Reply To: USB-B Muiltitrack Playback Problems – Qu24

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Phew, 10.5, thought my ‘2008 Mini running 10.7 is vintage… šŸ˜‰
In fact OSX changed significantly from 10.4 to 10.6 when it comes to USB AudioClass 2 support (the part below Core Audio). And if your hardware is of a similar age, it may as well be the mainboard chipset itself, which does not provide the required bandwith.
If streaming only IN (Device->Host) works well this may also indicate a low-level bandwidth issue.

From a technical standpoint the USB link will always stream all channels, even if only one stream is used. While recording only one stream is required, the DAW will just pick the channels of interest.
The opposite OUT direction (Host->Device) is slightly more complicated, since with the Qu as Clock Master the host (PC/Mac) needs to know how many samples to send in a particular time frame (1/8 mSec). The Mac simply opens the IN direction as well and sends the same quantity of samples back to the device, effectively synchronizing the understanding of samplerate on both ends.
In this situation two streams are required, effectively doubling the USB load.