WSJT-X/lib/ft8/ft8_downsample.f90

53 lines
1.3 KiB
Fortran

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