WSJT-X/lib/decode65a.f90

subroutine decode65a(dd,npts,newdat,nqd,f0,nflip,mode65,ntrials,     &
     naggressive,ndepth,sync2,a,dt,nsf,nhist,decoded)

! Apply AFC corrections to a candidate JT65 signal, then decode it.

  parameter (NMAX=60*12000)          !Samples per 60 s
  real*4  dd(NMAX)                   !92 MB: raw data from Linrad timf2
  complex cx(NMAX/8)                 !Data at 1378.125 samples/s
  complex c5x(NMAX/32)               !Data at 344.53125 Hz
  complex c5a(512)
  real s2(66,126)
  real a(5)
  logical first
  character decoded*22
  data first/.true./,jjjmin/1000/,jjjmax/-1000/
  data nhz0/-9999999/
  save

! Mix sync tone to baseband, low-pass filter, downsample to 1378.125 Hz
  call timer('filbig  ',0)
  call filbig(dd,npts,f0,newdat,cx,n5,sq0)
  call timer('filbig  ',1)

! NB: cx has sample rate 12000*77125/672000 = 1378.125 Hz

! Find best DF, drift, curvature, and DT.  Start by downsampling to 344.53125 Hz
  call timer('fil6521 ',0)
  call fil6521(cx,n5,c5x,n6)
  call timer('fil6521 ',1)

  fsample=1378.125/4.

  call timer('afc65b  ',0)
! Best fit for DF, drift, banana-coefficient, and dt. fsample = 344.53125 S/s
  dtbest=dt
  call afc65b(c5x,n6,fsample,nflip,a,ccfbest,dtbest)
  call timer('afc65b  ',1)

  sync2=3.7e-4*ccfbest/sq0                    !Constant is empirical 

! Apply AFC corrections to the time-domain signal
! Now we are back to using the 1378.125 Hz sample rate, enough to 
! accommodate the full JT65C bandwidth.
  a(3)=0 
  call timer('twkfreq ',0)
  call twkfreq65(cx,n5,a)
  call timer('twkfreq ',1)

! Compute spectrum for each symbol.
  nsym=126
  nfft=512
  j=int(dtbest*1378.125)
  if(j.lt.0) j=0

  c5a=cmplx(0.0,0.0)
  call timer('sh_ffts ',0)
  do k=1,nsym
     do i=1,nfft
        j=j+1
        if( j .le. NMAX/8 ) c5a(i)=cx(j)
     enddo
     call four2a(c5a,nfft,1,1,1)
     do i=1,66
        jj=i
        if(mode65.eq.2) jj=2*i-1
        if(mode65.eq.4) jj=4*i-3
        s2(i,k)=real(c5a(jj))**2 + aimag(c5a(jj))**2
     enddo
  enddo
  call timer('sh_ffts ',1)

  call timer('dec65b  ',0)
  call decode65b(s2,nflip,mode65,ntrials,naggressive,ndepth,nqd,nsf,   &
       nhist,decoded)
  dt=dtbest !return new, improved estimate of dt
  call timer('dec65b  ',1)

  return
end subroutine decode65a