I believe what happens is that in full duplex, either by request from Jack or on its own, snd-usb-audio attempts to synchronize the playback and recording streams by scheduling packets to move in specific USB (micro)frames. With playback only, the queued packets are only marked "ASAP" and the hardware can cheat by delaying them to the next frame if either: - it fails to do the transfer on time for internal reasons - we fail to queue the transfer before its due time See discussion in xHCI spec 4.10.3.1, 4.10.3.2. Regarding capture, snd-usb-audio seems to always queue about 10 packets or so. Latency depends only on the hardware returning them quickly and no delays in the IRO-drivers-application pipeline. There is almost zero risk that the URBs won't be resubmitted on time and the hardware will face either an empty queue or an already stale packet. That being said, I'm seeing Missed Service Errors on the recording side, so it's possible that in my case the ASmedia controller is to blame...
