Jambox distortion

[jazzjohn27]

I think there's an issue with Jamulus on Jambox. If a window (like the Connections or Settings window for example) using either NoVnc or Vnc is moved on my Win10 computer, distortion starts. Interestingly if the window is moved again slightly, the distortion disappears.
It seems like the Pi can't process rewriting the graphics and handling the audio at the same time. Increasing the buffer size to fix the problem adds delay so I'd rather not.
A friend with a Mac sees the same issue. However, he eliminated the problem by running X11 (aka X Windows) instead of NoVnc or Vnc. The theory is that by doing so, the Pi has less graphics processing to handle, since X11 does that on the connected computer instead of on the Pi.

[kdoren]

@jazzjohn27

Questions:

1. Are you using Pi4 or Pi3?
2. what are your settings for PERIOD and NPERIODS?
3. what audio interface are you using?
4. did you make any other config changes?
5. which jambox version, are you using v1.3.0?

In general:

1. moving windows around while playing with Jamulus on Pi4 has a slight audible impact while dragging the window. On Pi3 the impact is larger.
2. if you reduced NPERIODS in an attempt to minimize latency, you might have gone a bit too far. It's possible to find a setting that seems stable, but then the system can start distorting, then flip back to sounding OK. I don't know exactly what the failure mode is, only that the remedy, as you have found, is to increase NPERIODS.
3. certain audio interfaces seem to be more prone to this than others.

My sense is that you have taken the NPERIODS setting down to the point of instability for your interface, and the fix is to increase it until the system is stable.

I haven't seen this issue in recent testing, but then I don't try to crank NPERIODS down to find its lower limit.

If you are connecting via X11, then you are a technically sophisticated user, so the following info might be useful:

I suppose it's possible that there is a bug somewhere (in Jamulus, jack, jambox) that, if fixed, would allow lower NPERIODS settings.

I do my own builds of latest versions of jack audio and some alsa-related things, to pick up some bug fixes (Raspberry Pi distro versions are older).

Also, jambox uses my PREEMPT_RT kernel build - if you want to try the standard kernel, in /boot/config.txt, change [pi4] to [none] above the Pi4 kernel section at the bottom, and reboot. But my A/B testing indicates that PREEMPT_RT kernel provides slight performance improvement in i.e. window dragging.

In /etc/jackdrc.conf, there are settings for JACK_PRIORITY and JAMULUS_PRIORITY. These were tuned by trial and error to minimize xruns. You can try experimenting with them, as they could affect audio timing.

I don't think your problem is video-related, but FYI there is also a video driver that I have disabled because I believe it adds overhead and in rare cases can cause system to hang. If you want to try it, change [pi400] to [pi4] above the VC4 driver section at the bottom of /boot/config.txt.

-Kevin

[kdoren]

FYI a similar issue was reported here: #71
but I don't know a way to reproduce it.

Another thought: does "enable small network buffers" in jamulus settings make any difference?

[kdoren]

@jazzjohn27 I ran some tests on v1.3.0 on Pi4 with 8 different interfaces, restarting Jamulus maybe 50-60 times.

In general I was quite pleased the results, I could set NPERIODS down to 2 with almost all of the interfaces. I could drag windows around without problems.

However, on a few restarts with 2 of the interfaces, I did get it into the "distortion" state. Restarting Jamulus kicked it out of that state.

I have a theory. Jambox starts jack with the "-s" (softmode) switch - note lower case "s", NOT upper case. This is supposed to make jack ignore xruns from ALSA. (see: https://jack-devel.jackaudio.narkive.com/AWQVU91F/what-does-soft-mode-do). My theory is that an xrun with softmode can get the jack ring buffer into a state where samples can get played out of order (which would be a bug in jack).

So I tried removing "-s" from /etc/jackdrc, and I was not able to get it into the distortion mode. This is not definitive, because I had trouble getting it into distortion mode even with softmode enabled. But please give that a try and let me know if it helps.

The following commands should do it:

sudo cp -p /etc/jackdrc /etc/jackdrc.bak
sudo sed -i "s/ -s//" /etc/jackdrc

[jazzjohn27]

The above 2 commands seemed to help. Now when moving around the windows, slight distortion occurs only when moving. The noise and distortion doesn't "stick" like it did before. I need to test it for longer to confirm that it's fixed. I will also have others in my group test.
Here are answers to your questions:
Are you using Pi4 or Pi3? Pi3B
what are your settings for PERIOD and NPERIODS? The small buffer is 64. Offhand,I don't know how to check PERIODS and NPERIODS
what audio interface are you using? Focusrite Scarlet 2i2
did you make any other config changes? not that I remember
which jambox version, are you using v1.3.0? 1.2.0 is what I tested your commands on, but i also ran the latest version and distortion happened there, too. Didn't try your commands with it yet.

[kdoren]

@jazzjohn27 Thanks for the info. Pi3 has significantly lower performance than P4: https://magpi.raspberrypi.org/articles/raspberry-pi-4-specs-benchmarks

Pi3 can run Jambox, but it has a lot less headroom, so any other processes (i.e. dragging a window) are more likely to impact the audio, However the impact should or course be temporary.

Jambox v1.3.0 uses a newer build of jack, a newer kernel, and it also uses different settings for Jack PERIOD and NPERIODS based on Pi model. These are in /etc/jackdrc.conf. Pi3 uses PERIOD=128 for (vs 64 for Pi4). This was to minimize xruns (audio frames not being ready when needed), but comes at the expense of slightly longer delay (Jamulus "small network buffers" setting only works with PERIOD=64).

I'd suggest the following:
1, Use the Jambox v1.3.0 image, with the commands I have posted above to disable softmode.
2. edit the file /etc/jackdrc.conf and find the section for Pi3B settings of PERIOD and NPERIODS: ("sudo leafpad /etc/jackdrc.conf")
3. Try setting NPERIODS to 2 (leaving PERIOD=128). 2i2 should work with that.
4. For comparison, try PERIOD=64, and NPERIODS=3 (or 2 if it works).

PERIOD=64 will allow you to "enable small network buffers", which will give you lowest delay. However with Pi3 you will get a few xruns even when not dragging a window. You'll need to decide which settings you prefer. Please report back what you find.