I am working on an application using CoreAudio on the iPhone/iPad. The application both plays audio through the speakers (output) as well as records audio from the microphone (input) at the same time. For the purposes of this application it is extremely important that I be able to compare the input and output, specifically how well they "line up" in the time domain. Because of this, correctly calculating the total latency between the input and output channels is critical.
I am testing across 3 different devices. An iPhone, an iPad, and the simulator. I've been able to empirically determine that the latency for the iPhone is somewhere around 4050 samples, the iPad is closer to 4125 samples, and the simulator is roughly 2500 samples. After much research (aka googling) I found a smattering of discussions online about calculating latency in CoreAudio, but they generally pertain to using CoreAudio on OSX rather than iOS. Because of this, they refer to various functions that do not exist on iOS. However, it seems that for iOS the correct solution will be to use AVAudioSession and some combination of the inputLatency, outputLatency, and IOBufferDuration. However, no combinations of these values seem to add up to the empirically determined values above. In addition, I get wildly different values for each parameter when I check them before vs. after calling AudioUnitInitialize. Even more confusing is that the values are much closer to the expected latency before the call to AudioUnitInitialize, which is the opposite of what I would expect. Here are the values I am seeing. iPad (before): in 0.032375, out 0.013651, buf 0.023220, total samples 3054 iPad (after): in 0.000136, out 0.001633, buf 0.023220, total samples 1102 iPhone (before): in 0.065125, out 0.004500, buf 0.021333, total samples 4011 iPhone (after): 0.000354, out 0.000292, buf 0.021333, total samples 969 The simulator always returns 0.01 for in and out, but I suspect these aren't actual/correct values and that the simulator just doesn't support this functionality. One other potentially interesting note is that I'm using kAudioUnitSubType_VoiceProcessingIO rather than kAudioUnitSubType_RemoteIO which I do expect to add some additional latency. My assumption is that this would be included in the inputLatency value, but perhaps there's another value I need to query to include this? What's the correct way to determine the total latency between input and output in iOS?
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