Hi,
we have recently become aware of a problem with trigger sampling in the
Neuromag system. Externally delivered trigger values are not always read
correctly at the beginning or the end of a trigger pulse (both in the
digital trigger channels as well as the combined channel STI101). The
first and last sample point may show an intermediate value between 0 and
the actual trigger value (e.g. ...0 0 240 255 255... instead of ...0 0
255 255 255...). We tested sampling rates of 1000Hz and 5000Hz, and it
appears that there is always a delay of exactly one sampling point (but
more erroneous triggers occurred for the 5000Hz case). We therefore
conclude that the problem must lie on the acquisition (rather than the
presentation) side.
Has anyone else come across this problem before, and possibly found a
(hardware) solution? Not every analysis software automatically handles
this problem correctly, and it has already caused some confusion in the
Cambridge MEG community.
Regards,
Olaf Hauk
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