function xf = hlp(h,sp,x)
%HLP h-hour Low Pass filter
% HLP(H,SP,X) filters the input vector X, at the sampling
% rate SP, using a 2nd order lowpass digital Butterworth
% filter,  allowing ~> h-hour information to be "PASSED" to 
% the output.  All arguments are REQUIRED.  SP must be the
% sample rate in hours. The MATLAB routine FILTFILT is
% used to perform the actual filtering.
%
%   INPUT: h  -  Period to low-pass (hours)
%          sp -  sampling rate (hours)
%          x  -  signal
%
%  OUTPUT: y  -  h-hour low passed version of input x
%
% CALL AS: y=hlp(40,sp,x); % for 40-hour low pass
%
% Brian Blanton, Fall 1998
%

if nargin==0 & nargout==0
   disp('Call as: y=hlp(H,sample_rate,x);')  
   return
end

wn=cutoff(sp,h);
[b,a] = butter(2,wn);
xf = filtfilt(b, a, x);