The eSync is a PoE-powered device whose purpose is to put in phase the exposures of all cameras on the LAN. It has the additional capabilities to be externally triggered, accept linear SMPTE timecodes, and output certain signals. The NaturalPoint documentation of the eSync is nearly nonexistent, and this document is intended to be a supplement.
The device can accept trigger signals of several types:
- Hi-Z - 3.3V logic over short lines with high impedance
- Lo-Z - 3.3V logic over transmission line of 50 or 75-Ohm impedance
- Isolated - Logic levels higher than 3.3V, or with distinct ground potential
The trigger signal's frequency can be multiplied and divided by integer values using the Motive:Tracker software to provide the desired framerate. The resulting pulses demarcate the frame capture boundaries on either the rising or falling edge, which is also set in Motive:Tracker. The actual exposure of the frame is exactly centered inside the frame boundary. For example, if T = 1 / fps is the frame period, t0 is the time of the rising edge of the trigger pulse, and E < T is the exposure time, then the exposure will start at t0 + T / 2 − E / 2 and end at t0 + T / 2 + E / 2 by default. Motive:Tracker also allows you to further delay the exposure by arbitrary amounts.
This port is intended to be used to write timecodes onto the NatNet packets, which contain the time that the images are originally taken (as opposed to the time when the 3D reconstruction is made). This allows the motion capture data to be accurately synchronized with video frame numbers in post-processing. The input format is linear SMPTE timecodes encoded with biphase mark coding. The timecodes should reflect 24/25/29.97/30 fps video. Since biphase mark coding guarantees a level transition for every bit of data, this allows the eSync to perform clock recovery from the SMPTE input alone using a PLL or similar circuit.