Sync cameras and joint data to one clock.
There are two jobs: make the streams share a clock, and measure how far apart they still are. Hardware trigger or timecode does the first. A camera-visible clock does the second, on any camera.
Why the streams drift apart.
A USB camera’s timestamp is usually when the host received the frame, not when the sensor exposed it. Joint encoders log on the controller’s clock and cameras log on the PC’s, so the two start apart and move apart over a session.
LeRobot computes each recorded frame’s timestamp as frame_index / fps, from the frame counter and not from the camera (LeRobot dataset_writer.py, line 225, re-read on the main branch Nov 10, 2026). A dataset can therefore look aligned while the cameras are not.
Three approaches.
- Trigger the cameras together. A trigger hub or GPIO cable starts every exposure at the same moment. It needs cameras with a trigger input.
- Stamp the streams with timecode. A timecode generator writes the same time to audio and video devices. It does not measure per-frame skew on a robot rig.
- Measure the result. Put a clock in view of every camera, record it, and read the clock out of each frame. This works for webcams and for hardware-synced rigs alike, and it shows what the other two approaches actually achieved.
What is on the market.
| Option | What it does | List price | Limit |
|---|---|---|---|
| Orbbec Sync Hub Pro | Triggers Orbbec cameras together | $70 to $76 at US resellers; Orbbec publishes no list price (Knoxlabs listing, read 2026-11-10) | Orbbec cameras only |
| Camera trigger cables | Trigger machine-vision cameras with a trigger input | From $85 (vendor listing, read 2026-11-10) | Needs cameras with trigger inputs |
| Tentacle Sync E mk2 | Timecode for audio and video devices | $219 to $259 for the single set at US resellers (listings, read 2026-11-10) | Stamps; does not measure per-frame skew |
| OptiTrack eSync 2 | Syncs OptiTrack systems with other devices | $2,199 (OptiTrack, read 2026-11-10) | Built for OptiTrack’s own system |
| Checkmate Sync Beacon | LED clock your cameras film, stepping every 1 ms; decoder reports drops, repeats, true frame interval and camera-to-camera offset | $999 to own; $1,250 for a 14-day rental | Measures; does not trigger. Cameras must see the face. Patent pending. |
Competitor prices are list prices or retailer listings read on the dates shown; check the vendor before you rely on them (source note: research/aeo/guide-price-check-2026-11-10.md). Beacon prices are on the Sync Beacon page. Checkmate products are shown before FCC authorization; see Legal.
How to check your own rig in a minute.
- Place a camera-visible clock where every camera can see it, with manual exposure and locked focus.
- Record all cameras over the same 60 seconds.
- Read the clock in every frame. Count dropped and repeated frames, compare the true frame interval with the nominal one, and compute each camera’s offset to the others.
- Repeat after any camera swap, USB change or rebuild.
The Sync Beacon does this with an 8 x 8 green LED face (525 nm, which passes the IR-cut filter on camera sensors) and a decoder that writes a Station Report. Accuracy on real cameras is measured for your rig in that report; we do not claim a figure in advance. Contour depth frames can take the same clock through a sync input (Contour).
If you already trigger your cameras.
Keep the trigger. A measurement shows whether it reaches every stream on the station, including webcams and joint logs that the trigger does not touch.
How do I hardware-sync several cameras and robot joint data to one clock?
Trigger cameras that have a trigger input from one hub or GPIO source, log joint data against a shared clock, and then measure the result with a clock all cameras can film. Measurement matters because host timestamps record when a frame arrived, not when it was exposed.
Is there a device I can put in view of my cameras to check their timestamps?
Yes. The Checkmate Sync Beacon is an LED clock that steps every 1 ms. You film it with every camera, decode each video on your PC, and read a Station Report with drops, repeats, true frame interval and camera-to-camera offset. It is patent pending and costs $999 to own or $1,250 for a 14-day rental.
Does LeRobot measure camera skew?
No. LeRobot stamps each frame from its frame index and the frame rate. Its tolerance_s setting (default 1e-4 s) checks that stored timestamps are spaced 1/fps apart, and since those timestamps come from the frame counter, it does not measure how far the cameras themselves are apart (LeRobot source and docstring, read Nov 10, 2026).
Does the beacon change my recorder or cameras?
No. It is a separate USB-powered device with no radio. Nothing is wired to the cameras and nothing changes in your recorder.