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Apple does not secretly analyze sine waves to infer head motion. Instead, airpods pro/max/gen-3 include actual IMUs (inertial measurement units), and ios expose

by fiq100 1y ago

Apple does not secretly analyze sine waves to infer head motion. Instead, airpods pro/max/gen-3 include actual IMUs (inertial measurement units), and ios exposes their readings through core motion. It’s a known research technique called acoustic motion tracking (some labs use inaudible chirps to locate phones or headsets) you mentioned, but it’s not how airpods head tracking works

graypegg 1y ago
I think they're more so talking about measuring attenuation that apple applies for the "spatial audio" effect (after apple does all of the fancy IMU tracking for you), by using a known amplitude of signal in, and the ability to programmatically monitor the signal out after the effect, you can reverse engineer a crude estimated angle out of the delta between the two. I don't think that's how this app works though, after installing it I got a permission prompt for motion tracking. Looks like there is an API for this. Here's an example: https://github.com/tukuyo/AirPodsPro-Motion-Sampler/blob/8acf6198580690da5339d520ff4e2a0dbe7f8741/AirPodsProMotion/SK3DViewController.swift#L15C15-L15C39 https://github.com/tukuyo/AirPodsPro-Motion-Sampler/blob/8ac...
- anileated 1y ago
  Yup. Since the author of the app mentioned reverse engineering, analyzing audio is a way that immediately came to mind. It should be quite precise, too, only at the expense of extra CPU cycles. I did not imply that there is no API to get head tracking data (even though Google search overview straight up says that). It’s mostly a thought experiment. Kudos for digging up CMHeadphoneMotionManager.
anileated 1y ago
> Apple does not secretly analyze sine waves to infer head motion. Duh. The mechanism I described hinges on Apple being able to track head movements in the first place in order to convert that virtual 3D scene to stereo sound.

by fiq100 1y ago

graypegg 1y ago

I think they're more so talking about measuring attenuation that apple applies for the "spatial audio" effect (after apple does all of the fancy IMU tracking for you), by using a known amplitude of signal in, and the ability to programmatically monitor the signal out after the effect, you can reverse engineer a crude estimated angle out of the delta between the two. I don't think that's how this app works though, after installing it I got a permission prompt for motion tracking. Looks like there is an API for this. Here's an example: https://github.com/tukuyo/AirPodsPro-Motion-Sampler/blob/8acf6198580690da5339d520ff4e2a0dbe7f8741/AirPodsProMotion/SK3DViewController.swift#L15C15-L15C39 https://github.com/tukuyo/AirPodsPro-Motion-Sampler/blob/8ac...

anileated 1y ago
Yup. Since the author of the app mentioned reverse engineering, analyzing audio is a way that immediately came to mind. It should be quite precise, too, only at the expense of extra CPU cycles. I did not imply that there is no API to get head tracking data (even though Google search overview straight up says that). It’s mostly a thought experiment. Kudos for digging up CMHeadphoneMotionManager.

anileated 1y ago

> Apple does not secretly analyze sine waves to infer head motion. Duh. The mechanism I described hinges on Apple being able to track head movements in the first place in order to convert that virtual 3D scene to stereo sound.