Why is dt incorrect when TDMS log file is exported to ASCII text?

[chinchilla]

In brief, we do 6 hour recordings at 4096 samples per second over 6 channels. We use a different program to analyze the electrical data and so we export to Ascii text. However, when we import the Ascii text file the dt that is reported in the file is wrong and if I would convert to samples per second it would be more like 4098.2! As we sync this data to another system it is imperative that we make sure that the sampling times are correct. Does anyone know why we would get an incorrect dt reported in the ascii file?

If I go back to the DAQmx Acquire Module in Signal Express that we used to acquire the data, I see that the rate specified is 4096.0

We use Clampfit from Axon INstruments to import the Signal Express acquired data. 

We are potentially looking at days to weeks of reconverting data because of this discrepancy so if anyone can shed any light on the situaion I would be most grateful.

Regards,

smb

[will.i.am10]

Hi smb,

What hardware are you using to acquire your signals? Also, when you view the TDMS file does it report your correct sample rate at 4096? Or do you convert the data before viewing what is in the TDMS file?

-will.i.am10

[chinchilla]

I am using a USB-6218 DAQ. If I view the module in signal express it reports 4096 as the sampling rate. If I look at the header in the ASCII text file it only reports a dt value and when converted does not equal 4096.

[will.i.am10]

Have you tried opening the TDMS file in excel? You can do that by installing the TDMS Excel Add-In.

TDMS Excel Add-In: http://www.ni.com/example/27944/en/

Also, what timebase are you using for your sample clock, and what is the programmable clock divider number? This can be seen on page 4-11 of the user manual to see if the card can exactly sample at 4096.

USB-6218 User Manual: http://www.ni.com/pdf/manuals/371931f.pdf

-will.i.am10

[chinchilla]

We have not used the Excel spreadsheet export function as it usually tells us that we our data exceeds the size allowed. I have tried adjusting the clipboard size but it still kept stalling.

Checking timebase and clock issues now. With our other DAQs we always get an error or note about sampling rates being adjusted if the board cannot accomodate the requested rate.

[will.i.am10 · Accepted solution]

Take a look at the following documents, I think the first thing to verify would be that your card can achieve this sample rate. If it can, then we can look into if it is for some reason adjusting itself, or if something in the conversion is making the data appear different.

What Sample Rates Is my DAQ board Capable of Achieving: http://digital.ni.com/public.nsf/allkb/4BBE1409700F6CE686256E9200652F6B

How is the Actual Scan Rate Determined: http://digital.ni.com/public.nsf/allkb/5782F1B396474BAF86256A1D00572D6E?OpenDocument

Have you ever been succesful in converting this to ASCII before?

-will.i.am10