Quote:
Originally Posted by JFreak
USB 2.0 negotiate at 10 or 20 ms latency depending on features used by the drivers. Good luck going below that. Newer versions obviously faster, but the thing has not been designed for low latency to begin with.
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Uh no. As already pointed out in this thread USB 2 *is* capable of low latencies, and what you get is highly dependent on the driver implementation. Certainly latencies well below 10ms or 20ms RTL (let alone 10ms or 20ms one way latency) are possible.
It takes a development team that knows what they are doing. And we've already pointed to RME being a leader there. And since my RME UFX+ does Thunderbolt, USB 3.1, and USB 2 and USB 2 class compliant I can report those RTL measurements. Or at least I would have Thunderbolt measurements if I could find my long Thunderbolt cable. Anyhow these are some no-load measurements (Pro Tools on this system routinely gets used for tracking a few channels in small sessions in Pro Tools at 32 samples with this interface vi USB 3 @ 96 kHz).
Code:
Measured RTL @ 96 kHz, samples / ms
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I/O Buffer size: 32 sample 64 sample
USB 3 (RME driver) 221 / 2.30 (173 / 1.8)* 285 / 2.97 (237 / 2.47)*
USB 2 (RME driver) 221 / 2.30 (173 / 1.8)* 285 / 2.97 (237 / 2.47)*
USB 2 Class compliant -- 562 / 5.85
* Measurements in brackets were made with the RME driver set to use the short safety offset. Short safety offset mode may or may not be usable depending on load on the system/DAW etc. The other measurements not in brackets are made with the driver using the standard safety offset.These are actual RTL measurements made with RTL Utility using line level analog output to line level analog measured and checked with my own tools.
RME Fireface UFX+ Firmware v54
RME USB Driver version v3.27macOS Big Sur 11.6
The UFX+ allows you to switch between USB 3 or USB 2 or USB 2 class compliance modes, the Fireface USB Setting utility is used to check what USB mode the interface is in.
And whoa surprise surprise (it was a surprise to me) the USB 2 latencies are exactly equal to the USB 3 latencies. And yes that is real. Under the covers there are differences as well, USB 2 is always running to isochronous mode, USB 3 is defaulting to bulk mode, you can force the driver into isochronous mode (say if the interface is getting errors). Changing that mode made no difference to the latency measured.
And while YMMV as to how low an IO buffer size you can run with for your use, here is USB 2 and 3 showing impressive low RTL latency, that is far away from supposed USB 2 10ms or 20ms latency overhead.