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61 lines
1.6 KiB
Fortran
61 lines
1.6 KiB
Fortran
subroutine q65_sync(ss,i0,nts_q65,sync_ok,snr,xdt)
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! Test for presence of Q65 sync tone
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parameter (NFFT=32768)
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parameter (LAGMAX=33)
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real ss(322,NFFT) !Symbol spectra
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real ccf(0:LAGMAX) !The WSJT "blue curve", peak at DT
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logical sync_ok
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logical first
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integer isync(22),ipk(1)
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! Q65 sync symbols
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data isync/1,9,12,13,15,22,23,26,27,33,35,38,46,50,55,60,62,66,69,74,76,85/
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data first/.true./
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save first,isync
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tstep=2048.0/11025.0 !0.185760 s: 0.5*tsym_jt65, 0.3096*tsym_q65
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if(first) then
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fac=0.6/tstep !3.230
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do i=1,22 !Expand the Q65 sync stride
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isync(i)=nint((isync(i)-1)*fac) + 1
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enddo
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first=.false.
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endif
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m=nts_q65/2
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i1=max(1,i0-m)
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i2=min(NFFT,i0+m)
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ccf=0.
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do lag=0,LAGMAX !Search over range of DT
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do j=1,22 !Test for Q65 sync
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k=isync(j) + lag
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ccf(lag)=ccf(lag) + sum(ss(k,i1:i2)) + sum(ss(k+1,i1:i2)) &
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+ sum(ss(k+2,i1:i2))
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! Q: Should we use weighted sums, perhaps a Lorentzian peak?
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enddo
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enddo
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ccfmax=maxval(ccf)
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ipk=maxloc(ccf)
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lagbest=ipk(1)-1
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xdt=lagbest*tstep - 1.0
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xsum=0.
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sq=0.
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nsum=0
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do i=0,lagmax !Compute ave and rms of "blue curve"
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if(abs(i-lagbest).gt.2) then
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xsum=xsum+ccf(i)
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sq=sq+ccf(i)**2
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nsum=nsum+1
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endif
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enddo
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ave=xsum/nsum
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rms=sqrt(sq/nsum - ave*ave)
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snr=(ccfmax-ave)/rms
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sync_ok=snr.ge.5.0 !Require snr > 5.0 for sync detection
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return
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end subroutine q65_sync
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