How Accurate Is the Hexoskin Internal Clock, and What Level of Drift Should I Expect?
This article explains Hexoskin’s internal clock accuracy, expected time drift (up to 1.7 seconds per day), and how to synchronize multiple devices for reliable data alignment.
The Hexoskin Smart Device uses a 32.768 kHz crystal oscillator with a tolerance of 20 parts per million (ppm) to maintain their internal clock. This level of precision is generally sufficient for most research applications where second-level timing is acceptable.
Understanding 20 ppm Clock Drift
A tolerance of 20 ppm means that the internal clock may drift by up to approximately 1.7 seconds per day:
60 seconds × 60 minutes × 24 hours × 20 ppm = 1.7 seconds/day
Over a month (about 2.6 million seconds), this can add up to roughly 1 minute of accumulated error—typical for quartz-based systems without frequent synchronization.
Note: For Astroskin, the oscillator has a tolerance of 10 ppm, which means that the internal clock may drift by up to approximately 0.8 second per day.
Influence of Temperature and Device Differences
Clock drift can be affected by temperature. If two devices operate under different thermal conditions (e.g., one in a warm environment and one in a cold one), their respective drifts may go in opposite directions. In the worst-case scenario, one device could drift forward by 1.7 seconds per day while the other drifts backward by the same amount, leading to a total offset of up to 3.4 seconds per day between the two.
Synchronizing Devices and USB Timing Jitter
When multiple Hexoskin devices are synchronized using the OneSync tool on the same computer, they typically remain within 100 milliseconds of each other, even without internet connectivity. This is because both devices are aligned to the same host computer clock. However, USB communication may introduce some timing jitter due to scheduling and threading on the operating system, but this is usually well below 100 milliseconds.
How Long Does the Synchronization Hold?
When two devices are synced on the same day, they can typically stay within one second of each other for at least 24 hours. This level of precision is sufficient for most same-day data comparisons or aligned recordings.
Recommendations for Higher Precision Scenarios
If your application requires tighter synchronization (for example, under 100 milliseconds of timing offset), consider the following:
- Always synchronize both devices to the same computer before the start of each recording session.
- Use the manual annotation button simultaneously on both devices to create a clear alignment point in the data.
- If needed, apply post-processing methods to refine alignment based on known time markers.
Summary
- Hexoskin’s internal clock may drift by up to 1.7 seconds per day per device under normal conditions.
- A maximum offset of 3.4 seconds per day may occur between two devices in very different temperature environments.
- For most use cases, same-day synchronization and manual event annotations provide sufficient timing precision.
- If a formal specification is required, a 20 ppm tolerance should be used as the upper bound when estimating drift.