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Bug 1512 - read_nex_event sample + 1 missing

Reported 2012-06-11 07:49:00 +0200
Modified 2016-09-13 14:57:52 +0200
Product: FieldTrip
Component: fileio
Version: unspecified
Hardware: PC
Operating System: All
Importance: P3 normal
Assigned to: Martin Vinck
Depends on:
See also:

Martin Vinck - 2012-06-11 07:49:18 +0200

read_nex_event does the following: dum = fread(fid,hdr.varheader(mrkvarnum).cnt,'int32'); timestamp = dum; dum = dum ./(hdr.filheader.frequency./smpfrq); mrk.tim = round(dum); I think, this should be mrk.tim = round(dum+1); as the current definition would allow for a sample number of zero

Jörn M. Horschig - 2012-06-11 09:23:41 +0200

in that case I would rather use floor instead of round + 1, just to make it exactly rounding up (e.g. if dum would be 0.5, floor would result in 1, round(dum+1) would result in 2)

Robert Oostenveld - 2012-06-11 09:33:24 +0200

(In reply to comment #0) I think you are right about the +1 for the sample, but am not sure whether you should not also first subtract -1 from the timestamp. Say the data for this channel is acquired at 40kHz (equal to the system sampling rate), then timestamps equals samples, right? In that case sample = round(timestamp ./(hdr.filheader.frequency./smpfrq) + 1); results in sample = timestamp+1; which is incorrect. Regarding the comment of Jorn: the timestamp can be in-between two samples, and then you want it to to round to the nearest sample. Maybe as thought experiment: what about a sampling rate of 20kHz (half the system sampling rate), or 10kHz? You can draw the 10kHz case as follows |----|----|----|----|----|----| where each - is a timestamp point and each | is a boundary between two samples. So you get 4 timestamp points per sample, or stated differently: each sample is 4 timestamps wide.

Martin Vinck - 2012-06-11 09:49:25 +0200

(In reply to comment #2) sample = round(timestamp/40 + 1); results in 40 samples = timestamp + 40; samples does not equal timestamps here, because the lfp has a lower sampling rate. The first timestamp that can be registered is 0 if timestamp = 15, sample should be 1, not zero