WSJT-X/lib/ps4.f90

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subroutine ps4(dat,nfft,s)
parameter (NMAX=2520+2)
parameter (NHMAX=NMAX/2-1)
real dat(nfft)
real dat2(NMAX)
real s(NHMAX)
complex c(0:NMAX)
equivalence(dat2,c)
nh=nfft/2
do i=1,nh
dat2(i)=dat(i)/128.0 !### Why 128 ??
enddo
do i=nh+1,nfft
dat2(i)=0.
enddo
call four2a(c,nfft,1,-1,0)
fac=1.0/nfft
do i=1,nh
s(i)=fac*(real(c(i))**2 + aimag(c(i))**2)
enddo
return
end subroutine ps4