WSJT-X/lib/fchisq65.f90

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real function fchisq65(cx,npts,fsample,nflip,a,ccfmax,dtmax)
parameter (NMAX=60*12000) !Samples per 60 s
complex cx(npts)
real a(5)
complex w,wstep,z
real ss(3000)
complex csx(0:NMAX/8)
data twopi/6.283185307/a1,a2,a3/99.,99.,99./
save
call timer('fchisq65',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 signal
csx(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
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)
enddo
endif
! Compute whole-symbol powers at 1/16-symbol steps.
fac=1.e-4
do i=1,nout
j=nsps+(i-1)*nsps/16 !steps by 8 samples (1/16 of a symbol)
k=j-nsps
ss(i)=0.
if(k.ge.0 .and. j.le.npts) then
z=csx(j)-csx(k) ! difference over span of 128 pts
ss(i)=fac*(real(z)**2 + aimag(z)**2)
endif
enddo
ccfmax=0.
call timer('ccf2 ',0)
call ccf2(ss,nout,nflip,ccf,xlagpk)
call timer('ccf2 ',1)
if(ccf.gt.ccfmax) then
ccfmax=ccf
dtmax=xlagpk*dtstep
endif
fchisq65=-ccfmax
call timer('fchisq65',1)
return
end function fchisq65