Saving trigger input to BDF file...

[LINDBERGH]

Hi, I'm a newbie in EEG acquisition and ActiView. Currently I'm trying to send trigger bits to ActiView through USB2 receiver. However, although I can see the bits I sent in the bottom "trigger" display during scan, I could not find in the BDF file about the trigger information (all the other channel works well). The page of of trigger singals on Biosemi's site stated that "The input trigger signals are mixed with the digital data coming from the AD-box. The input trigger signals are saved in an extra channel (the status channel)", but I cannot see any signal in the status channel.

[Coen]

If the triggers are displayed in ActiView, then you can be 100% sure that they are also saved in the last (trigger/status) channel. Please check with our basic file reader available from http://www.biosemi.com/download/BDFReader.zip . If you see the triggers in our LabVIEW reader, then the error is in your particular review software.

Best regards, Coen (BioSemi)

[LINDBERGH]

Thank you for the help, the LabVIEW reader is very useful in viewing the data. However, I still cannot see the trigger value in the trigger bar in LabVIEW (supposely the horizontal color at the bottom shall be the trigger, right?). The trigger input I can see in the ActiView is of "Decimal" or "Hex" type, but not "Analogue" type (I can see nothing changing in "Analogue" display); so I'm wondering if the absense of trigger is due to the inability to handle the discrete values of LabVIEW and other software packages. Is there any way to handle that?

[Coen]

The trigger port expect TTL signals, see http://www.biosemi.com/faq/trigger_signals.htm . You can use between 1 and 16 lines (bits). Use the "analog" display mode to check the trigger setup (both in ActiView and in the BDF reader). The other display modes can be handy for seeing (familiar) trigger codes, but they do not actually influence how the trigger bits are stored in the file. Use positive trigger logic (line is Low when no trigger is sent, line is High when a trigger is sent). Return all lines to Low between the triggers. Ensure that each trigger pulse is longer than the selected hardware sample time (the sample rate selected by the speedmode switch on the AD-box, see http://www.biosemi.com/faq/adjust_samplerate.htm ).

If the triggers are not visible in ActiView with the "analog" display mode, then they are not received by the USB receiver (check the lines with a voltmeter/oscilloscope).

Best regards, Coen (BioSemi)

[lexvanrooij]

I have a related question, which is: what should the Status channel in the BDF file look like? We provide a 0V - 5V, 1 second duration TTL through the trigger port on the USB2 box. I have tested in the Analog window I do see trigger channel 1 change colour. However, the Status channel in de BDF file shows a very incomprehensive signal: very large (constant) values of very short duration.

Are we picking up noise in our trigger, are we sending the wrong signal, or am I interpreting the Status channel wrongly?

Many thanks,
Lex

[Coen]

You should not interpret the status channel as an 24-bit waveform. Instead, read out the status channel bit by bit, as described in http://www.biosemi.com/faq/trigger_signals.htm. Your trigger on line 1 is encoded as the value of bit 00 (LSB) of the status channel.

Best regards, Coen (BioSemi)