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My understanding of Bluetooth pairing in general is that the phone and the gadget exchange some encryption keys and other pairing data.

  • How can TWS headphones like AirPods be seen as one entity by the phone, although there are clearly two Earbuds?
  • Which of the Buds does the Phones communicate with during the pairing?
  • Do the buds exchange the pairing information, and if so, how?
  • How can the phone be connected with seemingly one device, but play stereo audio on both headphones?
Daniel D.
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I dont know about AirPods specifically, but most TWS products on the market use Qualcomm chips. Those work where each bud has its own Bluetooth radio. The buds are paired at the factory and have MAC addresses which are identical, save for the least sig bit. When you pair the buds to a phone, only one bud is actually performing pairing, and the phone sees just one MAC address. The buds are connected to each other and share some propriety Bluetooth commands which enable them to switch MAC addresses in real time and "handover" the logical link to the phone, depending on which bud is serving as the gateway to the phone. In practice, the phone will be streaming the music/voice data to one bud, which will forward it to the second bud alongside some synchronization metadata so that you end up hearing the audio data from both buds at the same time. Sorry I can't really go into more technical details than that.

By the way, I'm not 100% sure but I think that AirPods have some additional radio comms with iOS devices, besides just the Bluetooth spec. That's how they can do "magic" stuff like automatic pairing.

akatz
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  • Thank you, this is fascinating. So, for both headphones to work, they must be within bluetooth range of eachother? I was expecting both of them to be connected to the phone, not to each other... – Daniel D. Jan 04 '22 at 15:30
  • If they were both connected to the phone, you would see them as two devices in your Bluetooth settings. The next generation of Low Energy Bluetooth is being designed with support for TWS and audio sharing, so in a few years we may actually get products on the market that connect to the phone as you described. For now the TWS mechanism is just an a clever cheat to make two Bluetooth devices appear as one. – akatz Jan 05 '22 at 15:02
  • Is it not possible that both earbuds decode the audio data coming from the phone, while only one responds and acknowledges the data? – Daniel D. Jan 05 '22 at 17:37
  • The nature of Bluetooth "classic" is that connections are between two devices only. There is a mechanism to broadcast data to multiple devices, called "Connectionless Slave Broadcast", but as far as I know its a niche application that nobody uses. Most connections are between just two devices, so the phone is only connected to one earbud (in fact it only "knows" one MAC addresses.) That bud forwards the data to the second bud. Again, AirPods may be different, but this is how most TWS devices do it. – akatz Jan 09 '22 at 14:16
  • I see that BT is only supposed between to connect two devices. Still, if the buds exchange the bonding data like encryption keys, why can't the second bud just decode the BT data, without responding to it? – Daniel D. Jan 09 '22 at 15:15
  • I think I see what you're getting at Daniel. In theory yes, if you have complete control over the Bluetooth controller of two devices you could set them both to the same receive characteristics and intercept the same packets from the phone. – akatz Jan 10 '22 at 09:13