WSJT-X/filbig.f

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subroutine filbig(id,nmax,f0,newdat,c4a,c4b,n4)
C Filter and downsample complex data for X and Y polarizations,
C stored in array id(4,nmax). Output is downsampled from 96000 Hz
C to 1500 Hz, and the low-pass filter has f_cutoff = 375 Hz and
C f_stop = 750 Hz.
parameter (NFFT1=5376000,NFFT2=77175)
integer*2 id(4,nmax) !Input data
complex c4a(NFFT2),c4b(NFFT2) !Output data
complex ca(NFFT1),cb(NFFT1) !FFTs of input
real*8 df
C Impulse response of filter (one side)
real halfpulse(8)
complex cfilt(NFFT2)
!Filter (complex; imag = 0)
real rfilt(NFFT2) !Filter (real)
integer plan1,plan2,plan3,plan4,plan5
logical first
include 'fftw3.f'
equivalence (rfilt,cfilt)
data first/.true./
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
if(first) then
npatience=1
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
C Plan the FFTs just once
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)
C 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 sfftw_execute_(plan5)
base=cfilt(NFFT2/2+1)
do i=1,NFFT2
rfilt(i)=real(cfilt(i))-base
enddo
df=96000.d0/NFFT1
first=.false.
endif
C When new data comes along, we need to compute a new "big FFT"
C 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(float(int(id(1,i))),float(int(id(2,i))))
cb(i)=cmplx(float(int(id(3,i))),float(int(id(4,i))))
enddo
if(nmax.lt.NFFT1) then
do i=nmax+1,NFFT1
ca(i)=0.
cb(i)=0.
enddo
endif
call sfftw_execute_(plan1)
call sfftw_execute_(plan2)
newdat=0
endif
C NB: f0 is the frequency at which we want our filter centered.
C 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
c4a(i)=rfilt(i)*ca(j)
c4b(i)=rfilt(i)*cb(j)
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)
c4b(i)=rfilt(i)*cb(j)
enddo
C Do the short reverse transform, to go back to time domain.
call sfftw_execute_(plan3)
call sfftw_execute_(plan4)
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