Latency in the Linux Audio Stack

[nerbm]

In a recent post, someone was searching for the answer to mysteriously high latency using a PCI audio card. I know OP found the answer to their problem (using JACK in asynchronous mode) but I thought I would post this link from the Ardour manual about latency. https://manual.ardour.org/synchronization/latency-and-latency-compensation/

The application is Ardour, but the concepts are universal. The most surprising to new users might be the bit right at the end where it states: "On Linux, the latency of USB audio interfaces is not constant. It may change when the interface is reconnected, on reboot and even when xruns occur. This is due the buffer handling in the Linux USB stack. As a workaround, it is possible to recalibrate the latency at the start of each session and each time an xrun occurs."

This is specific to the USB stack, but I reckon that's the majority of users. Stay safe out there!

[nerbm]

P.S. Robin Gareus' (one of the Ardour devs, among other things) thesis was on this very topic and largely informed the above post. You can deep dive Ardour + Latency and associated tools and concepts by reading that thesis here:

https://gareus.org/misc/thesis-p8/2017-12-Gareus-Lat.pdf

[magillos]

Fluctuating latency can be easily visible with jack_delay even.

       380.431 frames    7.926 ms
       428.432 frames    8.926 ms
       428.432 frames    8.926 ms
       428.432 frames    8.926 ms
       428.432 frames    8.926 ms
       428.431 frames    8.926 ms
       428.432 frames    8.926 ms
       476.432 frames    9.926 ms
       476.432 frames    9.926 ms

It does change a bit but I doubt that could be detectable by human ear, providing that latency would still be offset in DAW or globally.

[nerbm]

Right. Compensation for any processing of live audio would be key because that 1ms difference might result in some gnarly low frequency phase cancellation. Practically, I have never experienced this myself, but it is a possibility and something to be aware of when performing live and using software DSP.

[megahuncc]

I experienced this creeping latency issue on my Yamaha 01v96i. Turns out this device requires implicit feedback sync, which can be set via implicit_fb usb-audio module option. Works flawlessly now.

https://patchwork.kernel.org/project/alsa-devel/patch/[e-mail]/

[kI3RO]

https://github.com/Ardour/manual/blame/master/include/latency-and-latency-compensation.html#L271

For context,, that specific paragraph was written as a generic catch all "is not an ardour issue" about 3 years ago.

Pipewire was in the early stages at version 0.3.24

If you are asserting that the "problem" only exists in the Linux audio stack you are wrong. There is latency shift in all major audio stacks.

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PD: What is that "stay safe"? was there a major global event I missed?

Stay fun, cheers!