WSJT-X/lib/fil6521.f90

subroutine fil6521(c1,n1,c2,n2)

! FIR lowpass filter designed using ScopeFIR

!                  Pass #1   Pass #2  
! -----------------------------------------------
! fsample    (Hz)  1378.125   Input sample rate
! Ntaps            21         Number of filter taps
! fc         (Hz)  40         Cutoff frequency
! fstop      (Hz)  172.266    Lower limit of stopband
! Ripple     (dB)  0.1        Ripple in passband
! Stop Atten (dB)  38         Stopband attenuation
! fout       (Hz)  344.531    Output sample rate

  parameter (NTAPS=21)
  parameter (NH=NTAPS/2)
  parameter (NDOWN=4)                !Downsample ratio = 1/4
  complex c1(n1)
  complex c2(n1/NDOWN)

! Filter coefficients:
  real a(-NH:NH+NTAPS/3)
  data a/                                                                &
       -0.011958606980,-0.013888627387,-0.015601306443,-0.010602249570,  &
        0.003804023436, 0.028320058273, 0.060903935217, 0.096841904411,  &
        0.129639871228, 0.152644580853, 0.160917511283, 0.152644580853,  &
        0.129639871228, 0.096841904411, 0.060903935217, 0.028320058273,  &
        0.003804023436,-0.010602249570,-0.015601306443,-0.013888627387,  &
        -0.011958606980,1.43370769e-019,2.64031087e-006,6.25548654e+028, &
        2.44565251e+020,4.74227538e+030,10497312.0e0000,7.74079654e-039/

  n2=(n1-NTAPS+NDOWN)/NDOWN
  k0=NH-NDOWN+1

! Loop over all output samples
  do i=1,n2
     c2(i)=0.
     k=k0 + NDOWN*i
     do j=-NH,NH
        c2(i)=c2(i) + c1(j+k)*a(j)
     enddo
  enddo

  return
end subroutine fil6521