View Full Version : Max. FPS in external trigger mode

July 5, 2007, 12:50:54
We have two DFK 21BF04 cameras, we want to use them synchronized.
I have written a small application to send signals through parallel port with the frequency I choose from the application.
I can make the cameras work synchronized without any problem at low FPS values,
but when I try with values over 15 FPS (15 FPS included) there occurs problems.
The camera drops some of the frames and cannot shoot exactly at the specified frame rate. Thus, causing to lose synchronization.

I found in a thread that you cannot trigger at 30 fps.

What is the max. frame rate that can be achieved in trigger mode in our camera model?
If the max. level is below 30, how is it possible to sync. the cameras at high frame rates?

Also, I have another question;
In the document on using the trigger signal, in the signal diagram, there is a period called image readout.
What is exactly image readout, and why it is dependent on the frame rate?
(Isn't image readout the transferring of the image data to buffers? if so, it must be a constant value because
the image is always the same size independent of the frame rate)

Thanks a lot.

Note: The entry I mention about

In July 4, 2005 in the thread at the link


If you need 30 fps, then you can not trigger the DFK 21BF04.
if you need less fps, then i suggest to use an external USB unit, that creates the TTL pulse, that creates the trigger signal.

The optimal solution would be the use of the GPIO of one of the DFK 21BF04 camera, that controlls an electronic, that creates the TTL pulse.

Johannes Vogel
July 6, 2007, 15:58:03

The post you are reffering was true for the first firmware revision of the camera. The current version can be operated in trigger at 30 fps. There are 2 things to be aware of:
1) The frame rate has to be set to 60 fps
2) the exposure time has to be slightly less than 1/30sec.

This is important because the transfer of the image data to the FireWire bus is dependent on the selected frame rate (1/60 Sec. if operated at 60 fps). As shown in the timing diagram you are refering to, the time required for one frame is: exposure time + readout time. If this sum is bigger than the time between 2 frames as determined by the frame rate, the camera does not "see" every second trigger because it is still transferring image data.

To make long story short: the maximum frame rate in trigger mode is determined by: exposure time + readout time.

July 11, 2007, 15:22:59
To make long story short: the maximum frame rate in trigger mode is determined by: exposure time + readout time.

I have a few questions about this:

Is write speed to harddisk not important, too?
How many frames are buffered in memory?

Thank you in advance,

Stefan Geissler
July 12, 2007, 08:10:39
Hi Fred,

Yes, the hard disc speed is important too. But this question was only about maximum trigger frequency. There was nothing about writing data to hard disc. Some software needs triggering only for image processing, not to save the images, e.g. find damaged glass bottles in a production line and sort them out. For this application you would need a high trigger frequency and the hard disk plays no rule.

Usually only one frame is buffered in memory for image processing, but this depends on the application. The sample "Iterating Through A Captured Image Sequence" (http://www.imagingcontrol.com/library/dotnet/section/image-aquisition/example/iterating-an-image-sequence/ ) buffers as many as needed frames in the memory. It is e.g. for 2 seconds at 30 fps 60 frames. These frames are displayed on screen and you may write them to an AVI file later (not implemented in this sample). Also hard disk speed plays no role in this application.

If you want to save the frames to hard disk with the used trigger frequency, then the hard disk plays a role too in order to avoid dropped frames.

July 12, 2007, 08:26:41
Hi Stefan,

I see.. Thank you for the answer.

Well.. It already works fine here at 6-10FPS with a simple micro swith for the trigger. I record uncompressed RGB24. But it drops a frame now and then. I am working on an optical switch. Then I am 100% sure about the trigger. If it still drops frames, then I must look at the harddisk.

PS: I am very happy with the new IC Capture.
It realy has everything we need. The preview of the captured files for example. Very well done! :)


Stefan Geissler
July 12, 2007, 09:45:48
Hi Fred,

Thank you for your approval for IC Capture.

If you have frame drops, you may check your hard disk. First of all, it must be unfragmented. You can achieve the highest speed, if you would use a free partition. (I think, we have already discussed this? I am not sure about this.)
The best is an own hard disk only for the capture AVI files. But 10 fps with RGB24 at 1024*768 should not be a problem in my opinion.