1
Diagnostic step 1
Avoid aggregate devices when possible: use the USB interface alone as both input and output or route internal mic through software if required (eliminates clock drift).
FULL AI ANALYSIS
USB audio interface shows intermittent crackling and dropouts only when macOS app uses CoreAudio aggregate device
Generated Oct 12, 2026, 10:57:59 PM
Analysis summary
Aggregate devices combine multiple hardware clocks into a virtual device. If one device's clock drifts or sample rates differ, CoreAudio uses drift correction, which resamples to keep streams in sync. Resampling and clock adjustments increase CPU load; under heavy GUI or kernel activity this can cause buffer underruns and dropouts. The fact single-device operation is stable and issues reduce with larger buffers indicates buffer underruns caused by increased CPU/IO from resampling and drift correction, or by mismatched USB isochronous scheduling.
Possible causes
Aggregate device resampling paired with host CPU spikes (WindowServer/kernel_task) causing processing delays and audio underruns.
90%Sample rate mismatch and drift between internal mic and USB interface requiring continuous resampling (CPU overhead) that leads to ring buffer underruns.
80%Driver-level interaction or bug in the audio interface driver when used in aggregate mode under macOS 13.x.
35%USB bus bandwidth or scheduling constraints when multiple isochronous streams are aggregated, introducing jitter.
15%Suggested fixes
2
Diagnostic step 2
If aggregation is needed, ensure all member devices are set to identical sample rate and bit depth (e.g., 48 kHz, 24-bit) at the OS and device firmware level to reduce resampling work.
3
Diagnostic step 3
Increase CoreAudio buffer size where acceptable; test the smallest buffer that eliminates underruns. Use dedicated audio performance mode: close background apps and disable energy-saving features.
4
Diagnostic step 4
Monitor CPU and kernel activity during glitches (in Activity Monitor and Console) to identify interfering processes; test with minimal session to confirm.
5
Diagnostic step 5
If problem persists, capture CoreAudio debug logs and engage interface vendor support; consider using an aggregate-capable interface that exposes a shared clock (word clock or ADAT) to avoid resampling.