var noteSource: (() -> Int)? = null
+
+    /** Receive callbacks (invoked on the MIDI handler thread; the ViewModel hops to
+     *  the main thread as needed). */
+    var onReceiveStart: (() -> Unit)? = null
+    var onReceiveStop: (() -> Unit)? = null
+    var onReceiveTempo: ((Float) -> Unit)? = null
+
+    private var thread: Thread? = null
+    @Volatile private var running = false
+
+    fun start() {
+        val mm = midiManager ?: return
+        mm.devices.forEach(::openDevice)
+        mm.registerDeviceCallback(object : MidiManager.DeviceCallback() {
+            override fun onDeviceAdded(device: MidiDeviceInfo) = openDevice(device)
+        }, handler)
+        running = true
+        thread = Thread({ sendLoop() }, "sizzle-midi-clock").apply {
+            priority = Thread.MAX_PRIORITY; isDaemon = true; start()
+        }
+    }
+
+    fun stop() {
+        running = false
+        thread?.join(200); thread = null
+        outPorts.forEach { runCatching { it.close() } }
+        outPorts.clear()
+    }
+
+    private fun openDevice(info: MidiDeviceInfo) {
+        if (info.inputPortCount == 0) return // need a port we can send TO
+        midiManager?.openDevice(info, { device ->
+            device?.openInputPort(0)?.let { outPorts += it }
+        }, handler)
+    }
+
+    private fun sendAll(status: Byte) {
+        val msg = byteArrayOf(status)
+        for (p in outPorts) runCatching { p.send(msg, 0, 1) }
+    }
+
+    /** Send one packed 3-byte channel message (note-on/off) to every device. */
+    private fun sendPacked(m: Int) {
+        val msg = byteArrayOf(
+            ((m shr 16) and 0xFF).toByte(), ((m shr 8) and 0x7F).toByte(), (m and 0x7F).toByte(),
+        )
+        for (p in outPorts) runCatching { p.send(msg, 0, 3) }
+    }
+
+    /** Drain and send any queued sequenced notes (bounded so a flood can't stall). */
+    private fun drainNotes() {
+        val src = noteSource ?: return
+        var guard = 0
+        var m = src()
+        while (m >= 0 && guard++ < moutDrainMax) { sendPacked(m); m = src() }
+    }
+
+    /** Generates the outgoing clock. Polls at 0.5 ms so pulse timing is steady enough
+     *  for typical gear; sends Start/Stop on the transport's play/stop edges. */
+    private fun sendLoop() {
+        var lastPlaying = false
+        var nextPulse = 0L
+        while (running) {
+            if (sendEnabled) {
+                val playing = playingProvider()
+                val now = System.nanoTime()
+                if (playing && !lastPlaying) { sendAll(START); nextPulse = now; lastPlaying = true }
+                else if (!playing && lastPlaying) { sendAll(STOP); lastPlaying = false }
+                if (playing && now >= nextPulse) {
+                    sendAll(CLOCK)
+                    val bpm = bpmProvider().coerceIn(20f, 300f)
+                    val interval = (60_000_000_000.0 / (bpm * 24.0)).toLong()
+                    nextPulse += interval
+                    if (nextPulse < now) nextPulse = now + interval // recover from any lag
+                }
+                drainNotes() // forward Sizzle's sequenced notes to external gear
+            } else if (lastPlaying) {
+                sendAll(STOP); lastPlaying = false // stop cleanly if send is switched off mid-play
+            }
+            // 0.5 ms when sending (steady pulse timing); relax to 10 ms when idle.
+            LockSupport.parkNanos(if (sendEnabled) 500_000L else 10_000_000L)
+        }
+        if (lastPlaying) sendAll(STOP)
+    }
+
+    // ---- receive: fed the real-time status bytes by MidiInput ----
+    private var lastPulseNs = 0L
+    private var pulseAccum = 0.0
+    private var pulseCount = 0
+
+    fun handleRealtime(status: Int) {
+        if (!receiveEnabled) return
+        when (status) {
+            0xFA, 0xFB -> { resetTempoEstimate(); onReceiveStart?.invoke() } // Start / Continue
+            0xFC -> onReceiveStop?.invoke()                                   // Stop
+            0xF8 -> onClockPulse()                                            // Clock
+        }
+    }
+
+    private fun resetTempoEstimate() { lastPulseNs = 0L; pulseAccum = 0.0; pulseCount = 0 }
+
+    /** Derive tempo from the incoming pulse rate, averaged over a quarter note (24
+     *  pulses) to smooth jitter. */
+    private fun onClockPulse() {
+        val now = System.nanoTime()
+        if (lastPulseNs != 0L) {
+            pulseAccum += (now - lastPulseNs)
+            if (++pulseCount >= 24) {
+                val avgNs = pulseAccum / pulseCount
+                val bpm = (60_000_000_000.0 / (avgNs * 24.0)).toFloat()
+                if (bpm.isFinite() && bpm in 20f..300f) onReceiveTempo?.invoke(bpm)
+                pulseAccum = 0.0; pulseCount = 0
+            }
+        }
+        lastPulseNs = now
+    }
+
+    private val moutDrainMax = 256 // cap notes sent per poll so a flood can't stall the loop
+
+    private companion object {
+        val CLOCK: Byte = 0xF8.toByte()
+        val START: Byte = 0xFA.toByte()
+        val STOP: Byte = 0xFC.toByte()
+    }
+}

app/src/main/java/space/rcmd/android/sizzle/input/MidiInput.kt

@@ -29,6 +29,9 @@ class MidiInput(
     /** CC number -> action id, editable from Settings (MIDI bindings section). */
     var ccBindings: MutableMap<Int, String> = mutableMapOf(),
 ) {
+    /** Sink for MIDI System Real-Time status bytes (0xF8..0xFF), notably clock (0xF8)
+     *  and Start/Continue/Stop (0xFA/0xFB/0xFC) — wired to [MidiClock] for clock sync. */
+    var onRealtime: ((Int) -> Unit)? = null
     private val midiManager = context.getSystemService(Context.MIDI_SERVICE) as? MidiManager
     private val handler = Handler(Looper.getMainLooper())
     private val openPorts = mutableListOf<MidiOutputPort>()
@@ -67,6 +70,9 @@ class MidiInput(
             while (i < end) {
                 val status = data[i].toInt() and 0xFF
                 if (status < 0x80) { i++; continue } // skip stray data bytes
+                // System Real-Time (0xF8..0xFF) are single-byte and may be interleaved
+                // anywhere; forward them (clock / start / stop) and move on one byte.
+                if (status >= 0xF8) { onRealtime?.invoke(status); i++; continue }
                 val type = status and 0xF0
                 when (type) {
                     0x90 -> { // Note On

app/src/main/java/space/rcmd/android/sizzle/io/SettingsStore.kt

@@ -29,6 +29,10 @@ data class AppSettings(
     val kbdOctave: Int = 3,
     /** Show the live VU meters inside the mixer volume faders. */
     val vuMeters: Boolean = true,
+    /** Send MIDI clock + start/stop to attached MIDI devices (drive external gear). */
+    val sendMidiClock: Boolean = false,
+    /** Follow an external MIDI clock's tempo + start/stop. */
+    val receiveMidiClock: Boolean = false,
 )
 
 /**
@@ -51,6 +55,8 @@ class SettingsStore(private val context: Context) {
             kbdChannel = prefs[KBD_CHANNEL] ?: 0,
             kbdOctave = prefs[KBD_OCTAVE] ?: 3,
             vuMeters = prefs[VU_METERS] ?: true,
+            sendMidiClock = prefs[MIDI_CLOCK_SEND] ?: false,
+            receiveMidiClock = prefs[MIDI_CLOCK_RECV] ?: false,
         )
     }
 
@@ -82,6 +88,14 @@ class SettingsStore(private val context: Context) {
         context.settingsDataStore.edit { prefs -> prefs[VU_METERS] = enabled }
     }
 
+    /** Persist the MIDI-clock send / receive toggles. */
+    suspend fun saveMidiClock(send: Boolean, receive: Boolean) {
+        context.settingsDataStore.edit { prefs ->
+            prefs[MIDI_CLOCK_SEND] = send
+            prefs[MIDI_CLOCK_RECV] = receive
+        }
+    }
+
     private companion object {
         val PALETTE = stringPreferencesKey("palette")
         val RECORD_DIR = stringPreferencesKey("recordDirUri")
@@ -89,5 +103,7 @@ class SettingsStore(private val context: Context) {
         val KBD_CHANNEL = intPreferencesKey("kbdChannel")
         val KBD_OCTAVE = intPreferencesKey("kbdOctave")
         val VU_METERS = booleanPreferencesKey("vuMeters")
+        val MIDI_CLOCK_SEND = booleanPreferencesKey("midiClockSend")
+        val MIDI_CLOCK_RECV = booleanPreferencesKey("midiClockRecv")
     }
 }

app/src/main/java/space/rcmd/android/sizzle/ui/AppViewModel.kt

@@ -50,6 +50,7 @@ class AppViewModel(
     private val songLibrary: space.rcmd.android.sizzle.io.SongLibrary,
     private val settingsStore: space.rcmd.android.sizzle.io.SettingsStore,
     private val themeLibrary: space.rcmd.android.sizzle.io.ThemeLibrary,
+    private val midiClock: space.rcmd.android.sizzle.input.MidiClock,
     private val appContext: android.content.Context,
 ) : ViewModel() {
 
@@ -108,6 +109,26 @@ class AppViewModel(
         touched()
     }
 
+    /** Send MIDI clock + start/stop to attached devices; follow an external clock's
+     *  tempo + transport. Both off by default; toggled from Settings and persisted. */
+    var sendMidiClock by mutableStateOf(false); private set
+    var receiveMidiClock by mutableStateOf(false); private set
+
+    fun updateSendMidiClock(enabled: Boolean) {
+        sendMidiClock = enabled; midiClock.sendEnabled = enabled
+        engine.midiOutEnabled = enabled // also forward the tracks' notes out
+        persistMidiClock(); touched()
+    }
+
+    fun updateReceiveMidiClock(enabled: Boolean) {
+        receiveMidiClock = enabled; midiClock.receiveEnabled = enabled
+        persistMidiClock(); touched()
+    }
+
+    private fun persistMidiClock() {
+        viewModelScope.launch { settingsStore.saveMidiClock(sendMidiClock, receiveMidiClock) }
+    }
+
     fun updateSkipStep(n: Int) { skipStep = n.coerceIn(0, 8) }
     fun togglePunchIn() { punchInMode = !punchInMode }
     fun updateKbdChannel(c: Int) { kbdChannel = c.coerceIn(0, Cell.MAX_CHANNEL); persistKeyboard(); touched() }
@@ -810,6 +831,24 @@ class AppViewModel(
             kbdChannel = s.kbdChannel.coerceIn(0, Cell.MAX_CHANNEL)
             kbdOctave = s.kbdOctave.coerceIn(0, 8)
             vuMeters = s.vuMeters
+
+            // MIDI clock sync: feed the send thread live transport, follow an external
+            // clock into the transport/tempo, and restore the persisted toggles.
+            midiClock.playingProvider = { engine.transport.value.isPlaying }
+            midiClock.bpmProvider = { project.tempoBpm }
+            midiClock.noteSource = { engine.pollMidiOut() } // forward sequenced notes out
+            midiClock.onReceiveStart = {
+                viewModelScope.launch(Dispatchers.Main) { if (!engine.transport.value.isPlaying) engine.play() }
+            }
+            midiClock.onReceiveStop = {
+                viewModelScope.launch(Dispatchers.Main) { engine.pause() }
+            }
+            midiClock.onReceiveTempo = { bpm ->
+                viewModelScope.launch(Dispatchers.Main) { setTempo(bpm) }
+            }
+            sendMidiClock = s.sendMidiClock
+            midiClock.sendEnabled = s.sendMidiClock; engine.midiOutEnabled = s.sendMidiClock
+            receiveMidiClock = s.receiveMidiClock; midiClock.receiveEnabled = s.receiveMidiClock
             engine.recTempDir = appContext.cacheDir
             engine.setRecordTailBars(recordTailBars)
             engine.onRecordingStarted = {

app/src/main/java/space/rcmd/android/sizzle/ui/settings/SettingsScreen.kt

@@ -98,6 +98,7 @@ fun SettingsScreen(vm: AppViewModel) {
             item { AudioDevicesCard(vm) }
             item { InterfaceCard(vm) }
             item { ThemeCard(vm) }
+            item { MidiSyncCard(vm) }
             item {
                 NavCard("Gamepad Bindings", "Rebind buttons & combos", Icons.Filled.SportsEsports) {
                     section = SettingsSection.GAMEPAD
@@ -501,26 +502,53 @@ private fun openUrl(context: Context, url: String) {
     runCatching { context.startActivity(Intent(Intent.ACTION_VIEW, Uri.parse(url))) }
 }
 
-/** Interface options: currently the mixer VU-meter toggle. */
+/** MIDI clock sync over USB + Bluetooth: drive external gear, or follow an external
+ *  clock. Both off by default. */
 @Composable
-private fun InterfaceCard(vm: AppViewModel) {
-    @Suppress("UNUSED_VARIABLE") val rev = vm.revision // refresh the switch state
-    SettingsCard("Interface", subtitle = "On-screen display options") {
+private fun MidiSyncCard(vm: AppViewModel) {
+    @Suppress("UNUSED_VARIABLE") val rev = vm.revision
+    SettingsCard("MIDI Sync", subtitle = "MIDI clock over USB & Bluetooth") {
+        ToggleRow(
+            "Send MIDI clock",
+            "Send clock, start/stop and the tracks' notes to attached MIDI devices, so " +
+                "Sizzle drives them.",
+            vm.sendMidiClock,
+        ) { vm.updateSendMidiClock(it) }
+        HorizontalDivider()
+        ToggleRow(
+            "Receive MIDI clock",
+            "Follow an external clock's tempo, and start/stop with it.",
+            vm.receiveMidiClock,
+        ) { vm.updateReceiveMidiClock(it) }
+    }
+}
+
+/** A labelled switch row (title + supporting text + trailing Switch). */
+@Composable
+private fun ToggleRow(title: String, subtitle: String, checked: Boolean, onChange: (Boolean) -> Unit) {
     Row(
         Modifier.fillMaxWidth(),
         horizontalArrangement = Arrangement.SpaceBetween,
         verticalAlignment = Alignment.CenterVertically,
     ) {
         Column(Modifier.weight(1f)) {
-                Text("Mixer VU meters", style = MaterialTheme.typography.bodyMedium)
-                Text(
+            Text(title, style = MaterialTheme.typography.bodyMedium)
+            Text(subtitle, style = MaterialTheme.typography.bodySmall, color = MaterialTheme.colorScheme.onSurfaceVariant)
+        }
+        Switch(checked = checked, onCheckedChange = onChange)
+    }
+}
+
+/** Interface options: currently the mixer VU-meter toggle. */
+@Composable
+private fun InterfaceCard(vm: AppViewModel) {
+    @Suppress("UNUSED_VARIABLE") val rev = vm.revision // refresh the switch state
+    SettingsCard("Interface", subtitle = "On-screen display options") {
+        ToggleRow(
+            "Mixer VU meters",
             "Show live level meters inside the mixer volume faders.",
-                    style = MaterialTheme.typography.bodySmall,
-                    color = MaterialTheme.colorScheme.onSurfaceVariant,
-                )
-            }
-            Switch(checked = vm.vuMeters, onCheckedChange = { vm.updateVuMeters(it) })
-        }
+            vm.vuMeters,
+        ) { vm.updateVuMeters(it) }
     }
 }