CODEWITHSUNDEEP
iosobjective-ctimestampcore-audio
estobbart
estobbart

Reputation: 1145

iOS Accurate AudioTimeStamp when rendering Audio Units

In my AudioInputRenderCallback I'm looking to capture an accurate time stamp of certain audio events. To test my code, I'm inputting a click track @120BPM or every 500 milliseconds (The click is accurate, I checked, and double checked). I first get the decibel of every sample, and check if it's over a threshold, this works as expected. I then take the hostTime from the AudioTimeStamp, and convert it to milliseconds. The first click gets assigned to that static timestamp and the second time through does a calculation of the interval and then reassigns to the static one. I expected to see a 500 interval. To be able to calculate the click correctly I have to be with in 5 milliseconds. The numbers seem to bounce back and forth between 510 & 489. I understand it's not an RTOS, but can iOS be this accurate? Is there any issues with using the mach_absolute_time member of the AudioUnitTimeStamp?

Upvotes: 2

Views: 1865

Answers (1)

hotpaw2
hotpaw2

Reputation: 70683

Audio Units are buffer based. The minimum length of an iOS Audio Unit buffer seems to be around 6 mS. So if you use the time-stamps of the buffer callbacks, your time resolution or time sampling jitter will be about +- 6 mS.

If you look at the actual raw PCM samples inside the Audio Unit buffer and pattern match the "attack" transient (by threshold or autocorrelation, etc.) you might be able get sub-millisecond resolution.

Upvotes: 1

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