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reference:ft_preproc_dftfilter [2018/08/23 14:43] (current)
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 +=====  FT_PREPROC_DFTFILTER =====
 +
 +Note that this reference documentation is identical to the help that is displayed in MATLAB when you type "help ft_preproc_dftfilter"​.
 +
 +<​html><​pre>​
 +  <a href=/​reference/​ft_preproc_dftfilter><​font color=green>​FT_PREPROC_DFTFILTER</​font></​a>​ reduces power line noise (50 or 60Hz) via two 
 +  alternative methods:
 +  A) DFT filter (Flreplace = '​zero'​) or
 +  B) Spectrum Interpolation (Flreplace = '​neighbour'​).
 + 
 +  A) The DFT filter applies a notch filter to the data to remove the 50Hz
 +  or 60Hz line noise components ('​zeroing'​). This is done by fitting a sine 
 +  and cosine at the specified frequency to the data and subsequently ​
 +  subtracting the estimated components. The longer the data is, the sharper ​
 +  the spectral notch will be that is removed from the data.
 +  Preferably the data should have a length that is a multiple of the
 +  oscillation period of the line noise (i.e. 20ms for 50Hz noise). If the
 +  data is of different lenght, then only the first N complete periods are
 +  used to estimate the line noise. The estimate is subtracted from the
 +  complete data.
 + 
 +  B) Alternatively line noise is reduced via spectrum interpolation ​
 +  (introduced by Mewett et al., 2004, Med. Biol. Eng. Comput. 42, 
 +  doi:​10.1007/​BF02350994). ​
 +  The signal is:
 +  I)   ​transformed into the frequency domain via a discrete Fourier ​
 +        transform (DFT), ​
 +  II)  the line noise component (e.g. 50Hz, Flwidth = 1 (1Hz): 49-51Hz) is 
 +        interpolated in the amplitude spectrum by replacing the amplitude ​
 +        of this frequency bin by the mean of the adjacent frequency bins 
 +        ('​neighbours',​ e.g. 49Hz and 51Hz). ​
 +        Neighwidth defines frequencies considered for the mean (e.g. 
 +        Neighwidth = 2 (2Hz) implies 47-49 Hz and 51-53 Hz). 
 +        The original phase information of the noise frequency bin is
 +        retained.
 +  III) the signal is transformed back into the time domain via inverse DFT
 +        (iDFT).
 +  If Fline is a vector (e.g. [50 100 150]), harmonics are also considered. ​
 +  Preferably the data should be continuous or consist of long data segments
 +  (several seconds) to avoid edge effects. If the sampling rate and the
 +  data length are such, that a full cycle of the line noise and the harmonics
 +  fit in the data and if the line noise is stationary (e.g. no variations
 +  in amplitude or frequency), then spectrum interpolation can also be 
 +  applied to short trials. But it should be used with caution and checked ​
 +  for edge effects.
 + 
 +  Use as
 +    [filt] = ft_preproc_dftfilter(dat,​ Fsample, Fline, varargin)
 +  where
 +    dat             data matrix (Nchans X Ntime)
 +    Fsample ​        ​sampling frequency in Hz
 +    Fline           line noise frequency (and harmonics)
 + 
 +  Additional input arguments come as key-value pairs:
 + 
 +    Flreplace ​      '​zero'​ or '​neighbour',​ method used to reduce line noise, '​zero'​ implies DFT filter, '​neighbour'​ implies spectrum interpolation  ​
 +    Flwidth ​        ​bandwidth of line noise frequencies,​ applies to spectrum interpolation,​ in Hz
 +    Neighwidth ​     width of frequencies neighbouring line noise frequencies,​ applies to spectrum interpolation (Flreplace = '​neighbour'​),​ in Hz 
 + 
 +  The line frequency should be specified as a single number for the DFT filter.
 +  If omitted, a European default of 50Hz will be assumed
 + 
 +  See also PREPROC
 +</​pre></​html>​