Building a metronome with GCD but sounds play unevenly when screen locked

Question

I am using GCD to fire a loop that play tick sound periodically.

I have also configured my AVAudioSession with AVAudioSessionCategoryPlayback and edited the info.plist file with audio background mode to enable background execution.

This worked very well when in the app or goes background. However, the sounds played very unevenly when the screen is locked. It sounds like dick-dick-dick---dickdick-dick-dick-dick

I run in debug mode with the screen locked and the log is outputted evenly as expected. So I guessed it may be the audio problem.

I've spent lots of time using different audio engines and libraries but none of them improved the situation.

Is making my own loop audio the way to go? I know AudioQueue may help but no idea where to get started.

Any idea is much appreciated.

Answer 1

GCD doesn't guarantee timings with the sort of precision that you need for doing audio timings. You need to get the jitter down to <20ms, ideally <10ms or <5ms if you want it to be useful for musicians, especially drummers.

AudioQueue or Audio Units is probably the way to go (I'd recommend AudioQueue). You need sample-level access to the sound stream, where you can precisely write samples with 22/44/48kHz resolution, which is what you'll need. If you count samples then you can guarantee that the ticks will happen at the right time, as long as the CPU can keep up with demands of the audio hardware.

If you go down this route, you'll need to quickly get an understanding of samples, interleaving and buffer sizes.