Aquisition delay

[Fedor.Ng]

Reposted sa per Charlies' recommendation

Acquisted Data delay

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‎04-15-2016 05:17 PM

Hi All

I am using the NI USB 6281 to generate and aquist  underwater acoustic signal. I sent and received 500 pings and attached spectrogram is from revceived data. Since my Hydrophone is 1m apart from my Transducer, I expect to see the first DARK RED points coming at about 0.67ms ( time =Distance/speed =1/1500) and all of them should be almost on the same line but they dont as you can see in attached file. Any hint will help.

Thanks 

Hi Fedor.Ng,

With the image you've provided, the only thing I can think of would be that your data acquisition or signal generation may not be happening at the exact intervals you expect. How are you generating the pings you are attempting to receive, and how are you receiving them? Are you using Hardware Timing for both of these portions of your code? You would need to be using a DAQmx Timing function call to use hardware timing I believe, and there are several examples included with LabVIEW that show how to use this functionality. You can find said examples in LabVIEW by going to "Help->Find Examples."
http://digital.ni.com/public.nsf/allkb/3E3D74E26B8A5B83862575CA0053E4B5
http://www.ni.com/product-documentation/2835/en/

If you are already using hardware timing, have you tried tying the NI USB 6281's input and output together? This should allow you to see if the problem is happening at the DAQ device or if there is some other interference.

I would also recommend posting your question in the Multifunction DAQ board in these forums. This specific forum board is for academic products such as the myDAQ or myRIO, while your device is generally better classified as a Multifunction DAQ. 
http://forums.ni.com/t5/Multifunction-DAQ/bd-p/250

Charlie J.
Applications Engineering
National Instruments

[stockson]

Hi Fedor,

as Charlie mentioned, if you don't use hardware timing you cannot be sure whether your signals are synchronized. As a result the time/distance data will be rather unreliable and dependent on the speed of your computer, tasks running in the background etc. Those are only assumptions but since I haven't seen your code I cannot say much more.

LabVIEW for sure gives more flexibility, but you can use hardware timing and synchronization options in SignalExpress (at least to some extent). Please refer to the help topics to get more information. I also found a tutorial that should make it more clear how to configure everything properly - http://www.ni.com/example/31286/en/