Q:

Time diff between AudioOutputUnitStart() and render callbackm for RemoteIO

This is a pretty "minutia" question regarding timing...
I'm using iOS's RemoteIO audio unit to do things. Just wonder how exactly system handles the timing: after calling AudioOutputUnitStart(), the unit should on "on", then render callbacks will be pulled by downstream units. Allow me to guess:
Possibility 1: the next render callback happens right after the execution of AudioOutputUnitStart(), then it goes on
Possibility 2: the system has its own render callback rhythm. After calling AudioOutputUnitStart(), the next render callback catches on one of the system's "next" tick, then start from there
1 or 2? or there's 3? Thanks in advance!

A:

The audio latency seems to depend on the specific device model, audio session and options, requested sample rate and buffer size, and whether any other audio (background or recently closed app) is or has recently been playing or recording on the system.  Whether or not the internal audio amplifier circuits (etc.) need to be powered up or are already turned on may make the biggest difference.  Requesting certain sample rates seems to also cause extra time due to the buffering potentially needed by the OS resampling and mixer code.
So likely (2) or (3).
The best way to minimize latency when using RemoteIO is to request very short buffers (1 to 6 mS) in the audio session setup, start the audio session and Audio Unit way ahead of time (at app startup, view load, etc.), then fill the callback buffers with zeros (or discard recorded callback data) until you need sound.

