Chronos => Chronos User Discussion => Topic started by: tretee on July 16, 2017, 02:31:01 AM

Title: external high speed trigg
Post by: tretee on July 16, 2017, 02:31:01 AM

Juts wanted to check if this use case is possible now or if it could be with a new firmware update?
(e.g. the hardware could do it but it's currently not supported)

I want to feed the exposure signal externally. The frame rate will be lower than the max frame rate for that resolution.
If the exposure duration is taken from the signal or set in the menu is not important. Both will work.
If there is a delay between the ext. trig signal and the actual sensor exposure. I would need a signal out from the camera to drive a synchronized flash.
Alternatively and even better. The delay is fixed and can be calculated upfront for any given setting.   

The background is this.
I have designed a Xenon flash that I can control in many different ways with a built in MCU. I would like to take a sequence of images (20 or so at 2k fps) with an exposure time of 6us.
I can't drive the Xenon flash continuously. It can withstand 20 flashes or so but needs to cool down before the next sequence.
I also have a LED based flash for the same purpose. To produce the same amount of light the LED flash needs to be much bigger. Since I am driving it very hard it also has a similar limitation of 40 or so flashes in sequence before it needs to be cooled down.   
Title: Re: external high speed trigg
Post by: tesla500 on July 16, 2017, 08:05:17 PM
This mode is now supported in V0.2, both triggered exposure, where a rising or falling edge triggers an exposure with the exposure duration controlled by the camera, and shutter gating, where the external signal controls both exposure start time and exposure duration.

To use triggered exposure mode, set your desired resolution in Record Settings, and set maximum frame rate for that resolution. Go into Trigger/IO Settings and select Exposure Trigger for IO1 (BNC), turn off the pullup and debounce options, and set Invert based on your signal. With Invert unchecked, it will start exposure on the rising edge of the input signal, and on the falling edge if checked. The shutter control will continue to control the exposure duration in this mode.

Setup for shutter gating is identical as for triggered exposure above, except the shutter is entirely controlled by the external signal, the exposure slider does nothing. When the external signal is active, the shutter is open, and is closed otherwise.

Note that you can't do a black cal unless the camera is receiving a proper repetitive exposure trigger, so you may need to set the IO 1 function in Trigger Settings to None before doing a black cal, then set it back to Exposure trigger afterwards.

The delay between the external input going active and start of exposure is very small, under 1us.

There's are a couple of bugs that need to be fixed with this new functionality. There's a horizontal line artifact depending on shutter speed, try to make the exposure close to minimum or maximum to avoid it being in the center of the image. There's also a larger black offset on the first frame following a large deadtime of no frame capture, it starts to get bad beyond about 100ms or so, so it's best to keep your trigger pulse repetition rate higher than 10Hz on average. Both of these are on the bugtracker to be fixed.

Title: Re: external high speed trigg
Post by: tretee on July 17, 2017, 12:46:07 PM