WSJT-X/libm65/lenmsk.f90

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Fortran
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subroutine lenmsk(r,npts,msglen)
! Determine length of the user message in a JTMS ping.
real r(60000)
real acf(4872)
integer np(9)
data np/5,7,9,11,13,17,19,23,29/ !Permissible message lengths
save acf !Why necessary? (But don't remove!)
msglen=0 !Use ACF to find msg length
if(npts.ge.8*168) then
r=r-sum(r(1:npts))/npts
acfmax=0.
acf0=dot_product(r(1:npts),r(1:npts))
kz=min(nint(0.75*npts),29*168)
do k=8,kz
fac=float(npts)/(npts-k)
acf(k)=fac*dot_product(r(1:npts),r(1+k:npts+k))/acf0
enddo
call hipass(acf(8),kz-7,50)
do k=8,kz !Find acfmax, kpk
if(acf(k).gt.acfmax) then
acfmax=acf(k)
kpk=k
endif
enddo
sumsq=0.
n=0
do k=8,kz !Find rms, skipping around kpk
if(abs(k-kpk).gt.10) then
sumsq=sumsq+acf(k)**2
n=n+1
endif
enddo
rms=sqrt(sumsq/n)
acf=acf/rms !Normalize the acf
amax2=0.
acflim=3.5
do i=1,9
k=168*np(i) !Check only the permitted lengths
if(k.gt.kz) go to 10
if(acf(k).gt.acflim .and. acf(k).gt.amax2) then
amax2=acf(k) !Save best value >3.5 sigma
msglen=np(i) !Save message length
kpk2=k
endif
enddo
10 continue
endif
return
end subroutine lenmsk