How to test MIDI clock messages for a loop

Your loop drifts, starts late, or seems out of step with another device. Before changing tempo or rebuilding your setup, check whether MIDI clock and transport messages are reaching the computer at all.

The MIDI Test can show incoming MIDI messages in your browser. Use it to check for clock pulses and transport commands, then compare what arrives with what your loop source is sending.

Test MIDI clock messages

1. Open the MIDI Test in a browser that supports MIDI access. It does not work in Safari.
2. Connect the MIDI device or interface that receives the clock. If the browser asks for MIDI access, allow it, then choose the incoming port in the test if a port choice is available.
3. Set your sequencer, groovebox, or other clock source to send MIDI clock to that input. Make sure the source is running and the clock is routed to the computer, not just to another device.
4. Start the loop and watch the incoming message list. Look for repeated clock or system real-time messages. If the test shows raw status values, MIDI Clock is F8.
5. Stop playback, start it again, and try a locate or continue action if your setup uses one. Check whether the matching transport messages appear alongside the clock.

MIDI clock does not send a tempo number. It sends 24 pulses for every quarter note, so the pulse interval depends on the tempo. For example, at 120 beats per minute, pulses are about 20.8 milliseconds apart. At 60 beats per minute, they are about 41.7 milliseconds apart.

What the messages mean

Clock pulses repeat while the source runs

Repeated F8 messages mean clock is reaching the browser. A steady stream is a useful first check, but the browser test is not a precision hardware analyzer. Browser scheduling and how the page displays events can make closely spaced messages look uneven or arrive in visible batches. Do not judge a performance by tiny apparent timing differences in the list alone.

Clock appears, but there is no Start or Stop

The clock source may be sending timing pulses without transport commands, or its settings may send transport separately. MIDI Start is FA, Continue is FB, and Stop is FC. Some programs send these only when transport sync is enabled. Their absence does not by itself mean the clock pulses are faulty.

Start appears, but there are no repeating clock messages

A transport command arrived, but timing pulses did not. Check whether the source is configured to send MIDI clock, and whether it sends clock to the same port the test is monitoring. A single Start message cannot keep another device synchronized.

No messages appear

The browser may be listening to the wrong input, access may be blocked, or the source may not be sending MIDI to the computer. Confirm the route before assuming the MIDI device is broken.

A locate or loop restart does not show a position message

Song Position Pointer, status value F2, can indicate a position when a sender supports and uses it. Not every sequencer sends it on each loop restart, and some use Start or Continue instead. Check your source's sync behavior before treating a missing position message as a fault.

How to fix missing or irregular clock

1. Enable clock output at the source. In your sequencer or hardware's MIDI sync settings, turn on MIDI clock transmission and select the output port connected to the computer. The exact menu name depends on the device.
2. Check the route from output to input. Clock must reach a MIDI input the browser can access. If you use an interface, check its MIDI cable direction and connect the source's MIDI Out to the interface's MIDI In. A software instrument's internal clock does not automatically appear as an external MIDI input.
3. Confirm the browser has access to the device. Reopen the test and approve its MIDI access request if it appears. Close and reopen the page after changing access or connecting a device. The test needs permission to read incoming MIDI.
4. Pair a Bluetooth MIDI device again if needed. On Windows 11, check Settings › Bluetooth & devices › Devices. On macOS, check System Settings › Bluetooth. Pairing confirms the wireless connection, but you still need to route clock from your music source to the browser's MIDI input.
5. Check for a filter or a second clock source. Disable any setting that filters system real-time messages, and avoid sending clock from two sources at once. Competing clocks can make connected gear behave unpredictably even if messages appear in the test.
6. Reconnect the interface and retest. Unplug and reconnect the MIDI interface, then select its input again. Try another cable or port if messages still do not arrive. If the device is missing from the computer in other MIDI software too, investigate the interface, cable, or device connection.

Use the result without overreading it

A browser test can confirm that the computer receives MIDI messages, but it cannot prove that every instrument follows the clock tightly or that a performance has no audible timing drift. The receiving device, its settings, the MIDI route, and the loop playback engine all affect what you hear.

For a practical check, send clock from one source, start the loop, and listen to the receiving device over several repetitions. If the message stream is present but the loop still drifts, test the receiving device's sync settings and MIDI route next. Keep the MIDI Test open in a supported browser while the source is running, since Safari does not support this test.