50 lines
2.0 KiB
Fortran
50 lines
2.0 KiB
Fortran
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subroutine decode9w(nfqso,ntol,nsubmode,ss,id2,sync,nsnr,xdt1,f0,decoded)
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! Decode a weak signal in a wide/slow JT9 submode.
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parameter (NSMAX=6827,NZMAX=60*12000)
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real ss(184,NSMAX) !Symbol spectra at 1/2-symbol steps
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real ccfred(NSMAX) !Best sync vs frequency
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real ccfblue(-9:18) !Sync vs time at best frequency
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real a(5) !Fitted Lorentzian params
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integer*2 id2(NZMAX) !Raw 16-bit data
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integer*1 i1SoftSymbols(207) !Binary soft symbols
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character*22 decoded !Decoded message
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df=12000.0/16384.0 !Bin spacing in ss()
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nsps=6912 !Samples per 9-FSK symbol
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tstep=nsps*0.5/12000.0 !Half-symbol duration
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npts=52*12000
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limit=10000 !Fano timeout parameter
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ia=max(1,nint((nfqso-ntol)/df)) !Start frequency bin
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ib=min(NSMAX,nint((nfqso+ntol)/df)) !End frequency bin
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lag1=-int(2.5/tstep + 0.9999) !Start lag
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lag2=int(5.0/tstep + 0.9999) !End lag
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nhsym=184 !Number of half-symbols
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! First sync pass finds approximate Doppler spread; second pass does a
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! good Lorentzian fit to determine frequency f0.
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do iter=1,2
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nadd=3
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if(iter.eq.2) nadd=2*nint(0.375*a(4)) + 1
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call sync9w(ss,nhsym,lag1,lag2,ia,ib,ccfred,ccfblue,ipk,lagpk,nadd)
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sum1=sum(ccfblue) - ccfblue(lagpk-1)-ccfblue(lagpk) -ccfblue(lagpk+1)
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sq=dot_product(ccfblue,ccfblue) - ccfblue(lagpk-1)**2 - &
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ccfblue(lagpk)**2 - ccfblue(lagpk+1)**2
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base=sum1/25.0
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rms=sqrt(sq/24.0)
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sync=(ccfblue(lagpk)-base)/rms
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nsnr=nint(db(sync)-29.7)
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xdt0=lagpk*tstep
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call lorentzian(ccfred(ia),ib-ia+1,a)
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f0=(ia+a(3))*df
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ccfblue=(ccfblue-base)/rms
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enddo
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call softsym9w(id2,npts,xdt0,f0,a(4)*df,nsubmode,xdt1-1.05,i1softsymbols)
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call jt9fano(i1softsymbols,limit,nlim,decoded)
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return
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end subroutine decode9w
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