PM_K-1: on-screen MIDI indicator + auto-mute buzzer when a host is listening

The editor's 'Device audio' now sends a MIDI Active-Sensing heartbeat (0xFE, every 250ms)
to the device while on. The firmware reads usb_midi.ports[0]; while it hears the heartbeat
(<1s) it shows a green 'MIDI' badge top-right and silences the buzzer (the computer plays);
~1s after it stops, it reverts to the buzzer and hides the badge. Manual MUTE_BUZZER still
works. Verified headless: host detected -> MIDI shown + buzzer duty 0; timeout -> reverts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

editor.html

@@ -1117,8 +1117,14 @@ async function loadFromDevice() {
 
 /* Device audio (Phase 3): a connected PM_K-1 sends a USB-MIDI note per click; we voice it through
    this page's synth, so the device drives sound out the computer's speakers, locked to its clock. */
-let _midiAccess = null, _midiOn = false, _midiFlash = 0;
+let _midiAccess = null, _midiOn = false, _midiFlash = 0, _midiBeat = 0;
 function _midiInputs() { return _midiAccess ? [..._midiAccess.inputs.values()] : []; }
+function _heartbeat(on) {   // tell the device a host is listening, so it shows "MIDI" + mutes its buzzer
+  clearInterval(_midiBeat); _midiBeat = 0;
+  if (on) _midiBeat = setInterval(() => {
+    if (_midiAccess) for (const out of _midiAccess.outputs.values()) { try { out.send([0xFE]); } catch (_) {} }  // Active Sensing
+  }, 250);
+}
 function onDeviceMidi(e) {
   const d = e.data; if (!d || d.length < 3) return;
   if ((d[0] & 0xf0) === 0x90 && d[2] > 0) {                       // Note On
@@ -1136,12 +1142,12 @@ function updateMidiBtn() {
   b.classList.add("primary");
 }
 async function toggleDeviceAudio() {
-  if (_midiOn) { _midiOn = false; _bindMidi(); updateMidiBtn(); return; }
+  if (_midiOn) { _midiOn = false; _heartbeat(false); _bindMidi(); updateMidiBtn(); return; }
   if (!navigator.requestMIDIAccess) return alert("Playing the device through this computer needs the Web MIDI API — use Chrome or Edge.");
   try { if (!_midiAccess) { _midiAccess = await navigator.requestMIDIAccess(); _midiAccess.onstatechange = () => { _bindMidi(); updateMidiBtn(); }; } }
   catch (e) { return alert("MIDI access was denied."); }
   ensureAudio(); if (audioCtx && audioCtx.state === "suspended") audioCtx.resume();
-  _midiOn = true; _bindMidi(); updateMidiBtn();
+  _midiOn = true; _heartbeat(true); _bindMidi(); updateMidiBtn();
   const names = _midiInputs().map((i) => i.name || "MIDI");
   alert(names.length
     ? "Device audio ON.\nMIDI input(s): " + names.join(", ") + "\nPress play on the device — the button pulses green on each note received."

pico-cp/code.py

@@ -289,6 +289,8 @@ class App:
         self.i2c = busio.I2C(scl=P_SCL, sda=P_SDA, frequency=400_000)
         self.touch = GT911(self.i2c)
         self.midi = usb_midi.ports[1] if (MIDI_ENABLED and usb_midi and len(usb_midi.ports) > 1) else None
+        self.midi_in = usb_midi.ports[0] if (MIDI_ENABLED and usb_midi and len(usb_midi.ports) > 0) else None
+        self._mbuf = bytearray(16); self.midi_host = False; self.last_midi_in = 0.0
         self.led = RGB(P_RGB)
         self.buz = pwmio.PWMOut(P_BUZ, frequency=1600, variable_frequency=True, duty_cycle=0)
         self.buz_off = 0
@@ -321,6 +323,7 @@ class App:
         self.g_bpm = displayio.Group(); root.append(self.g_bpm)     # big tempo (right)
         self.g_run = displayio.Group(); root.append(self.g_run)     # RUN / STOP (left)
         self.g_name = displayio.Group(); root.append(self.g_name)   # item index + name
+        self.g_midi = displayio.Group(); root.append(self.g_midi)   # "MIDI" indicator (top-right) when a host is listening
         self.g_grid = displayio.Group(); root.append(self.g_grid)   # lanes × step pads
         # buttons (rects static; labels in per-button groups so play can toggle)
         bw, bh = 96, 56; gap = (WIDTH - 3*bw)//4; xs = [gap, gap*2+bw, gap*3+bw*2]
@@ -426,7 +429,7 @@ class App:
                 if adv and li < len(self.lane_pads): self._move_playhead(li, L['step'])
             if fired:
                 best = max(fired, key=lambda l: PRIO.get(l, 0))
-                if not MUTE_BUZZER: self.click(best)
+                if not MUTE_BUZZER and not self.midi_host: self.click(best)   # computer plays it instead
                 self.flash(best)
             if self.rgb != (0, 0, 0):
                 r, g, b = self.rgb; r = r*7//10; g = g*7//10; b = b*7//10
@@ -461,6 +464,16 @@ class App:
                 self._touchDown = True; self.hit(pt[0], pt[1])
         elif self._touchDown and (nowms - self._touchSeen) > 0.14:
             self._touchDown = False
+        # MIDI host present? the editor sends a heartbeat (Active Sensing) while "Device audio" is on
+        if self.midi_in is not None:
+            try:
+                if self.midi_in.readinto(self._mbuf): self.last_midi_in = nowms
+            except Exception: pass
+            host = bool(self.last_midi_in) and (nowms - self.last_midi_in) < 1.0
+            if host != self.midi_host:
+                self.midi_host = host
+                if host: self.buz.duty_cycle = 0          # silence the buzzer when the computer takes over
+                self.led_off(); self.draw_midi()
 
     def hit(self, x, y):
         for bx, by, bw, bh, key in self.buttons:
@@ -481,6 +494,8 @@ class App:
                     C_GREEN if self.running else C_MUTE, C_BG, FONT_M)
         self._place(self.g_name, "%d/%d  %s" % (self.idx+1, len(self.programs), self.name[:18]),
                     12, 112, C_TXT, C_BG, FONT_M)
+    def draw_midi(self):
+        self._place(self.g_midi, "MIDI" if self.midi_host else "", 0, 12, C_GREEN, C_BG, FONT_M, right_edge=WIDTH-12)
 
     # ---------- pad grid (each lane = a row of step pads; playhead lit as it plays) ----------
     def _padbase(self, L, s):