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https://github.com/saitohirga/WSJT-X.git
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70 lines
1.7 KiB
FortranFixed
70 lines
1.7 KiB
FortranFixed
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subroutine synct(data,jz,jstart,f0,smax)
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C Gets a refined value of jstart.
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parameter (NMAX=30*11025)
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parameter (NB3=3*512)
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real data(jz)
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real*8 dpha,twopi
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complex*16 z,dz
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complex c,c1,zz
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common/hcom/c(NMAX)
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ps(zz)=real(zz)**2 + imag(zz)**2 !Power spectrum function
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C Convert data to baseband (complex result) using quadrature LO.
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twopi=8*atan(1.d0)
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dpha=twopi*f0/11025.d0
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dz=cmplx(cos(dpha),-sin(dpha))
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z=1.d0/dz
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do i=1,jz
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z=z*dz
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c(i)=data(i)*z
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enddo
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C Detect zero-beat sync tone in 512-sample chunks, stepped by 1.
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C Sums replace original values in c(i).
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zz=0
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do i=1,512 !Compute first sum
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zz=zz+c(i)
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enddo
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c1=c(1)
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c(1)=zz
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do i=2,jz-512 !Compute the rest recursively
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zz=c(i-1)+c(i+511)-c1
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c1=c(i) !Save original value
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c(i)=zz !Save the sum
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enddo
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C Iterate to find the best jstart.
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jstart=jstart+NB3
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nz=(jz-jstart)/NB3 -1
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smax=0.
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jstep=256
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jbest=jstart
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10 jstep=jstep/2
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jstart=jbest
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do j=jstart-jstep,jstart+jstep,jstep
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s=0.
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r=0.
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do n=1,nz
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k=(n-1)*NB3 + j
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s=s + ps(c(k))
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r=r + ps(c(k+512)) + ps(c(k+1024))
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enddo
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s=2*s/r !Better to use s/r or s-r?
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if(s.gt.smax) then
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smax=s
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jbest=j
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endif
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enddo
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if(jstep.gt.1) go to 10
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jstart=jbest
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if(jstart.gt.NB3) jstart=jstart-NB3
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return
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end
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