subroutine detectmsk144(cbig,n,pchk_file,lines,nmessages,nutc)
!nutc and t0 are for debug output 
  use iso_c_binding, only: c_loc,c_size_t
  use packjt
  use hashing
  use timer_module, only: timer

  parameter (NSPM=864, NPTS=3*NSPM)
  integer, parameter :: nstep=693
  character*22 msgreceived,allmessages(20)
  character*80 lines(100)
  character*512 pchk_file,gen_file
  complex cbig(n)
  complex cdat(NPTS)                    !Analytic signal
  complex cdat2(NPTS)
  complex c(NSPM)
  complex ctmp(6000)                  
  complex cb(42)                        !Complex waveform for sync word 
  complex cfac,cca,ccb
  complex cc(NPTS)
  complex cc1(NPTS)
  complex cc2(NPTS)
  complex bb(6)
  integer s8(8),hardbits(144)
  integer*1, target:: i1Dec8BitBytes(10)
  integer, dimension(1) :: iloc
  integer*4 i4Dec6BitWords(12)  
  integer*1 decoded(80)   
  integer*1 i1hashdec
  integer indices(nstep)
  integer ipeaks(10)
  logical ismask(6000)
  real cbi(42),cbq(42)
  real detmet(-2:nstep+3)
  real detfer(nstep)
  real tonespec(6000)
  real rcw(12)
  real dd(NPTS)
  real ferrs(20)
  real pp(12)                          !Half-sine pulse shape
  real snrs(20)
  real times(20)
  real*8 dt, df, fs, pi, twopi
  real softbits(144)
  real*8 unscrambledsoftbits(128)
  real lratio(128)
  logical first
  data first/.true./

  data s8/0,1,1,1,0,0,1,0/
  save df,first,cb,fs,nfft,pi,twopi,dt,s8,rcw,pp

  if(first) then
     i=index(pchk_file,".pchk")
     gen_file=pchk_file(1:i-1)//".gen"
     call init_ldpc(trim(pchk_file)//char(0),trim(gen_file)//char(0))
! define half-sine pulse and raised-cosine edge window
     pi=4d0*datan(1d0)
     twopi=8d0*datan(1d0)
     fs=12000.0
     dt=1.0/fs
     nfft=6000      !using a zero-padded fft to get 2 Hz bins
     df=fs/nfft

     do i=1,12
       angle=(i-1)*pi/12.0
       pp(i)=sin(angle)
       rcw(i)=(1-cos(angle))/2
     enddo

! define the sync word waveform
     s8=2*s8-1  
     cbq(1:6)=pp(7:12)*s8(1)
     cbq(7:18)=pp*s8(3)
     cbq(19:30)=pp*s8(5)
     cbq(31:42)=pp*s8(7)
     cbi(1:12)=pp*s8(2)
     cbi(13:24)=pp*s8(4)
     cbi(25:36)=pp*s8(6)
     cbi(37:42)=pp(1:6)*s8(8)
     cb=cmplx(cbi,cbq)

     first=.false.
  endif

  ! fill the detmet, detferr arrays
  detmet=0
  do istp=1,nstep
    ns=1+256*(istp-1)
    ne=ns+NPTS-1
    if( ne .gt. n ) exit
    tt=(ns+ne)/2.0/12000.0
    cdat=cbig(ns:ne)

! Coarse carrier frequency sync - seek tones at 2000 Hz and 4000 Hz in 
! squared signal spectrum.
! search range for coarse frequency error is +/- 100 Hz

    ctmp=cmplx(0.0,0.0)
    ctmp(1:NPTS)=cdat**2
    ctmp(1:12)=ctmp(1:12)*rcw
    ctmp(NPTS-11:NPTS)=ctmp(NPTS-11:NPTS)*rcw(12:1:-1)
    call four2a(ctmp,nfft,1,-1,1)
    tonespec=abs(ctmp)**2

    ismask=.false.
    ismask(1901:2101)=.true.  ! high tone search window
    iloc=maxloc(tonespec,ismask)
    ihpk=iloc(1)
    ah=tonespec(ihpk)
    ismask=.false.
    ismask(901:1101)=.true.   ! window for low tone
    iloc=maxloc(tonespec,ismask)
    ilpk=iloc(1)
    al=tonespec(ilpk)
    fdiff=(ihpk-ilpk)*df
    ferrh=(ihpk-2001)*df/2.0
    ferrl=(ilpk-1001)*df/2.0
    if( abs(fdiff-2000) .le. 16.0 ) then
      if( ah .ge. al ) then
        ferr=ferrh
      else
        ferr=ferrl
      endif
    else
      ferr=-999.99
    endif
    detmet(istp)=ah+al
    detfer(istp)=ferr
  enddo  ! end of detection-metric, snr, and frequency error estimation loop

  call indexx(detmet(1:nstep),nstep,indices) !find median of detection metric vector
  xmed=detmet(indices(nstep/2))
  detmet=detmet/xmed ! noise floor of detection metric is 1.0

  ndet=0

  do ip=1,20 ! use something like the "clean" algorithm to find candidates
    iloc=maxloc(detmet(1:nstep))
    il=iloc(1)
    if( (detmet(il) .lt. 2.0) .or. (abs(detfer(il)) .gt. 100.0) ) cycle 
    ndet=ndet+1
    times(ndet)=((il-1)*256+NPTS/2)*dt
    ferrs(ndet)=detfer(il)
    snrs(ndet)=10.0*log10(detmet(il))/2-5.0 !/2 because detmet is a 4th order moment
    detmet(il-3:il+3)=0.0
  enddo

  nmessages=0
  allmessages=char(0)
  lines=char(0)

  do ip=1,ndet  !run through the candidates and try to sync/demod/decode
    imid=times(ip)*fs
    t0=times(ip)
    cdat=cbig(imid-NPTS/2+1:imid+NPTS/2)
    ferr=ferrs(ip)
    nsnr=snrs(ip)

! remove coarse freq error - should now be within a few Hz
    call tweak1(cdat,NPTS,-(1500+ferr),cdat)

! attempt frame synchronization
! correlate with sync word waveforms
    cc=0
    cc1=0
    cc2=0
    do i=1,NPTS-(56*6+41)
      cc1(i)=sum(cdat(i:i+41)*conjg(cb))
      cc2(i)=sum(cdat(i+56*6:i+56*6+41)*conjg(cb))
    enddo
    cc=cc1+cc2
    dd=abs(cc1)*abs(cc2)

! Find 5 largest peaks
    do ipk=1,5
      iloc=maxloc(abs(cc))
      ic1=iloc(1)
      iloc=maxloc(dd)
      ic2=iloc(1)
      ipeaks(ipk)=ic2
      dd(max(1,ic2-7):min(NPTS-56*6-41,ic2+7))=0.0
    enddo

    do ipk=1,5

! we want ic to be the index of the first sample of the frame
      ic0=ipeaks(ipk)

! fine adjustment of sync index
! bb lag used to place the sampling index at the center of the eye
      do i=1,6
        if( ic0+11+NSPM .le. NPTS ) then
          bb(i) = sum( ( cdat(ic0+i-1+6:ic0+i-1+6+NSPM:6) * conjg( cdat(ic0+i-1:ic0+i-1+NSPM:6) ) )*2 )
        else
          bb(i) = sum( ( cdat(ic0+i-1+6:NPTS:6) * conjg( cdat(ic0+i-1:NPTS-6:6) ) )*2 )
        endif
      enddo
      iloc=maxloc(abs(bb))
      ibb=iloc(1)
      bba=abs(bb(ibb))
      if( ibb .le. 3 ) ibb=ibb-1
      if( ibb .gt. 3 ) ibb=ibb-7
      do id=1,3             ! slicer dither
        if( id .eq. 1 ) is=0
        if( id .eq. 2 ) is=-1
        if( id .eq. 3 ) is=1

! Adjust frame index to place peak of bb at desired lag
        ic=ic0+ibb+is
        if( ic .lt. 1 ) ic=ic+864

! Estimate fine frequency error. 
! Should a larger separation be used when frames are averaged?
        cca=sum(cdat(ic:ic+41)*conjg(cb))
        if( ic+56*6+41 .le. NPTS ) then
          ccb=sum(cdat(ic+56*6:ic+56*6+41)*conjg(cb))
          cfac=ccb*conjg(cca)
          ferr2=atan2(imag(cfac),real(cfac))/(twopi*56*6*dt)
        else
          ccb=sum(cdat(ic-88*6:ic-88*6+41)*conjg(cb))
          cfac=cca*conjg(ccb)
          ferr2=atan2(imag(cfac),real(cfac))/(twopi*88*6*dt)
        endif

! Final estimate of the carrier frequency - returned to the calling program
        fest=1500+ferr+ferr2

! Remove fine frequency error
        call tweak1(cdat,NPTS,-ferr2,cdat2)

! place the beginning of frame at index NSPM+1
        cdat2=cshift(cdat2,ic-(NSPM+1))

        do iav=1,7 ! try each of 7 averaging patterns, hope that one works

          if( iav .eq. 1 ) then
            c(1:NSPM)=cdat2(NSPM+1:2*NSPM)  !avg 1 frame to the right of ic
          elseif( iav .eq. 2 ) then
            c=cdat2(NSPM-431:NSPM+432)      !1 frame centered on ic
            c=cshift(c,-432)
          elseif( iav .eq. 3 ) then         !1 frame to the left of ic
            c=cdat2(1:NSPM)
          elseif( iav .eq. 4 ) then
            c=cdat2(NSPM+432:NSPM+432+863)  !1 frame beginning 36ms to the right of ic
            c=cshift(c,432)
          elseif( iav .eq. 5 ) then
            c=cdat2(1:NSPM)+cdat2(NSPM+1:2*NSPM)
          elseif( iav .eq. 6 ) then
            c=cdat2(NSPM+1:2*NSPM)+cdat2(2*NSPM+1:NPTS)
          elseif( iav .eq. 7 ) then
            c=cdat2(1:NSPM)+cdat2(NSPM+1:2*NSPM)+cdat2(2*NSPM+1:NPTS)
          endif

! Estimate final frequency error and carrier phase. 
          cca=sum(c(1:1+41)*conjg(cb))
          ccb=sum(c(1+56*6:1+56*6+41)*conjg(cb))
          cfac=ccb*conjg(cca)
          ffin=atan2(imag(cfac),real(cfac))/(twopi*56*6*dt)
          phase0=atan2(imag(cca+ccb),real(cca+ccb))

! Remove phase error - want constellation such that sample points lie on re,im axes
          cfac=cmplx(cos(phase0),sin(phase0))
          c=c*conjg(cfac)

! sample to get softsamples
!  do i=1,72
!    softbits(2*i-1)=imag(c(1+(i-1)*12))
!    softbits(2*i)=real(c(7+(i-1)*12))  
!  enddo

! matched filter - might be possible to improve this
          softbits(1)=sum(imag(c(1:6))*pp(7:12))+sum(imag(c(864-5:864))*pp(1:6))
          softbits(2)=sum(real(c(1:12))*pp)
          do i=2,72
            softbits(2*i-1)=sum(imag(c(1+(i-1)*12-6:1+(i-1)*12+5))*pp)
            softbits(2*i)=sum(real(c(7+(i-1)*12-6:7+(i-1)*12+5))*pp)
          enddo

          hardbits=0
          do i=1,144
            if( softbits(i) .ge. 0.0 ) then
              hardbits(i)=1
            endif
          enddo
          nbadsync1=(8-sum( (2*hardbits(1:8)-1)*s8 ) )/2
          nbadsync2=(8-sum( (2*hardbits(1+56:8+56)-1)*s8 ) )/2
          nbadsync=nbadsync1+nbadsync2
          if( nbadsync .gt. 4 ) cycle

! normalize the softsymbols before submitting to decoder
          sav=sum(softbits)/144
          s2av=sum(softbits*softbits)/144
          ssig=sqrt(s2av-sav*sav)
          softbits=softbits/ssig

          sigma=0.65
          lratio(1:48)=softbits(9:9+47)
          lratio(49:128)=softbits(65:65+80-1)
          lratio=exp(2.0*lratio/(sigma*sigma))
  
          unscrambledsoftbits(1:127:2)=lratio(1:64)
          unscrambledsoftbits(2:128:2)=lratio(65:128)

          max_iterations=20
          max_dither=50
          call ldpc_decode(unscrambledsoftbits, decoded, &
                           max_iterations, niterations, max_dither, ndither)

          if( niterations .ge. 0.0 ) then
            goto 778
          endif

        enddo ! frame averaging loop
      enddo   ! sample-time dither loop
    enddo     ! peak loop - could be made more efficient

    msgreceived=' '
    phase0=-98
    i1hashdec=0
    goto 999

778 continue
! The decoder found a codeword - compare decoded hash with calculated
! Collapse 80 decoded bits to 10 bytes. Bytes 1-9 are the message, byte 10 is the hash
    do ibyte=1,10   
      itmp=0
      do ibit=1,8
        itmp=ishft(itmp,1)+iand(1,decoded((ibyte-1)*8+ibit))
      enddo
      i1Dec8BitBytes(ibyte)=itmp
    enddo

! Calculate the hash using the first 9 bytes.
    ihashdec=nhash(c_loc(i1Dec8BitBytes),int(9,c_size_t),146)
    ihashdec=2*iand(ihashdec,255)

! Compare calculated hash with received byte 10 - if they agree, keep the message.
    i1hashdec=ihashdec

    if( i1hashdec .eq. i1Dec8BitBytes(10) ) then
! Good hash --- unpack 72-bit message
      do ibyte=1,12
        itmp=0
        do ibit=1,6
          itmp=ishft(itmp,1)+iand(1,decoded((ibyte-1)*6+ibit))
        enddo
        i4Dec6BitWords(ibyte)=itmp
      enddo
      call unpackmsg(i4Dec6BitWords,msgreceived)
    else
      msgreceived=' '
      phase0=-99
    endif
    if( msgreceived .ne. ' ' ) then
      ndupe=0
      do im=1,nmessages
        if( allmessages(im) .eq. msgreceived ) ndupe=1
      enddo
      if( ndupe .eq. 0 ) then
        nmessages=nmessages+1
        allmessages(nmessages)=msgreceived
        write(lines(nmessages),1020) nutc,nsnr,t0,nint(fest),msgreceived
1020    format(i6.6,i4,f5.1,i5,' & ',a22)
      endif
    endif  
    
999 continue
!    write(78,1001) nutc,t0,iav,ipk,is,fest,ffin,nbadsync1,nbadsync2, &
!               phase0,niterations,ndither,i1hashdec,i1Dec8BitBytes(10),msgreceived
!1001 format(i6,f8.2,i4,i4,i4,f8.2,f8.2,i4,i4,f8.2,i4,i4,i4,i4,2x,a22)

  enddo
  return
end subroutine detectmsk144