25 Sep
2019
25 Sep
'19
11:45 a.m.
Hello Lorenzo,
I have anecdotally, but recurrently observed on
laptops - including a
rather recent one - that there is always a 'best' USB port for external
sound cards.
For instance, on my latest machine with a decent 'realtime audio'
configuration/set-up (real-time kernel, /etc/limits stuff, 'performance'
CPU governor, (wireless) network switched off), I'm able to have a
pleasant xrun-free session recording in Ardour including a bunch of
tracks with effects playing at 64 frames and period of 3 with a
relatively cheap card (UMC202) on one of the USB ports.
On the other hand, in the exact same conditions I get incidental xruns
at even 128 frames and xrun instability at 64 frames on the other USB
ports.
I wonder:
1. Is there a more scientific (well, precise at least) method to assess
this USB port performance? What to test or look into?
You can check with lsusb what sits on which bus and if your soundcard
sits on the same bus with a device that could interfere (Wifi,
Bluetooth) then either try moving it to a different bus or try freeing
that bus. A bus is not the same as an USB port, multiple USB ports could
be hooked to the same bus.
2. Is there a way to change (e.g. improve the
not-so-good USB port
performance) OS/software wise, or is this usually hard-wired in notebooks?
You can prevent kernel modules from loading by blacklisting them or even
unbinding them:
https://autostatic.com/2013/12/29/resolved-jack-issues-on-notebook/
2a. Are IRQs relevant on laptops and if so can a
whole USB port (or
the device attached to it) be optimised from the OS?
Yes they are and they can be optimised if you use threaded IRQ's (i.e.
hardware IRQ's that have a software counterpart). There is useful info
about that on https://wiki.linuxaudio.org/wiki/system_configuration
Of course I _can_ live with one 'good sound-card
port' on a laptop but
I'm quite curious about people's experiences and the gurus' wisdom -
albeit on my former machine this was the left-side port which was closer
to where the sound-card usually sits, now it's on the right, too bad!
Hopefully other LAU have mused about such USB-related mysteries in the
past...
Well I have so if you'd like to improve your current situation I might
be able to help out.
Jeremy