Audio dropouts with external USB interface when connected through Thunderbolt dock while laptop is charging

Environment

MacBook Pro M1 Pro running macOS 13.4. External audio interface (Focusrite Scarlett 18i8) connected to a CalDigit TS4 Thunderbolt dock. Audio routed in Logic Pro via an aggregate device created in Audio MIDI Setup combining interface and system audio. The dock provides power (PD) to the Mac and runs other peripherals.

Symptoms

Periodic audio dropouts and crackles in multitrack recording sessions that coincide with audible glitches. Dropouts occur more frequently when the Mac is plugged into the dock and charging through the same TB4 cable. Recording continues but artifacts appear; CoreAudio-based system sounds do not always glitch.

Observations

Console logs and system.log show entries around the time of dropouts like 'IOUSBHostDevice: device disconnected' or 'IOAudioEngine: underrun detected'. When the audio interface is connected directly to the Mac (bypassing the dock) the dropouts disappear in long test sessions. Using a different Thunderbolt cable yielded reduced frequency but did not eliminate the issue. Resetting CoreAudio (sudo killall coreaudiod) temporarily removes the symptom until the next docking cycle. Dock firmware is up to date and macOS is fully patched.

Already attempted

Updated dock firmware and macOS. Recreated aggregate device with matching sample rates, tried different buffer sizes in Logic Pro (higher buffer reduced but did not eliminate glitches), killed coreaudiod when issues began, tested alternate TB4 cables and different wall outlet for dock power, connected interface directly which eliminated dropouts.

Question

Is the behaviour consistent with a Thunderbolt PD/power negotiation or USB re-enumeration issue from the dock causing the audio interface to momentarily disconnect? What diagnostic steps and reversible mitigations will help identify whether it's a dock firmware/PD problem, cable, or CoreAudio/driver issue, and what should I do to avoid losing project data while troubleshooting?

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AI DIAGNOSTICS

Incident analysis

Updated 1w ago• Trouble ID

Most likely causes

Thunderbolt dock firmware or USB controller firmware causing USB device re-enumeration on power negotiation or load changes. 85%

Dock power delivery events (voltage/current changes) triggering hub resets on the dock. 80%

Thunderbolt/USB-C cable not rated for full TB/USB host power and data stability, causing intermittent disconnects under PD negotiation. 60%

Summary

The logs showing USB device disconnect/reconnect alongside dropouts and the direct-connection success point to the dock or Thunderbolt link introducing transient USB host resets, likely during PD or link re-negotiation. Possible contributory factors are cable quality (not full TB4 spec), dock firmware bugs handling USB device power, or power draw anomalies when the internal battery is charging via the dock. CoreAudio treats device re-enumeration as device removal which can cause underruns and audible glitches.

Suggested Fixes

Diagnostic step 1

Back up all Logic Pro projects before further tests. Avoid destructive changes to audio files during diagnostic testing.

Diagnostic step 2

Reproduce the issue with logging enabled: capture Console logs (filter for IOUSBHostDevice and IOAudioEngine) while reproducing a dropout to get precise timestamps. Note whether PD negotiation occurs at the same time (Console may show related messages).

Diagnostic step 3

Test with a Thunderbolt-certified cable known to support required PD and data rates; compare with a verified direct TB4 cable to rule out cable limitations.