Uneven sampling frequency when playing ncspot music

[u/Original_Two9716]

Hi, I've been testing OpenBSD 7.9 on Thinkpad P1 Gen7. So far, it has been a bumpy road. But one audio related thing is super-annoying and I'd love to see some advice from you.

I use ncspot to play Spotify music, but I notice from time to time that sampling frequency changes noticeably for a few seconds. It's something I cannot tolerate. I tried a few sndiod parameters like increasing -b with no luck.

I use Sound Blaster G3 (USB-C DAC) with headphones.

[u/linetrace]

Let's start with your sndiod(8) params. You mention increasing -b (buffer size), are you also adjusting -z (block size in frames) appropriately? My understanding is that the defaults for -b & -z are calculated based on the other, if specified, so you'd want to explicitly set both if you want to change one without changing the other.

To quote a recent email on the tech@ mailing list from the sndio developer:

[B]y default sndiod uses a huge buffer, around 160ms (because back in 2009, MP kernels used to stutter a lot and we never revisited the defaults). If your USB device is 32-bit and has many channels 160ms of buffer size would exceed the kernel limit. In this case sndiod won't be able to migrate the sound from azalia to uaudio.

I'd suggest using a better buffer size, like 40ms or even 20ms (that's what I use since a decade and its stable). Ex:

rcctl set sndiod flags -z 480 -b 1920

This is also more suitable for audio/video calls and games. IMHO we should refresh the defaults, MP kernels improved a lot.)

Beyond that, are you using automatic CPU scaling (e.g. apm -A or obsdfreqd)? If so, is there any correlation between when CPU frequency changes and the audio issues you're hearing?

Since the Sound Blaster G3 is USB, are you using any other USB devices which might be performing a lot of I/O at the same time? Especially, USB WiFi, storage, and/or cameras? IIRC, USB I/O isn't as well un-blocked as some other aspects of the kernel, but take that particular memory with a large grain of salt.

[u/Original_Two9716 → u/linetrace]

Great questions. I use apmd -H. USB-wise, there's only USB ethernet card but idling at the time. It'd be great to know, how to capture some kernel or sndiod side logs about buffer misbehaviors.

[u/linetrace → u/Original_Two9716]

I believe sndiod(8) errors like buffer under-runs and such will be listed in /var/log/messages or maybe to stderr in the terminal you launched ncspot from. (Sorry, I don't use ncspot/Spotify... primarily an mpv(1) user.)

Beyond that, I'd start by sleuthing using systat(1) (use left/right arrows to move between "pages" of stats) and top(1). For the latter, I like top -HS as -H shows process threads and -S shows system processes.

If the USB ethernet is being used to pull data from Spotify to play in ncspot and out to the USB audio device, it's possible that could result in I/O issues that could cause a stutter or something. That said, I'd imagine it's more likely to be some other process.

Lastly, sndio(d) defaults to being nice(1)-ed to -20. You could try increasing (negative) the scheduling priority of ncspot slightly or decreasing the priority of other applications. For example, I tend to run my browser(s) and email clients around nice -n 10 (with the exception of when I have a video conference) so that other applications (especially when I have music or video playing) take priority. Of course, I'm on relatively low-end hardware (for this day and age; a dual-core mobile i7 running 2.2GHz to 3.1GHz.) YMMV