Time Stamp in CSV tracking export

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jcase
Posts: 4
Joined: Thu Nov 21, 2013 6:57 pm

Time Stamp in CSV tracking export

Post by jcase »

I am using the timestamp in exported CSV tracking data from Motive 1.5 for synchronization with another data source. I would like to have sub millisecond accurate time stamping. I am having the following issues however with the accuracy of this time stamp.

1) It only has 2 digits after the decimal or 10 millisecond precision. Is there a way to get a higher precision time stamp?

2) It appears that this time stamp is simply the frame index divided by 100. Is this true? Is this really an accurate time base?

In general is the time stamp in the exported CSV file the best source of time information, or can it be improved?

Best Regards,
Philip
beckdo
Posts: 520
Joined: Tue Jan 02, 2007 2:02 pm

Re: Time Stamp in CSV tracking export

Post by beckdo »

Hi jcase,

Everything boils down to having accurate timing information. This is why timing information is a first class citizen all the way through our processing pipeline.

The quality of timing information depends on what devices you're using. All of our Ethernet devices implement what we call hardware timestamps which allow us to report very accurate timing information that compensates for very small differences between cameras as well as accurate measurements of information transit times.

1. We report more decimal places but they are not showing in your particular case because they are all zeroes.

2. For a standard USB setup you'll end up with 'idealized time' which is effectively ideally '1/frame rate' increments like you are seeing. Because of the accuracy of the camera triggers it's much more accurate to report idealized time than attempt to measure incoming frame times directly because that is subject to information transit times and CPU load.

3. You're not likely going to improve upon the timing information reported in the CSV file. All time reported is accurate within +/- a few microseconds. If you're running very long takes with USB cameras without any external time code or synchronization you might end up with a small amount of drift by the end.
jcase
Posts: 4
Joined: Thu Nov 21, 2013 6:57 pm

Re: Time Stamp in CSV tracking export

Post by jcase »

Hi Beckdo,

I am using flex-V100 USB cameras and doing runs which are approximately 30 minutes long. I am optimizing for time offset and scale relationship with another data source which is hardware time stamped based on a crystal oscillator with +/- 30ppm stability. In many runs I am seeing 100s of milliseconds of time error peak to peak. I care more about integral time error and not so much about differential error, so perhaps using PC system time could improve upon the 100s of milliseconds of integral error. Is there any option in Motive to use PC generated timestamps rather than idealized time?

Best Regards,
Philip
jcase
Posts: 4
Joined: Thu Nov 21, 2013 6:57 pm

Re: Time Stamp in CSV tracking export

Post by jcase »

Is there some documentation about how external synchronization can be done? It sounds like this could solve my issue.

Thanks,
Philip
beckdo
Posts: 520
Joined: Tue Jan 02, 2007 2:02 pm

Re: Time Stamp in CSV tracking export

Post by beckdo »

Hi jcase,

I agree with you that 30 minute takes are in the 'very long' category and idealized time stamps are not ideal for you. I'll make sure this topic gets flagged so that we might be able to address this in the next version of the software.

Additionally, if you want to use an external '100 Hz' signal to trigger the camera shutters, you just need to drive that signal into the Signal In of the OptiHub and under Tools->Synchronization you need to click the Custom tab and select External Sync In as your Synchronization Source. Once you click apply, triggering of the camera's shutters are controlled by the square wave you're sending into the OptiHub.

The OptiHub User Guide is on this page:

http://www.naturalpoint.com/optitrack/d ... dware.html
jcase
Posts: 4
Joined: Thu Nov 21, 2013 6:57 pm

Re: Time Stamp in CSV tracking export

Post by jcase »

Hi Beckdo,

Thanks for the information. I ordered an optihub after scoping the sync signals between cameras and realizing it is not a simple signal. I have been using a standard USB hub so far.

I am trying to understand better the delay between the sync signal input to the optihub and camera exposure start. I see the parameter "External Sync Input Trigger Delay" specified as 0-65 ms, but later there are formulas which imply that given a parameter selection the delay can be better specified than this. Static delay is not a problem for me, but dynamic delay would be. Given a exposure, imager scan rate, and use of WiredSync, how much jitter/drift should I expect on this delay?

Thanks,
Philip
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