additional triggers changing sampling rate from 2048 Hz to 256 Hz

[ronken]

Hi Coen!

We currently have the issue that additional triggers are added once we change our sampling rate from 2048 Hz to 256 Hz shortly before the onset of either a visual stimulus presentation or the display of given responses. They are displayed while recording in ActiView and are also added as events in EEGLAB.
Can you help us with this issue?

Best,
Rebecca

[Coen]

ActiView does not allow you to change the sample rate while saving data to file (decimation selector is grayed out while saving). Any unexpected triggers seen in EEGLAB (that reads the BDF file) must have another explanation.

Best regards, Coen (BioSemi)

[ronken]

Hi Coen,

thanks for your reply. I think we might have not communicated well considering our problem. Our problem is that additional triggers occur whenever we have changed the sample rate to 256 Hz prior to recording/saving the file. At a sample rate of 2048 Hz these additional triggers do not occur. These are present in actiview while recording as well as in EEGLAB. Hope this makes it more clear.

Best,
Rebecca

[Coen]

When decimating from 2048 to 256 Hz, an OR function is applied to trigger bits within each group of 8 original samples. A repetition of very short (millisecond) triggers can therefore appear as a single trigger in the decimated file. So, no triggers are missed (provided positive trigger logic is used) and no extra triggers are generated.

As always: use the "analog'' trigger display mode to inspect unexpected triggers

Best regards, Coen ( BioSemi)