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Fabrice
June 9, 2014, 11:10:39
Hi,
I'm working with DMM 42BUC03-MLPB 3 cameras and I'm facing some difficulties to get picture with low exposure time.
We're trying to retrieve picture (resolution 1280x960) from 6 cameras at low sample rate (1 Hz, external trigger ) and as we are working under sunlight, we need low exposure time.
We develop our own soft in vb net. We use the ImageAvailable event to retrieve picture. With one single camera there is no problem, I can set low integration time (minimum for example) and the event is always fired. Using two camera and default framerate (~25 fps), when the exposure time goes under 200 ms the ImageAvailable event is not fired. I need to decrease the framerate to ~7 fps using ICImagingControl.DeviceFramerate property (then I can use the minimum integration time). For 5 camera with 7 fps setting I'm not able to use integration time less than 250 ms. Note that for all that we use the same external trigger at 1Hz.
It looks like a bandwidth issue but we only asking 1 picture per second per camera with very low integration time compared to fps. It looks like each camera is sending picture even when they are not triggered. Is that true ? Is there a workaround ?
(I've tried the Processor Idle state manager with no success, the same behaviour can be reproduced using the ICcapture software version 2.3)

Best regards,
Fabrice

Stefan Geissler
June 10, 2014, 15:46:31
Fabrice

there is a similar thread in the forum somewhere.

However: The usage of bandwidth is determined by the frame rate set in the camera by DeviceFramerate. The higher the frame rate, the higher the bandwidth usage.

The camera has no memory on board for the image. That means, the image must be sent out as soon as exposure ends. The image data is send with the speed determined by the above mentioned frame rate. This leads to not picked up datablocks in the camera and therefore to dropped images.

If now 6 cameras start to send images at the same point of time, then the USB bandwidth is exhausted very soon for this burst of data to be send.

Only solution is: Using several USB controllers.

Fabrice
June 12, 2014, 16:27:35
Ok thanks for the answer I thought that in external trigger mode the camera would send image only on trigger signal. Nevertheless, we solved the problem by using a second usb controller.
Regards,

Fabrice

Stefan Geissler
June 12, 2014, 17:09:11
Fabrice,

this is correct. But if two cameras receive the trigger pulse at the same point of time, they start sending image data on the USB Bus also on the same point of time.
Second USB controller is fine, or lower frame rates.