converting 24 bit data

[miriam]

Hi all,

we have recently bought a Biosemi EEG and Brain Vision as analyzing software. We also downloaded EEG Lab to compare the data converted by the Biosemi converter (downloaded from the Biosemi homepage) with the original data. However, one thing that puzzles me and that's the reason for this post is that we record our raw EEG with 24 bit (using Activ II electrodes), however the converter (and thus the resulting EDF file) has only 16 bit. Can somebody please tell me which sort of information we loose by converting data and how we can circumvent this problem? many thanks in advance, miriam

[Coen]

By converting from 24-bit BDF to 16-bit EDF, you loose a certain amount of low-frequency information in the the data. The original BDF file shows data recorded with a true DC amplifier. The EDF conversion shows the data as if it was recorded with an AC amplifier (amplifier with high-pass filter). Please note that the high-pass cutoff frequency is adjustable in the converter tool (select appropriate value before starting the conversion).

Whether the loss of low-frequency information is a problem depends on your particular research application. If DC information is essential for your application, then the Polerex converter developed by Juergen Kayser is a better alternative, see http://psychophysiology.cpmc.columbia.e ... e/PolyRex/

Best regards, Coen (BioSemi)