how to keep the beginning and ending information after band filtering?

Dear everyone,

I'd like to use the following code to do the band-pass filter,

;--------------------
; FFT filter (band filter) interannual...
;--------------------
  x = vara
  ntim = dimsizes(vara&time)
  fca = 1./(8.0*12) ; minimum freq 8-year, 8*12 months
  fcb = 1./13. ; maximum freq 13-month
  xtaper = taper(x(lev|:,lat|:,lon|:,time|:), 0.1, 0) ; taper
  xdtrend = dtrend(xtaper,False) ; detrend
  cf = ezfftf (xdtrend) ; sub period (2, ntim/2)
  fcf = fspan(0, 0.5, ntim/2)
  ifcf = ind(fcf.lt.fca .or. fcf.gt.fcb)
  cf(:,:,:,:,ifcf) = 0.0 ; set coef to zero
  xfft = ezfftb (cf, cf_at_xbar)

After filtering, the magnitudes for the beginning and ending points are very low, and even not correct. I know there must be something unreal for those points due to FFT, but why and how the FFT method cause such kind of problem? and how to reduce the errors and keep the information?

Thanks.

Best regards,
Xuguang

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Received on Tue Aug 25 2009 - 07:30:11 MDT