FW: Sync time between phone and Earables

[DennisMoschina]

Problem: No Reliable Way to Align Timestamps Across Two Earables

At the moment, aligning sensor data recorded on two earables requires manual synchronization methods (e.g., tapping both devices or generating a sharp sound). This is error-prone and unsuitable for automated data collection.

The Bluetooth specification includes the Current Time Service (CTS), which allows an earable to query the phone for a wall-clock timestamp. However, CTS is not highly accurate:

• The timestamp is generated when the phone prepares the response
• The read operation is delayed by the BLE connection interval and additional OS/BLE stack latencies
• Typical end-to-end accuracy is tens to hundreds of milliseconds, which is insufficient for precise multi-device alignment

Bluetooth Isochronous Streams (CIS/BIS) can provide much higher-precision timing, because the controller exposes microsecond-level anchor points and event counters. These can be used to derive a shared high-precision timebase between two earables.
However, this approach has two major limitations:

• Many phones and platforms do not support LE Audio isochronous streams yet
• An isochronous stream must be explicitly created and started (e.g., by a broadcaster or central), which adds complexity and may require running a stream even when no audio is transmitted

Proposed Approach: Multi-Layer Time Synchronization

A combined solution could provide robust, cross-platform synchronization:

1. Coarse synchronization via CTS:
   Each earable reads the phone’s CTS once on connect (or periodically) to align its wall-clock time. This gives a shared absolute reference with low precision, but high convenience and universal support.

2. High-precision synchronization via isochronous timestamps:
   The two earables exchange or follow a custom isochronous stream (not necessarily carrying audio) to derive a shared monotonic clock with sub-millisecond accuracy.

Combining both layers allows:

• Wall-clock alignment across devices (CTS)
• Precise inter-ear synchronization (isochronous timestamps)

Next Steps

We should define:

• The required accuracy for cross-device timestamp alignment
• Whether a custom BIS/CIS broadcaster is feasible
• The trade-offs between universal compatibility (CTS) and high accuracy (ISO-based sync)

[TobiasRoeddiger]

We need BLE 5.2 to function and the isochronous streams appear to be part of the BLE 5.2 spec? Can we build a simple prototype to fetch time via CIS and check if it can work independet of LEAudio streams? Especially w.r.t. iOS.

Check this example: https://docs.nordicsemi.com/bundle/ncs-3.0.0/page/nrf/samples/bluetooth/iso_time_sync/README.html

[TobiasRoeddiger]

Upon further investigation it seems like iOS does not support CIS at this time. Hence CTS is the preferred option, but the peripheral should not read from the phone, CTS should be implemented on the earable side and we write the time to it.

[DennisMoschina]

According to the CTS specification, it is optional to implement write access to the time characteristic. So implementing the service on the Earable is possible.
Apple's Accessory Design Guidelines for Apple Devices in section '55.12.4 Available Services' mentions, that the CTS is already implemented and available to accessories.
I was not able to find comparable information about it for Android for now. If it is available, I suppose the best approach IMO would be to use the existing CTS, as we would be independent of our app writing the local time. This might be especially useful for possible future features where the earable does not rely on configuration via the app to record data

[TobiasRoeddiger]

But how would the OE know which central it should connect to? We don't want it to connect to random phones. Only one can act as central/peripheral, and it is usually always the earphone that act as the peripheral.

[DennisMoschina]

I don't fully understand how the BLE connection works in this case, but using the CTS client example of nRF Connect I was able to sync the time immediately with an iPhone 15 Pro Max. I also tried it with a MacBook Pro M2, but there was no CTS found. According to Apple's documentation this might be due to the service not being immediately available, but for now this looks like a promising approach. Next steps will include testing this on Pixel 7 and other Android devices

[TobiasRoeddiger]

Great! Will have to read more into how this actually works under the hood. Also try with the Pixel phones :)