diff --git a/PROTOCOL.md b/PROTOCOL.md
index 587bf0b..36213bb 100644
--- a/PROTOCOL.md
+++ b/PROTOCOL.md
@@ -55,7 +55,7 @@ The rest of the 64 bytes is zero-padded.
 | 0x2026 | headphone monitoring | 2000 = on, 3000 = off |
 | 0x2027 | noise cancellation | 0 / 1 |
 | 0x2028 | noise cancellation strength | 0-100 |
-| 0x202A | RGB ring brightness | 0-100 (the app does not show it for this model) |
+| 0x202A | labelled RGB ring brightness in the app's shared code | stores 0-100, but the LEDs do not change on firmware 2.1.2; not exposed |
 | 0x202C | vocal enhancement | 0 / 1 |
 | 0x204B | level-meter enable | not answered by firmware 2.1.2 |
 
diff --git a/README.md b/README.md
index a966666..2974d85 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,6 @@ this covers the same settings with a local web page and a command-line tool.
 - Vocal enhancement
 - Hear yourself (headphone monitoring)
 - Mute, kept in sync with the mic's touch pad
-- RGB ring brightness
 - Factory reset
 
 Pure Python, no dependencies. Nothing leaves your machine: the page is served from your own
@@ -80,7 +79,7 @@ keeps its current settings.
 dgm20-web --open             # open the control page (http://localhost:8720)
 dgm20ctl                     # show current settings
 dgm20ctl set nc on nc-level 80
-dgm20ctl set monitor off enhance on brightness 40
+dgm20ctl set monitor off enhance on
 dgm20ctl watch               # print changes made on the mic itself
 dgm20ctl info                # model and firmware
 dgm20ctl factory-reset --yes
@@ -93,7 +92,6 @@ dgm20ctl factory-reset --yes
 | `nc-level` | 0-100: noise cancellation strength |
 | `enhance` | on, off: vocal enhancement |
 | `monitor` | on, off: hear yourself in headphones plugged into the mic |
-| `brightness` | 0-100: RGB ring |
 
 Settings are stored on the mic, so they stay the same after a replug and on other computers.
 
@@ -124,6 +122,11 @@ model that isn't supported yet (see above).
 **`dgm20-web: cannot listen on 127.0.0.1:8720`**: it's already running, for example as the
 service. Open http://localhost:8720, or pick another port with `--port`.
 
+**"the mic is connected but has stopped answering"**: the mic's settings channel froze. This can
+happen if you hold or tap the mic's buttons while settings are being changed from the computer.
+Unplug the mic and plug it back in; its settings are kept. A software USB reset is not enough,
+the mic needs to lose power.
+
 **Commands not found after installing**: `~/.local/bin` isn't on your `PATH`. Add
 `export PATH="$HOME/.local/bin:$PATH"` to your shell profile.
 
diff --git a/docs/screenshot.png b/docs/screenshot.png
index 458eed0..4784a4a 100644
Binary files a/docs/screenshot.png and b/docs/screenshot.png differ
diff --git a/pyproject.toml b/pyproject.toml
index 9dd91ec..9f464e8 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "dgm20-linux"
 version = "0.1.0"
-description = "Control the Maono DGM20 USB microphone on Linux: noise cancellation, monitoring, vocal enhancement, mute, RGB brightness"
+description = "Control the Maono DGM20 USB microphone on Linux: noise cancellation, monitoring, vocal enhancement, mute"
 readme = "README.md"
 license = "MIT"
 requires-python = ">=3.9"
diff --git a/src/dgm20/device.py b/src/dgm20/device.py
index e2e7c17..ace8d79 100644
--- a/src/dgm20/device.py
+++ b/src/dgm20/device.py
@@ -20,6 +20,7 @@ Both share one framing:
 Settings are 16-bit registers. Input gain and headphone volume are standard
 USB audio controls and are not part of this channel.
 """
+import fcntl
 import glob
 import os
 import select
@@ -37,7 +38,6 @@ SETTINGS = {
     "nc-level":   (0x2028, (0, 100), "Noise cancellation strength"),
     "enhance":    (0x202C, "bool", "Vocal enhancement"),
     "monitor":    (0x2026, {"on": 2000, "off": 3000}, "Hear yourself in headphones"),
-    "brightness": (0x202A, (0, 100), "RGB ring brightness"),
 }
 REG_FACTORY_RESET = 0x100A
 REG_DFU = 0x1008
@@ -48,6 +48,14 @@ class DeviceError(Exception):
     pass
 
 
+class NotResponding(DeviceError):
+    """The mic is plugged in but its settings channel has stopped answering."""
+
+    def __init__(self):
+        super().__init__("the mic is connected but has stopped answering. Unplug it and plug it back in; "
+                         "your settings are kept")
+
+
 def find_hidraw():
     for node in sorted(glob.glob("/sys/class/hidraw/hidraw*")):
         try:
@@ -152,8 +160,11 @@ class Mic:
             os.read(self.fd, 64)
 
     def _io(self, typ, words, accept, timeout=0.5):
+        # the flock makes other processes (CLI, web service) wait their turn, so nobody
+        # mistakes another process's reply or ack for their own
         with self.lock:
             try:
+                fcntl.flock(self.fd, fcntl.LOCK_EX)
                 self._drain()
                 os.write(self.fd, frame(typ, words))
                 while select.select([self.fd], [], [], timeout)[0]:
@@ -162,6 +173,11 @@ class Mic:
                         return p[1]
             except OSError as e:
                 raise DeviceError(f"mic disconnected ({e.strerror})") from None
+            finally:
+                try:
+                    fcntl.flock(self.fd, fcntl.LOCK_UN)
+                except OSError:
+                    pass
         return None
 
     def read(self, addr):
@@ -188,7 +204,10 @@ class Mic:
         return self.get(name)
 
     def state(self):
-        return {name: self.get(name) for name in SETTINGS}
+        state = {name: self.get(name) for name in SETTINGS}
+        if all(v is None for v in state.values()):
+            raise NotResponding()
+        return state
 
     def info(self):
         model = self.read_range(REG_MODEL, 8)
diff --git a/src/dgm20/static/index.html b/src/dgm20/static/index.html
index a007c6b..52308f8 100644
--- a/src/dgm20/static/index.html
+++ b/src/dgm20/static/index.html
@@ -179,11 +179,6 @@ footer { margin-top: 28px; display: flex; justify-content: space-between; flex-w
       <p>Plays the mic through headphones plugged into the DGM20.</p>
       <button class="switch ctl" role="switch" aria-checked="false" aria-labelledby="l-monitor" data-name="monitor" disabled></button>
     </li>
-    <li class="row slider">
-      <label for="brightness">Ring brightness</label>
-      <input type="range" id="brightness" min="0" max="100" step="1" data-name="brightness" disabled>
-      <output for="brightness" id="brightness-out">–</output>
-    </li>
   </ul>
 
   <footer>
@@ -243,6 +238,9 @@ function render(snap) {
     if (/permission/i.test(err)) {
       notice("The mic is plugged in, but this page can't talk to it.",
         "Install the udev rule, then unplug and replug the mic:", "sudo cp udev/70-maono-dgm20.rules /etc/udev/rules.d/");
+    } else if (/stopped answering/i.test(err)) {
+      notice("The mic stopped answering.",
+        "Unplug it and plug it back in. Your settings are kept, and this page reconnects on its own.");
     } else {
       notice("Mic not found.", "Plug in the DGM20. This page reconnects on its own.");
     }
@@ -258,7 +256,7 @@ function render(snap) {
     $("fw").textContent = `Firmware ${snap.info.firmware}. Unofficial tool, not affiliated with Maono.`;
   }
 
-  if (!busy.has("mute")) {
+  if (!busy.has("mute") && state.mute != null) {
     pad.setAttribute("aria-pressed", String(!!state.mute));
     pad.setAttribute("aria-label", state.mute ? "Unmute microphone" : "Mute microphone");
     $("pad-state").textContent = state.mute ? "Muted" : "Live";
@@ -269,6 +267,7 @@ function render(snap) {
     const n = s.dataset.name;
     if (busy.has(n)) continue;
     const v = state[n];
+    if (v == null) continue;          // mic busy for a moment: keep the last known value
     s.setAttribute("aria-checked", String(v === true || v === "on"));
   }
   for (const s of sliders) {
@@ -278,7 +277,6 @@ function render(snap) {
     $(`${n}-out`).textContent = state[n];
   }
   $("nc-level-row").dataset.disabled = state.nc ? "0" : "1";
-  document.documentElement.style.setProperty("--ring-glow", ((state.brightness ?? 100) / 100 * 0.85 + 0.15).toFixed(2));
   lastConnected = true;
 }
 
@@ -314,7 +312,6 @@ for (const s of sliders) {
   s.addEventListener("input", () => {
     busy.add(n);
     $(`${n}-out`).textContent = s.value;
-    if (n === "brightness") document.documentElement.style.setProperty("--ring-glow", (s.value / 100 * 0.85 + 0.15).toFixed(2));
     clearTimeout(timer);
     timer = setTimeout(() => api("/api/set", { name: n, value: Number(s.value) }).catch(() => {}), 120);
   });
diff --git a/tests/test_protocol.py b/tests/test_protocol.py
index 3a19c23..407c913 100644
--- a/tests/test_protocol.py
+++ b/tests/test_protocol.py
@@ -60,5 +60,21 @@ class Encoding(unittest.TestCase):
             encode("volume", 3)
 
 
+
+class NotRespondingDetection(unittest.TestCase):
+    def test_all_unanswered_reads_raise(self):
+        from dgm20.device import Mic, NotResponding
+        mic = Mic.__new__(Mic)          # no device needed: reads always time out
+        mic.read = lambda addr: None
+        with self.assertRaises(NotResponding):
+            mic.state()
+
+    def test_partial_answers_are_fine(self):
+        from dgm20.device import Mic
+        mic = Mic.__new__(Mic)
+        mic.read = lambda addr: 1 if addr == 0x2022 else None
+        self.assertTrue(mic.state()["mute"])
+
+
 if __name__ == "__main__":
     unittest.main()