European Data Format
Specs
 Downloads
 Companies
Home EDF > Specs > DC attenuator
 
A DC-attenuator that fits large DC offsets in traditional amplifier ranges and EDF(+)
 

The original article was published by Elsevier and provides all details:
Bob Kemp, Teunis van Beelen, Marion Stijl, Paul van Someren, Marco Roessen, J. Gert van Dijk. Clin Neurophysiol. 121 (12), 2010:1992-7.  A DC attenuator allows common EEG equipment to record fullband EEG,  and fits fullband EEG into standard European Data Format.
 


Some biosensors produce large DC offsets that can drive the signal outside the range of both amplifiers and 16-bit EDF(+). Examples are respiration sensors or EEG electrodes. Therefore, respiration and EEG amplifiers usually have a high-pass filter that rejects the DC component. However, the DC component sometimes is essential, for instance when recording  respiration for the diagnosis of Rett syndrome or Fullband EEG.

 In those cases, the DC component can be attenuated rather than rejected. The DC-attenuated signal then fits in the range of the amplifiers and EDF(+). The original Fullband or Respiration signal is then reconstructed by a simple digital de-attenuator in the review software. In summary (but check the publication for a few important details such as initial conditions), it works as follows:
During recording
Attenuate the DC component by an electronic filter (pick one on the left), or by its digital equivalent (on the right):
Save the DC-attenuated signal in EDF or EDF+.
Save the applied values of DCgain and DCbandwidth in the EDF(+) header.
 
 

During review
Read the values of DCgain and DCbandwidth.
Read the DC-attenuated signal. 
Reconstruct the original signal, i.e. de-attenuate the DC component, by the inverse filter:



This is a valuable add-on for those cases in which either the amplifier and/or the 16 EDF bits can not accommodate large DC components or very slow drifts. The method is not part of the formal EDF or EDF+ specification, so the de-attenuator may not be implemented in all EDF(+) software. Therefore, the DC-attenuator should only be applied when absolutely necessary.

Your implementation of both the attenuator and the de-attenuator can be tested by our DC-attenuated EEG file and the de-attenuator in our review software Polyman. Both are on the download page.