WSJT-X/map65/libm65/fchisq.f90

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real function fchisq(cx,cy,npts,fsample,nflip,a,ccfmax,dtmax)
use timer_module, only: timer
parameter (NMAX=60*96000) !Samples per 60 s
complex cx(npts),cy(npts)
real a(5)
complex w,wstep,za,zb,z
real ss(3000)
complex csx(0:NMAX/64),csy(0:NMAX/64)
data twopi/6.283185307/a1,a2,a3/99.,99.,99./
save
call timer('fchisq ',0)
baud=11025.0/4096.0
nsps=nint(fsample/baud) !Samples per symbol
nsph=nsps/2 !Samples per half-symbol
ndiv=16 !Output ss() steps per symbol
nout=ndiv*npts/nsps
dtstep=1.0/(ndiv*baud) !Time per output step
if(a(1).ne.a1 .or. a(2).ne.a2 .or. a(3).ne.a3) then
a1=a(1)
a2=a(2)
a3=a(3)
! Mix and integrate the complex X and Y signals
csx(0)=0.
csy(0)=0.
w=1.0
x0=0.5*(npts+1)
s=2.0/npts
do i=1,npts
x=s*(i-x0)
if(mod(i,100).eq.1) then
p2=1.5*x*x - 0.5
! p3=2.5*(x**3) - 1.5*x
! p4=4.375*(x**4) - 3.75*(x**2) + 0.375
dphi=(a(1) + x*a(2) + p2*a(3)) * (twopi/fsample)
wstep=cmplx(cos(dphi),sin(dphi))
endif
w=w*wstep
csx(i)=csx(i-1) + w*cx(i)
csy(i)=csy(i-1) + w*cy(i)
enddo
endif
! Compute 1/2-symbol powers at 1/16-symbol steps.
fac=1.e-4
pol=a(4)/57.2957795
aa=cos(pol)
bb=sin(pol)
do i=1,nout
j=i*nsps/ndiv
k=j-nsph
ss(i)=0.
if(k.ge.1) then
za=csx(j)-csx(k)
zb=csy(j)-csy(k)
z=aa*za + bb*zb
ss(i)=fac*(real(z)**2 + aimag(z)**2)
endif
enddo
ccfmax=0.
call timer('ccf2 ',0)
call ccf2(ss,nout,nflip,ccf,lagpk)
call timer('ccf2 ',1)
if(ccf.gt.ccfmax) then
ccfmax=ccf
dtmax=lagpk*dtstep
endif
fchisq=-ccfmax
call timer('fchisq ',1)
return
end function fchisq