Synchronizing 353

Why do you need synchronizing

There are applications of the cameras when precise frame rate is required as there are some applications when several cameras need to have their frames locked - acquired at exactly the same time or with controlled delays between them. It is important for stereo imaging or other multi-camera setups. The following method applies to the Model 353 cameras with 10338 sensor front ends that use Micron 5MPix sensors that allow synchronizing in so called "snapshot" mode.

Snapshot vs. continuous mode with ERS

Most of the CMOS image sensors (including those used in Elphel Model 353 cameras) use so-called Electronic Rolling Shutter (ERS) mode of image acquisition where there are two running "pointers" in the sensor - first erases pixels, second reads out the pixel values accumulated since the first pointer passed. Those pointers scan all the pixels, row by row with the constant delay between them. That delay defines the exposure time and can be as short as a single scan line (about 1/30,000 of a second for the 5MPix sensors), but it happens at different times for different scan lines - the difference between the top and the bottom ones is about 1/15 of a second (that may lead to significant distortions of the fast moving objects (or the camera itself).

Normally such sensors run continuously and the vertical gap between frames can be very small - smaller than the exposure time, because while erase pointer moves across next frame, readout pointer scans the current one, and without significant vertical blanking (gap between frames) the exposure time can be as long as the frame readout time. In some sensors (including the ones used in 10338 sensor front end) it is possible to erase all pixels at once, but end of exposure is always defined by the pixel readout moment, so global erase is only useful with additional mechanical shutter (not available in model 353 cameras) or a special case of the flash light illumination. In most cases to achieve identical exposure for all pixels we have to erase with the rolling pointer.

So what kind of synchronization is possible with the ERS mode? It is possible to start the erase pointer running at arbitrary moment, wait exposure time and then start reading out the frame. Because sensor "does not know" when the trigger will arrive overlapping of the exposure and readout (as in continuous ERS mode) is not possible and the minimal frame period (reverse of the frame rate) is defined is equal to the frame readout time (i.e. 1/15 sec for a full frame of the 5MPix sensor used) plus required exposure time.