Hi Eric, I did some reverse engineering on this issue about a year ago and this is what I found:
Note that it only applies to RemoteIO in the context of AudioUnits / AudioGraphs. I believe that AVAudioEngine uses the same graph under the hood but haven’t replicated measurements there yet. Any insights from CoreAudio people would be welcome! :) You get the following from AVAudioSession: inputLatency outputLatency ioBufferDuration Then your throughput latency, assuming a Stereo Stream, would be: inputLatency + outputLatency + 4*ioBufferDuration In case inputLatency > ioBufferDuration, you add one more ioBufferDuration. Same with outputLatency! This means that when lowering your ioBufferDuration, your Session mode becomes important (which directly affects input and output latencies). The lowest you can achieve would thus be with the Measurement Mode. Now all the above values are in Seconds (sigh!) but you should be able to convert back to samples easily using your Session sample rate. This all makes sense if you visualise the audio graph and imagine a Pull mechanism (which is what’s happening in CoreAudio). If anyone else has insights on this, it would be very welcome! Cheers, Arshia Cont www.antescofo.com <http://www.antescofo.com/> > On 20 Jan 2020, at 15:09, Eric Herbrandson via Coreaudio-api > <coreaudio-api@lists.apple.com> wrote: > > 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? > > > > _______________________________________________ > Do not post admin requests to the list. They will be ignored. > Coreaudio-api mailing list (Coreaudio-api@lists.apple.com) > Help/Unsubscribe/Update your Subscription: > https://lists.apple.com/mailman/options/coreaudio-api/arshiacont%40antescofo.com > > This email sent to arshiac...@antescofo.com
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_______________________________________________ Do not post admin requests to the list. They will be ignored. Coreaudio-api mailing list (Coreaudio-api@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/coreaudio-api/archive%40mail-archive.com This email sent to arch...@mail-archive.com
