[FieldTrip] general questions about the FieldTrip

Robert Oostenveld r.oostenveld at donders.ru.nl
Fri Mar 27 16:53:34 CET 2015

Hi Moran

1) The fieldtrip buffer by itself has a delay that can be as small as 5ms. But the actual delay in the data stream from the electronics to the MATLAB side of the buffer depends on quite a number of parameters and can be much larger. Sending data in blocks (i.e. multiple samples are bufferend and sent in one go), bluetooth and USB all cause delays before you are able to process the samples in the data block. Even the USB connection and whether you are sharing it with another device (i.e. mouse or USB hard drive on the same USB hubor port) can affect the delay and can cause jitter (variation in the delay). I suggest you look at http://www.fieldtriptoolbox.org/example/measuring_the_timing_delay_and_jitter_for_a_real-time_application for a demonstration how you can quantify the That page has data for our CTF MEG system, which has a blocksize of ~100ms and does some online processing (head localization) prior to forwarding the data, hence the delay of 150ms that we see here makes sense. 

2) On http://www.fieldtriptoolbox.org/getting_started/realtime you can find some getting started instructions. Reading data from a file rather than from a realitme buffer just means that you specify the name of the file. If you look at http://www.fieldtriptoolbox.org/example/ft_realtime_signalviewer you can see how you can simulate a real time data stream. Just replace cfg.dataset with an EEG file name in ft_realtime_signalviewer and you are not plotting real time data, but data from file.

3) your P300 application can send events to the fieldtrip buffer and the signal processing application can read them and act upon the data referred to in the events. You can also make a single application that does both, or make two applications that communicate events separate from the data stream.

4) I recall that some emotiv software needed to be installed, but don’t know whether that requires the full SDK to be installed. I don’t have an emotiv epoc myself. Please see http://www.fieldtriptoolbox.org/development/realtime/emotiv and feel free to add your findings to that (wiki) page.

best regards,

On 27 Mar 2015, at 16:28, moran abilea <mor2451 at gmail.com> wrote:

> hi again, 
> i have another question/s, but this time they are genra.
> but first let me explain what i need from field trip: i'm working for extracting from 14 channels EPOC Emotive the EEG raw data as contious data, send trigger if i want to display the raw data or not and then process the data with SVM. all this process is for creating EEG speller in Matlab.
> so my questions are:
> 1. i assume the field trip has constant "delay", the delay i reffer to is between the time of view the signal and the real time data transfer in the buffer. does anyone knows what is the delay time somehow?
> 2. where can i find how to open stream for a file? 
> 3. using triggers: can i send time stamp in my configuration in order to know when to display the data that i need for the EEG speller?
> 4. what to do in order to use the EPOC Emotive? do i need to set some new configurations for matlab? to download any SDK? etc
> any kind of help will be welcomed,
> Moran Abilea
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> fieldtrip at donders.ru.nl
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