[Eeglablist] BDF data range

[Andreas Widmann]

Hi Quentin,

> Two questions on the BDF data range:
> 
> - After opening  a bdf file recorded with Biosemi in EEGlab
> EEG = pop_readbdf(filename);
> what is the range of the data in EEG.data ? Are they between physical min
> and physical max or between digital min and digital max ?
Which EEGLAB version? In EEGLAB 5.03 pop_readbdf only gives an error to 
use pop_biosig. BIOSIG scales (and shifts, see below) the data to 
physical units (at least for EEG channels, not 100% sure about other 
sensors).

> - While checking the BIOSIG files, I realised than sometimes the bdf data
> are multiplied by the gain :
> (physmax -physmin) ./ (digmax - digmin)
> and sometimes they are multiplied by the gain and then the following
> factor is added:
> physmin - digmin*gain
> 
> This depends on a flag but I did not go far enough to understand the flag.
> The first solution was also proposed recently on this list.
> Why such a difference ?
I did not find the flag you mention in a quick search in the biosig 
code, anyway I would strongly recommend to correct for the offset in all 
EDF based formats where physical units do not directly correspond to 
digital units (as e.g., in the Biosemi status channel where the offset 
is zero).

In EDF physical and digital units are connected by an arbitrary 
relationship, e.g. if you encode data from a temperature sensor ranging 
from 34 to 40 degree Celsius in a 2 byte signed integer (ranging from 
-32768 to 32767) you have to correct for the 37 degree offset when 
converting back the integer to physical units.

In Biosemi BDF EEG channels this offset is rather small (-262144 + 
(262143 + 262144) / (8388607 + 8388608) * 8388608 = -0.4844 µV), 
however, in order to get the same output from different readers you 
should correct for it anyway.

Best,
Andreas
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