function xf=pl66tn(x,dt,T);
% PL66TN: pl66t for variable dt and T
% xf=PL66TN(x,dt,T) computes low-passed series xf from x
% using pl66 filter, with optional sample interval dt (hrs)
% and filter half amplitude period T (hrs) as input for
% non-hourly series.
%
% INPUT: x=time series (must be column array)
% dt=sample interval time [hrs] (Default dt=1)
% T=filter half-amp period [hrs] (Default T=33)
%
% OUTPUT: xf=filtered series
% NOTE: both pl64 and pl66 have the same 33 hr filter
% half-amplitude period. pl66 includes additional filter weights
% upto and including the fourth zero crossing at 2*T hrs.
% The PL64 filter is described on p. 21, Rosenfeld (1983), WHOI
% Technical Report 85-35. Filter half amplitude period = 33 hrs.,
% half power period = 38 hrs. The time series x is folded over
% and cosine tapered at each end to return a filtered time series
% xf of the same length. Assumes length of x greater than 132.
%
% Program written in Matlab v7.1.0 SP3
% Program ran on PC with Windows XP Professional OS.
%
% "Although this program has been used by the USGS, no warranty,
% expressed or implied, is made by the USGS or the United States
% Government as to the accuracy and functioning of the program
% and related program material nor shall the fact of distribution
% constitute any such warranty, and no responsibility is assumed
% by the USGS in connection therewith."
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% 10/30/00
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% default to pl64
if (nargin==1); dt=1; T=33; end
cutoff=T/dt;
fq=1./cutoff;
nw=2*T./dt;
nw=round(nw);
disp(['number of weights = ',int2str(nw)])
nw2=2.*nw;
[npts,ncol]=size(x);
if (nptsnw2)
% detrend time series, then add trend back after filtering
%save temp
xdt=detrend(x(jgd,ic));
trnd=x(jgd,ic)-xdt;
y=[cs(jm).*xdt(jm);xdt;cs(j).*xdt(npts+1-j)];
% filter
yf=filter(wts,1.0,y);
% strip off extra points
xf(jgd,ic)=yf(nw2+1:npts+nw2);
% add back trend
xf(jgd,ic)=xf(jgd,ic)+trnd;
else
'warning time series is too short'
end
end