WSJT-X/lib/decode9w.f90

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subroutine decode9w(nfqso,ntol,nsubmode,ss,id2,sync,nsnr,xdt1,f0,decoded)
! Decode a weak signal in a wide/slow JT9 submode.
parameter (NSMAX=6827,NZMAX=60*12000)
real ss(184,NSMAX) !Symbol spectra at 1/2-symbol steps
real ccfred(NSMAX) !Best sync vs frequency
real ccfblue(-9:18) !Sync vs time at best frequency
real a(5) !Fitted Lorentzian params
integer*2 id2(NZMAX) !Raw 16-bit data
integer*1 i1SoftSymbols(207) !Binary soft symbols
character*22 decoded !Decoded message
df=12000.0/16384.0 !Bin spacing in ss()
nsps=6912 !Samples per 9-FSK symbol
tstep=nsps*0.5/12000.0 !Half-symbol duration
npts=52*12000
limit=10000 !Fano timeout parameter
ia=max(1,nint((nfqso-ntol)/df)) !Start frequency bin
ib=min(NSMAX,nint((nfqso+ntol)/df)) !End frequency bin
lag1=-int(2.5/tstep + 0.9999) !Start lag
lag2=int(5.0/tstep + 0.9999) !End lag
nhsym=184 !Number of half-symbols
! First sync pass finds approximate Doppler spread; second pass does a
! good Lorentzian fit to determine frequency f0.
do iter=1,2
nadd=3
if(iter.eq.2) nadd=2*nint(0.375*a(4)) + 1
call sync9w(ss,nhsym,lag1,lag2,ia,ib,ccfred,ccfblue,ipk,lagpk,nadd)
s=0.
sq=0.
ns=0
do i=-9,18
if(abs(i-lagpk).gt.3) then
s=s+ccfblue(i)
sq=sq+ccfblue(i)**2
ns=ns+1
endif
enddo
base=s/ns
rms=sqrt(sq/ns - base**2)
sync=(ccfblue(lagpk)-base)/rms
xdt0=lagpk*tstep
call lorentzian(ccfred(ia),ib-ia+1,a)
f0=(ia+a(3))*df
enddo
ccfblue=(ccfblue-base)/rms
call softsym9w(id2,npts,xdt0,f0,a(4)*df,nsubmode,xdt1-1.05,snrdb,i1softsymbols)
nsnr=nint(snrdb)
call jt9fano(i1softsymbols,limit,nlim,decoded)
!###
! do i=-9,18
! write(81,3081) i,ccfblue(i)
!3081 format(i3,f10.3)
! enddo
! do i=1,NSMAX
! write(82,3082) i*df,ccfred(i)
!3082 format(f10.1,e12.3)
! enddo
!###
return
end subroutine decode9w