View source for ESync

The [http://www.naturalpoint.com/optitrack/products/esync/ eSync] is a [http://en.wikipedia.org/wiki/Power_over_Ethernet 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 [http://en.wikipedia.org/wiki/Linear_timecode linear SMPTE timecodes], and output certain signals. The [http://www.naturalpoint.com/optitrack/static/documents/eSync%20Quick%20Start%20Guide.pdf NaturalPoint documentation] of the eSync is nearly nonexistent, and this document is intended to be a supplement.

==Trigger Input==

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 [http://www.naturalpoint.com/optitrack/products/motive/tracker/ 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 <math>T = 1/\text{fps}</math> is the frame period, <math>t_0</math> is the time of the rising edge of the trigger pulse, and <math>E<T</math> is the exposure time, then the exposure will start at <math>t_0 + T/2 - E/2</math> and end at <math>t_0 + T/2 + E/2</math> by default. Motive:Tracker also allows you to further delay the exposure by arbitrary amounts.

==SMPTE Input==

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 [http://en.wikipedia.org/wiki/Linear_timecode linear SMPTE timecodes] encoded with [http://en.wikipedia.org/wiki/Biphase_mark_code 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 [http://en.wikipedia.org/wiki/Clock_recovery clock recovery] from the SMPTE input alone using a [http://en.wikipedia.org/wiki/Phase_locked_loop PLL] or similar circuit.