Synchronized video and analog acquisition

[josborne]

Hello Diadem experts,

I am curious about this Diadem DAC example:

http://zone.ni.com/reference/en-XX/help/370858M-01/explonl/explonl/explonl_dac_video/

It claims to be able to acquire synchronized video and analog data.  Can anyone determine how well synchronized these two data streams are?  Given that we are talking about desktop machine, probably Windows, and no hardware trigger between the analog and video devices, I am guessing the two streams are, at best, about within +-1 second.  

Can anyone address this issue?  I see in the Diadem user manual (here) that NI has done some work to investigate how accurate the clocks are within Diadem.  And they even mention some benchmarks of 100 msec.  But I don't really trust that number when it comes to acquiring video and analog data simultaneously.

Thanks, John

[Otmar · Accepted solution]

Hello John,

You are asking an interesting question, and I got on touch with the R&D developer responsible for this feature and got an extensive explanation from him on how this feature works.

The target application of this specific feature is not for use cases where exact synchronization between data acquisition and measurement are required (i.e. a vehicle crash test with 1,000+ frame per second video recording). This feature is implemented for users that want to have the ability to use low cost Web cams to get insight into what their machine or test are doing while recoding data at the same time. We should be calling this "parallel acquisition of data and video" and not "synchronous acquisition" - which would indicate that there is some way of ensuring that the data and video are actually synchronized during the acquisition process. The original German Help System entry talks about "parallel" instead of "synchronous" acquisition, the English translation incorrectly uses the word "synchronous" instead of "parallel". I have contacted our documentation team to clarify this section of the help system. 

The achieve exact synchronization, especially between more than one data acquisition stream and one camera, the application will have to use frame grabber hardware - hardware specifically designed to support video or image acquisition.

This is how DIAdem DAC approaches this type of application, I hope this information is helpful to you and others looking at this feature:

DIAdem DAC places top priority on the acquisition and output of data to and from the data acquisition hardware. The video recording feature has less priority and isn't monitored by DIAdem continuously. The video recording process is triggered as closely as possible to the data acquisition process. The exact time stamps of both triggers (data and video) is recorded which allows us to determine the exact start time for both signals for playback in DIAdem VIEW, i.e. if there was a delay in the start of the video recording, this will be considered for the playback in DIAdem VIEW.

Our testing with done with one camera (not multiple, which is supported in DIAdem VIEW) and one NI data acquisition device (not multiple devices, which is also supported by DIAdem). In these tests we were able to see that video and data were generally synchronous within a few frames (no true 100% synchronicity) - which was enough for the purposes that this solution based on low cost USB camera technology was designed for. It's usually better than +/- 1 second depending on the application, but also not synchronized to the millisecond such as hardware based systems would be able to achieve.

There are many factors that influence the results of tests performed in this application space: The PC hardware (processor, RAM, disk system), the measurements tasks, other software running on the computer, the camera and frame frame,  the software drivers, etc. There are too many variables outside of DIAdem's control that can influence the synchronicity of the data and video acquisition based on software only, thus true synchronicity can only be achieved using hardware designed for that purpose.

I hope this clarifies the DIAdem DAC feature set a little better. Thank your for pointing out the misuse of the work "synchronous" in the help system, we'll have that corrected for the next DIAdem release. Please let us know if you have any other questions.

Best regards,

     Otmar

[josborne]

Thanks so much Otmar.  This was very helpful, and the is kind of what I expected.  Kudos!

[Otmar]

John,

The team that creates the help system just made me aware of some additional information on the video acquisition in the help system, it can be found here:

http://zone.ni.com/reference/en-XX/help/370858M-01/genvis/genvis/video_infos/

I though this might be helpful ...

Thank you for the Kudos!

    Otmar