ActiView TCP server output signals have arbitrary units?

khild · Post by **khild** » Mon Jun 22, 2009 9:18 pm

It appears that there are two main options to extract data from a bdf file using the Matlab functions found on the BioSemi website. The first option produces signed integers (this result is obtained when calling readbdf.m with the third input parameter set to "1"). The second option produces real numbers (this result is obtained when readbdf.m is called using only 2 input parameters).

It makes sense for the ADC in the Biosemi to try to use as much of the dynamic range as possible and this likely explains the two choices (signed integer data and data scaled and shifted to have units of volts or microvolts). Hence, I suspect that the first option above produces signals having arbitrary units and the second option above has units of microvolts. According to our bdf files, the multiplicative constant can be different for each channel so that the arbitrary units are different for each channel.

The information needed to convert between the two sets of outputs is contained in the bdf file (after using openbdf.m it is stored in the variable DAT.Head.Calib). However, the current interface supplied by BioSemi's ActiView TCP data server (which we use instead of using the bdf files) does not contain this information and it appears that it only produces the signed integers (having arbitrary units). Before we can reference the data to a particular channel it is imperative that, at the very least, all the channels in question have the same (arbitrary or known) units, but it currently appears that we do not have access to the required information using the ActiView TCP server.

Is this correct?

Post by **Coen** » Wed Jun 24, 2009 2:03 pm

The signals sent by TCP-IP have the same scaling as the signals in the BDF file. The scaling of the channels is listed in the BDF header, see http://www.biosemi.com/faq/file_format.htm.

All electrode channels have the same scaling (24 bit signed integers with LSB = 31.25 nV). The sensor channels use different units and scaling, see file header.

Best regards, Coen (BioSemi)