MIDI clock drift after changing sustain polarity: checks

Your notes or tempo seem to drift after you changed the sustain pedal's polarity setting. These can be separate problems: polarity changes how the keyboard reads the pedal, while MIDI clock carries timing information between devices.

First check what the keyboard is sending. The MIDI Test can show incoming pedal, note, and other MIDI messages in your browser. It can help find a pedal or connection problem, though it is not a precision clock-timing analyzer.

Check the pedal and MIDI messages

1. Open the MIDI Test in a browser other than Safari. Connect the keyboard or controller, allow MIDI access if the browser asks, and select its MIDI input if needed.
2. With the pedal connected, press and release it several times without playing notes. Look for a pedal message that changes with each press. Sustain commonly uses Control Change 64, often shown as CC64.
3. Play several notes with the pedal released, then hold it down and play again. Check whether notes arrive and the pedal message changes as expected. Repeat with the pedal disconnected.
4. If your device sends MIDI clock, check whether clock messages arrive steadily while the device is playing. Compare the behavior with the pedal connected and disconnected. Use your music software or receiving device to judge timing over a longer passage.

Browser MIDI access depends on your browser and device. If the keyboard does not appear, check its connection and try again before treating that as a clock fault.

What the result means

The pedal message changes, but notes do not drift with the pedal unplugged

The pedal is probably being read, and the polarity change is unlikely to be causing clock drift. Check the clock source and sync settings in the keyboard, sequencer, or other device that is playing the timing. If you need a closer look at clock messages, see how to investigate MIDI clock drift on a USB-C adapter.

The pedal message stays on or off, or changes backward

The keyboard may be interpreting the pedal's normal state as pressed. This is a polarity or pedal-recognition issue, not proof of clock drift. Check the keyboard's pedal polarity setting and test again with the pedal released, then pressed.

Clock messages stop or become irregular when the pedal is connected

The pedal should not normally change MIDI clock timing. A shared connection, controller mode, or device setting may be involved. Repeat the test with the pedal disconnected, then connect it directly rather than through a hub if possible. Check whether the keyboard is set to send clock or only receive it.

The messages look consistent, but the music still drifts

The browser test confirms that messages reach the computer, but it cannot establish exact clock stability from that alone. Check whether the sequencer is following the intended master clock and whether the playback device is set to receive sync. A tempo mismatch can sound like drift even when the tempo is simply different.

How to fix it

1. Set pedal polarity with the pedal released. Use the keyboard's pedal polarity option, if it has one, and check the device manual for the expected setting. Some keyboards detect polarity at startup, so turn the keyboard off before connecting the pedal and turn it back on afterward.
2. Check the pedal's normal state. With the pedal untouched, the keyboard should register sustain as off. If the state is reversed, try the other polarity option. For more on the expected behavior, see how to test sustain pedal polarity.
3. Choose one clock source. If a keyboard and sequencer can both send clock, set one as the master and the other to receive external sync. Two competing clock sources can cause unstable tempo or unexpected timing.
4. Check the tempo and sync mode. Confirm that the master tempo is the one you expect and that the receiving device is not using its own internal clock. A stable but different tempo is not clock drift.
5. Reconnect the MIDI path. Reseat the pedal plug and MIDI or USB cable. Try a different USB port and, if possible, connect the keyboard directly instead of through a hub. Change one connection at a time so you can see what affects the result.
6. Restart the devices in order. Stop playback, switch off the keyboard and connected equipment, then power on the clock master before the receiving device. Recheck the pedal state and timing before restoring other connections.

When the pedal or keyboard may be faulty

If the pedal sends no change with the correct polarity selected, test it on another compatible keyboard, or test a known-working pedal with yours. A damaged cable, plug, or pedal switch can cause a stuck sustain state. If the pedal works elsewhere but your keyboard continues to misread it, consult the keyboard maker or a repair shop. Avoid a factory reset until you have saved any custom settings, because it can erase unrelated controller assignments.

Why sustain and clock are different

A sustain pedal usually sends a control message that tells an instrument whether to let notes continue after you release the keys. MIDI clock is a stream of timing messages used to keep sequencers and other music devices in time. A polarity change affects how the pedal's on and off state is interpreted; it does not normally change the clock rate.

For a useful comparison, keep the same tempo, clock source, and cables, then test once with the pedal disconnected and once with it connected. If the timing changes only when the pedal or its cable is connected, investigate that signal path. If it drifts in both tests, focus on clock settings and the receiving device.