To answer that question let’s assume the following:
- You want to do real-time audio, for instance you would like to do an overdub in a song project by using a MIDI keyboard and a soft synth.
- You’re using a USB audio interface.
- You’re experiencing xruns when using lower buffer sizes (64 or even lower) that result in crackling audio.
Basically all components of a desktop computer or a laptop have an IRQ. Let’s say IRQs are some sort of communication lines between the brains of your computer (the CPU) and those components. All these communication lines are more or less the same as in, none of these communication lines has priority over the other. But if you suffer from bad audio quality caused by xruns then giving the communication line that is connected to your USB device more priority than the rest could alleviate the audio issues.
This is where rtcirqus could be helpful. What rtcirqus does is detecting your USB audio interface when it gets connected, then it figures out what the IRQ of the USB bus is that your audio interface is connected to and raises the priority of the IRQ attached to that USB bus. In other words, rtcirqus gives the communication line between the USB bus that your audio interface is connected to and the CPU of your computer a higher priority. This higher priority translates into other communication lines being throttled and giving the communication line of our USB audio interface more room to transfer and receive its audio data.
rtcirqus needs to be able to prioritize that communication line though and this is where the threadirqs kernel option is for, it creates so-called threads of all available IRQs on a computer. Threads are more or less like processes running on your computer, with threadirqs enabled you can list them as such with a command like ps -eLo cmd | grep '^[irq'. So make sure this kernel option has been added to the kernel command line. In case you’re using a real-time kernel this isn’t needed as a real-time kernel uses threaded IRQs by default.
So if you find yourself in such a situation, head over to https://codeberg.org/autostatic/rtcirqus and give rtcirqus a try, maybe it helps you achieving lower latencies with your USB audio interface while retaining a clean audio output signal.
