[Eeglablist] time dilation on Tucker-Davis-via-neuroscan CNT import

[Hamish Innes-Brown]

Hi Ross, thanks for that.

I wonder what this means for the NS data then - it appears to be all correct
in terms of timing of events when I compare (though I think I will check
again now!).  To work out the time of an event in seconds, wouldn't the NS
system have to use the sampling rate stored in the CNT file, and thus report
an incorrect value?

Anyway thanks for the info, looks like I've got a day of checking ahead of
me!

-hamish-


On Feb 17, 2008 11:54 PM, Ross Fulham <Ross.Fulham at newcastle.edu.au> wrote:

> Hamish,
>
> Neuroscan CNT files store sampling rates as a 16 bit integer quantity (in
> Hz). So the problem with sampling rates being rounded down is occurring when
> you convert the data to Neuroscan, not when you import the neuroscan file
> into EEGLAB.
>
> Ross
>
>
>
>
> ------------------------------------------------------------
> Dr Ross Fulham
> Research Officer
> Room 1025, Ph: 492 46636, Fax: 492 46608
> Centre For Mental Health Studies
> James Fletcher Hospital
> Newcastle, NSW. Australia.
>
> Functional Neuroimaging Laboratory,
> Discipline of Psychology
> School of Behavioural Sciences,
> The University of Newcastle.
>
> e-mail Ross.Fulham at newcastle.edu.au
> ------------------------------------------------------------
> >>> Hamish Innes-Brown <hinnesbrown at gmail.com> 02/15/08 4:37 PM >>>
> Hi all, I am having a strange problem on importing continuous EEG where
> the
> time of events (and the whole EEG) is stretched a little bit on importing
> the data.
>
> It's a bit of a convoluted process getting the data from a TDT system into
> EEGlab, what we do is this:
>
> 1) Export the channel data from the TDT tanks (to CSV files)
> 2) Read the CSV data into Neuroscan and add the events from the TDT event
> log
> 3) Load the CNT file in to EEGlab using pop_loadcnt
>
> The weird thing is that the data is all slightly "slowed down" when read
> into EEGlab - events and EEG features are delayed in time.  I think this
> might be a problem with how a sampling rate that's not a multiple of one
> is
> read from the CNT file.  The TDT sampling rate is 488.28125, and once read
> into EEGlab, the EEG.srate value has dropped all the decimals and become
> "488".
>
> Has anyone else had problems importing data with such strange sampling
> rates?
>
> I'm aware there is a direct TDT import plugin but so far I haven't been
> able
> to get that to work (I think our TDT installation is missing some vital
> parts), and we also use the Neuroscan CNT files for another parallel
> analysis...
>
> Thanks.
>
>
> -hamish-
>
>
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