subroutine filbig(dd,nmax,f0,newdat,nfsample,xpol,c4a,c4b,n4)

! Filter and downsample complex data stored in array dd(4,nmax).  
! Output is downsampled from 96000 Hz to 1375.125 Hz.

  parameter (MAXFFT1=5376000,MAXFFT2=77175)
  real*4  dd(4,nmax)                         !Input data
  complex ca(MAXFFT1),cb(MAXFFT1)            !FFTs of input
  complex c4a(MAXFFT2),c4b(MAXFFT2)          !Output data
  real*8 df
  real halfpulse(8)                 !Impulse response of filter (one sided)
  complex cfilt(MAXFFT2)                     !Filter (complex; imag = 0)
  real rfilt(MAXFFT2)                        !Filter (real)
  integer*8 plan1,plan2,plan3,plan4,plan5
  logical first,xpol
  include 'fftw3.f'
  common/cacb/ca,cb
  equivalence (rfilt,cfilt)
  data first/.true./,npatience/1/
  data halfpulse/114.97547150,36.57879257,-20.93789101,                &
       5.89886379,1.59355187,-2.49138308,0.60910773,-0.04248129/
  save

  if(nmax.lt.0) go to 900

  nfft1=MAXFFT1
  nfft2=MAXFFT2
  if(nfsample.eq.95238) then
     nfft1=5120000
     nfft2=74088
  endif

  if(first) then
     nflags=FFTW_ESTIMATE
     if(npatience.eq.1) nflags=FFTW_ESTIMATE_PATIENT
     if(npatience.eq.2) nflags=FFTW_MEASURE
     if(npatience.eq.3) nflags=FFTW_PATIENT
     if(npatience.eq.4) nflags=FFTW_EXHAUSTIVE
     
! Plan the FFTs just once
     call timer('FFTplans ',0)
     call sfftw_plan_dft_1d(plan1,nfft1,ca,ca,FFTW_BACKWARD,nflags)
     call sfftw_plan_dft_1d(plan2,nfft1,cb,cb,FFTW_BACKWARD,nflags)
     call sfftw_plan_dft_1d(plan3,nfft2,c4a,c4a,FFTW_FORWARD,nflags)
     call sfftw_plan_dft_1d(plan4,nfft2,c4b,c4b,FFTW_FORWARD,nflags)
     call sfftw_plan_dft_1d(plan5,nfft2,cfilt,cfilt,FFTW_BACKWARD,nflags)
     call timer('FFTplans ',1)

! Convert impulse response to filter function
     do i=1,nfft2
        cfilt(i)=0.
     enddo
     fac=0.00625/nfft1
     cfilt(1)=fac*halfpulse(1)
     do i=2,8
        cfilt(i)=fac*halfpulse(i)
        cfilt(nfft2+2-i)=fac*halfpulse(i)
     enddo
     call timer('FFTfilt ',0)
     call sfftw_execute(plan5)
     call timer('FFTfilt ',1)

     base=cfilt(nfft2/2+1)
     do i=1,nfft2
        rfilt(i)=real(cfilt(i))-base
     enddo

     df=96000.d0/nfft1
     if(nfsample.eq.95238) df=95238.1d0/nfft1
     first=.false.
  endif

! When new data comes along, we need to compute a new "big FFT"
! If we just have a new f0, continue with the existing ca and cb.

  if(newdat.ne.0) then
     nz=min(nmax,nfft1)
     do i=1,nz
        ca(i)=cmplx(dd(1,i),dd(2,i))
        if(xpol) cb(i)=cmplx(dd(3,i),dd(4,i))
     enddo

     if(nmax.lt.nfft1) then
        do i=nmax+1,nfft1
           ca(i)=0.
           if(xpol) cb(i)=0.
        enddo
     endif
     call timer('FFTbig  ',0)
     call sfftw_execute(plan1)
     if(xpol) call sfftw_execute(plan2)
     call timer('FFTbig  ',1)
     newdat=0
  endif

! NB: f0 is the frequency at which we want our filter centered.
!     i0 is the bin number in ca and cb closest to f0.

  i0=nint(f0/df) + 1
  nh=nfft2/2
  do i=1,nh                                !Copy data into c4a and c4b,
     j=i0+i-1                              !and apply the filter function
     if(j.ge.1 .and. j.le.nfft1) then
        c4a(i)=rfilt(i)*ca(j)
        if(xpol) c4b(i)=rfilt(i)*cb(j)
     else
        c4a(i)=0.
        if(xpol) c4b(i)=0.
     endif
  enddo
  do i=nh+1,nfft2
     j=i0+i-1-nfft2
     if(j.lt.1) j=j+nfft1                  !nfft1 was nfft2
     c4a(i)=rfilt(i)*ca(j)
     if(xpol) c4b(i)=rfilt(i)*cb(j)
  enddo

! Do the short reverse transform, to go back to time domain.
  call timer('FFTsmall',0)
  call sfftw_execute(plan3)
  if(xpol) call sfftw_execute(plan4)
  call timer('FFTsmall',1)
  n4=min(nmax/64,nfft2)
  go to 999

900 call sfftw_destroy_plan(plan1)
  call sfftw_destroy_plan(plan2)
  call sfftw_destroy_plan(plan3)
  call sfftw_destroy_plan(plan4)
  call sfftw_destroy_plan(plan5)

999 return
end subroutine filbig