[Eeglablist] BDF data range

[Lloyd Smith]

A full description of the BDF file format (based on the familiar EDF
format) can be found at http://www.biosemi.com/faq/file_format.htm.  For
those not familiar with BDF, here is a general explanation of the
relevant header parameters:

Physmin/max and digmin/max encode (indirectly) the gain and
quantization-resolution (LSB) of each channel in the BDF file.  Gain is
the ratio of digital range to the physical range, and LSB (in
microvolts) is the inverse of the gain.  Active electrode channels and
channels with other types of  sensors normally have different gain/LSB
values.  For example, here is a table outlining the entries you would
find for a GSR sensor as compared to a typical active electrode channel
with +-262 uV range:

				GSR sensor	active electrode
physical max (uV)		8388607	262143
physical min (uV)		-8388608	-262144
digital max (uV)		8388607	8388607
digital min (uV)		-8388608	-8388608
physical range (V)	16.78		0.52
gain (uV/uV)		1		32
LSB (uV)			1.000		0.031

Data values stored in the BDF file are in A/D units rather than
microvolts.  Data values read from the BDF file must be multiplied by
the gain factor to be represented in the original units.  It is a good
idea to be consistent in reading and using the gain information for BDF
channels, since almost any type of sensor can be connected to the
system.  The 13th field of the header (physical dimension) contains the
measurement unit.  ActiView is programmed to store the proper unit for
each channel in the BDF file. So, the BDF file headers actually list
"uV" for the electrode channels, and "Ohm" or "Siemens" for the GSR
channels, and Celsius for the temperature channel, etc..  While active
electrode channels rescaled by the gain factor are scaled in microvolts,
you need to know some additional information about non-active-electrode
channels to rescale the units properly.  The GSR sensor example above
would be scaled in units of Siemens (16 Hz SC coupler) or Ohms (512 Hz
GSR coupler), but other standard sensors for the ActiveTwo system
obviously would have different units.

Lloyd Smith
Owner
Cortech Solutions, LLC

321 N. Front Street, Suite 113
Wilmington, NC 28401 USA
Office: 910-362-1143
Mobile: 910-431-2811
Fax: 910-362-1147
E-mail: LSmith at cortechsolutions.com
Web: www.cortechsolutions.com
 
 

> -----Original Message-----
> From: Quentin Noirhomme [mailto:Noirhomme at tele.ucl.ac.be]
> Sent: Friday, February 23, 2007 6:54 AM
> To: eeglablist at sccn.ucsd.edu
> Subject: [Eeglablist] BDF data range
> 
> Dear all,
> 
> Two questions on the BDF data range:
...