Precisely scheduling sound in iOS 7

Question

I'm working on an iOS7-only app that needs to display a clock complete with ticking sound. I've used a NSTimer of 1s and I use AVAudioPlayer to play the tick sound every second.

Unfortunately, there's something slightly off with the timing. I've measured that timer is off by between 2 and 22 thousands of a second, which you wouldn't think would matter a great deal, but the lag creates a nail biting tension.. kind of like a heart flutter :-)

I've looked around a bit but it sounds like using audio queue services is the only way to go.. and I really don't fancy delving into the depths of that particular framework again.

My question: Is there some other way of getting precisely scheduled sound events in iOS 7 and failing that is there a decent wrapper framework for audio queue services available somewhere? Or better still is there a way of more precisely scheduling NSTimers?

Answer 1

Using any of NSTimer, libdispatch, or spawning a thread that sleeps for the tick duration rely on the underlying thread getting scheduled in time. The kernel provides no guarantee of this, and it is not surprising that the you observe timing jitter; the latency you observe looks reasonable.

NSTimer running on the main thread is likely to perform worst of these as you are also contending against other events delivered through it.

I think your options here are either to use audio queue services, a real-time thread to schedule the events with AVAudioPlayer, or render the audio yourself to a remoteIO unit.

I don't think AVPlayer provides any particular guarantees about timing either.