[Eeglablist] The meaning of dB

[Thomas Ferree]

Antoine,
In eeglab 4.515 at least, the timef function does not return power exactly.
To start with, the Matlab FFT function is not normalized for the number of
points.  When computing power or amplitude in uV, one needs to normalize
by the number of data points.  We went through a series of steps to remedy
this and other related things in the timef fuction.  Attached please find a
slightly modified version of the timef function.  Search on the string 'TF
and MC' to find our changes.  Note these changes are only multiplicative
factors, so they do not affect the ERSP which is a power ratio.  With
this function, the power is the so-called average power, not total power
or power spectral density (see Numerical Recipes), and is corrected for
number of data points, windowing and negative frequencies.  We made
these edits only for the basic FFT settings, but took care that the
normalization for data points is correct when zero-padding, i.e., one
should normalize by the number of non-zero data points only.  In order
to compute the amplitude in uV, take sqrt(2*P) where P is power.  We
tested this by overwriting an EEG.data array with sine waves of known
amplitude (use something other than unit amplitude) and verified that
we were getting the right answer.  You should verify that yourself too.
Hope this helps, Tom.
-- 
Thomas Ferree, PhD
Department of Radiology
UT Southwestern Medical Center
Email: tom.ferree at utsouthwestern.edu
Voice: (214) 648-9767
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