WSJT-X/lib/ft8/ft8_downsample.f90

subroutine ft8_downsample(dd,newdat,f0,c1)

! Downconvert to complex data sampled at 200 Hz ==> 32 samples/symbol

  parameter (NMAX=15*12000,NSPS=1920)
  parameter (NFFT1=192000,NFFT2=3200)      !192000/60 = 3200
  
  logical newdat,first
  complex c1(0:NFFT2-1)
  complex cx(0:NFFT1/2)
  real dd(NMAX),x(NFFT1),taper(0:100)
  equivalence (x,cx)
  data first/.true./
  save cx,first,taper

  if(first) then
     pi=4.0*atan(1.0)
     do i=0,100
       taper(i)=0.5*(1.0+cos(i*pi/100))
     enddo
     first=.false.
  endif
  if(newdat) then
! Data in dd have changed, recompute the long FFT
     x(1:NMAX)=dd
     x(NMAX+1:NFFT1)=0.                       !Zero-pad the x array
     call four2a(cx,NFFT1,1,-1,0)             !r2c FFT to freq domain
     newdat=.false.
  endif

  df=12000.0/NFFT1
  baud=12000.0/NSPS
  i0=nint(f0/df)
  ft=f0+8.5*baud
  it=min(nint(ft/df),NFFT1/2)
  fb=f0-1.5*baud
  ib=max(1,nint(fb/df))
  k=0
  c1=0.
  do i=ib,it
   c1(k)=cx(i)
   k=k+1
  enddo
  c1(0:100)=c1(0:100)*taper(100:0:-1)
  c1(k-1-100:k-1)=c1(k-1-100:k-1)*taper
  c1=cshift(c1,i0-ib)
  call four2a(c1,NFFT2,1,1,1)            !c2c FFT back to time domain
  fac=1.0/sqrt(float(NFFT1)*NFFT2)
  c1=fac*c1

  return
end subroutine ft8_downsample