function x = hhp(h,sp,x)
%HHP h-Hour High Pass filter
% HHP(H,SP,X) filters the input vector X, at the sampling
% rate SP, using a 2nd order highpass 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 high-pass (hours)
%         sp -  sampling rate (hours)
%         x  -  signal
% OUTPUT: y  -  h-hour high passed version of input x
%
% CALL AS: y=hhp(40,sp,x); % for 40-hour high pass

% Brian Blanton, Fall 1998
%

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

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