Adding a USB hub fixed my audio dropouts

Guitar into Ableton through a Scarlett 2i4, audible dropouts every few seconds. Buffer size didn’t matter. Closing Chrome didn’t matter.

coreaudiod had a confident answer: 555 overloads blamed on a background audio daemon, 73 on Ableton. I disabled the daemon. Nothing changed. CoreAudio names the clients affected by a reset, not the thing that caused it — every attached client gets flagged, so the loudest complainer is just the busiest one.

One layer down, the kernel was repeating this every nine seconds:

mismatched event TRB 0x0000000080538970
IOUSBHostPipe::abortGated: ... endpoint 0x82: aborting 64 requests
AppleUSBAudio: (Resetting engine due to error returned in Read Handler)

196 engine resets in 30 minutes. The Fresco Logic FL1100 controller in my CalDigit dock was desyncing its event ring and dumping every pending isochronous transfer. Each dump reset the audio engine. Each reset was a dropout.

Moving the interface behind a cheap USB 2.0 hub — still on the same dock — took it to zero. A USB 2.0 hub has a Transaction Translator that schedules isochronous traffic locally instead of handing that job upstream. I can’t prove that’s the mechanism; I never tested the Mac’s built-in port. But zero is zero.

The number worth keeping:

/usr/bin/log show --last 10m --predicate 'process == "kernel"' --style compact \
  | grep -c "Resetting engine due to error"

Earlier the same day I’d blamed a USB 2.0 hub for a set of flaky external drives. Opposite transfer type, opposite fix.