subroutine syncf0(data,jz,NFreeze,NTol,jstart,f0,smax) C Does 512-pt FFTs of data with 256-pt step size. C Finds sync tone and determines aproximate values for jstart and f0. real data(jz) !Raw data real s2(128,6) !Average spectra at half-symbol spacings real x(512) complex cx(0:511) complex z equivalence (x,cx) ps(z)=real(z)**2 + imag(z)**2 !Power spectrum function call zero(s2,6*128) !Clear average df=11025./512. ia=(f0-400)/df ib=(f0+400)/df + 0.999 if(NFreeze.eq.1) then ia=(f0-NTol)/df ib=(f0+Ntol)/df + 0.999 endif C Most of the time in this routine is in this loop. nblk=jz/256 - 6 do n=1,nblk !Accumulate avg spectrum for j=256*(n-1)+1 !512-pt blocks, stepping by 256 call move(data(j),x,512) call xfft(x,512) do i=ia,ib x(i)=ps(cx(i)) enddo k=mod(n-1,6)+1 call add(s2(ia,k),x(ia),s2(ia,k),ib-ia+1) !Average at each step enddo C Look for best spectral peak, using the "sync off" phases as reference. smax=0. do i=ia,ib do k=1,6 k1=mod(k+1,6)+1 k2=mod(k+3,6)+1 r=0.5*(s2(i,k1)+s2(i,k2)) s=s2(i,k)/r if(s.gt.smax) then smax=s jstart=(k-1)*256 + 1 !Best starting place for sync f0=i*df !Best sync frequency endif enddo enddo return end