WSJT-X/lib/xcor4.f90

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subroutine xcor4(s2,ipk,nsteps,nsym,ich,mode4)
! Computes ccf of the 4-FSK spectral array s2 and the pseudo-random
! array pr2. Returns peak of CCF and the lag at which peak occurs.
! The CCF peak may be either positive or negative, with negative
! implying a message with report.
use jt4
parameter (NHMAX=1260) !Max length of power spectra
parameter (NSMAX=525) !Max number of half-symbol steps
real s2(NHMAX,NSMAX) !2d spectrum, stepped by half-symbols
real a(NSMAX)
save
nw=nch(ich)
do j=1,nsteps
n=2*mode4
if(mode4.eq.1) then
a(j)=max(s2(ipk+n,j),s2(ipk+3*n,j)) - max(s2(ipk ,j),s2(ipk+2*n,j))
else
kz=max(1,nw/2)
ss0=0.
ss1=0.
ss2=0.
ss3=0.
wsum=0.
do k=-kz+1,kz-1
w=float(kz-iabs(k))/nw
wsum=wsum+w
ss0=ss0 + w*s2(ipk +k,j)
ss1=ss1 + w*s2(ipk+ n+k,j)
ss2=ss2 + w*s2(ipk+2*n+k,j)
ss3=ss3 + w*s2(ipk+3*n+k,j)
enddo
a(j)=(max(ss1,ss3) - max(ss0,ss2))/sqrt(wsum)
endif
enddo
do lag=1,65
x=0.
do i=1,nsym
j=2*i-1+lag
if(j.ge.1 .and. j.le.nsteps) x=x+a(j)*float(2*npr(i)-1)
enddo
zz(ipk,lag,ich)=x
enddo
return
end subroutine xcor4