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function [sm,cwp]=cspect(nlags,ntp,wtype,x,y,wpar)
//<sm,cwp>=cspect(nlags,ntp,wtype,x,y,wpar)
//Spectral estimation using the modified periodogram method.
//Cross-spectral estimate of x and y is calculated when both
//x and y are given. Auto-spectral estimate of x is calculated
//if y is not given.
// x :data if vector, amount of input data if scalar
// y :data if vector, amount of input data if scalar
// nlags :number of correlation lags (pos. integer)
// ntp :number of transform points (pos. integer)
// wtype :window type ('re','tr','hm','hn','kr','ch')
// wpar :optional window parameters
// : for wtype='kr', wpar>0
// : for wtype='ch', 0<wpar(1)<.5, wpar(2)>0
// sm :power spectral estimate in the interval [0,1]
// cwp :calculated value of unspecified Chebyshev
// :window parameter
//
//!
//author: C. Bunks date: 16 Sept 1988
// Copyright INRIA
[lhs,rhs]=argn(0);
cross=0;
//construct window
if rhs==4 then,
w=window(wtype,2*nlags-1);
else if rhs==5 then,
if wtype=='kr' then,
wpar=y;
w=window(wtype,2*nlags-1,wpar);
else if wtype=='ch' then,
wpar=y;
[w,cwp]=window(wtype,2*nlags-1,wpar);
else,
cross=1;
w=window(wtype,2*nlags-1);
end,
end,
else,
[w,cwp]=window(wtype,2*nlags-1,wpar);
cross=1;
end,
end,
//estimate correlations
if maxi(size(x))==1 then,
nsects=int(x/(3*nlags));
xlen=int(x/nsects);
ss=0*ones(1,2*nlags);
if cross==1 then,
for k=1:nsects,
xk=getx(xlen,1+(k-1)*xlen);
yk=gety(xlen,1+(k-1)*xlen);
ss=corr('update',xk,yk,ss);
end,
re=fft(ss,1)/x;
re=[re(nlags:-1:1) re(2:nlags)];
else,
for k=1:nsects,
xk=getx(xlen,1+(k-1)*xlen);
ss=corr('update',xk,ss);
end,
re=fft(ss,1)/x;
re=[re(nlags:-1:1) re(2:nlags)];
end,
else,
if cross==1 then,
[re,me]=corr(x,y,nlags);
re=[re(nlags:-1:1) re(2:nlags)];
else,
[re,me]=corr(x,nlags);
re=[re(nlags:-1:1) re(2:nlags)];
end,
end,
//window correlation estimate
wre=w.*re;
//fourier transform to obtain spectral estimate
wree=[wre 0*ones(1,ntp-2*nlags+1)];
sm=abs(fft(wree,-1));
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