WSJT-X/lib/ft4/subtractft4.f90

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subroutine subtractft4(dd,itone,f0,dt)
! Subtract an ft4 signal
!
! Measured signal : dd(t) = a(t)cos(2*pi*f0*t+theta(t))
! Reference signal : cref(t) = exp( j*(2*pi*f0*t+phi(t)) )
! Complex amp : cfilt(t) = LPF[ dd(t)*CONJG(cref(t)) ]
! Subtract : dd(t) = dd(t) - 2*REAL{cref*cfilt}
use timer_module, only: timer
parameter (NMAX=21*3456,NSPS=576,NFFT=NMAX,NFILT=1400)
parameter (NFRAME=(103+2)*NSPS)
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real*4 dd(NMAX), window(-NFILT/2:NFILT/2), xjunk
complex cref,camp,cfilt,cw
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integer itone(103)
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logical first
data first/.true./
common/heap8/cref(NFRAME),camp(NMAX),cfilt(NMAX),cw(NMAX),xjunk(NFRAME)
save first
nstart=dt*12000+1-NSPS
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nsym=103
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fs=12000.0
icmplx=1
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bt=1.0
nss=NSPS
call gen_ft4wave(itone,nsym,nss,fs,f0,cref,xjunk,icmplx,NFRAME)
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camp=0.
do i=1,nframe
id=nstart-1+i
if(id.ge.1.and.id.le.NMAX) camp(i)=dd(id)*conjg(cref(i))
enddo
if(first) then
! Create and normalize the filter
pi=4.0*atan(1.0)
fac=1.0/float(nfft)
sum=0.0
do j=-NFILT/2,NFILT/2
window(j)=cos(pi*j/NFILT)**2
sum=sum+window(j)
enddo
cw=0.
cw(1:NFILT+1)=window/sum
cw=cshift(cw,NFILT/2+1)
call four2a(cw,nfft,1,-1,1)
cw=cw*fac
first=.false.
endif
cfilt=0.0
cfilt(1:nframe)=camp(1:nframe)
call four2a(cfilt,nfft,1,-1,1)
cfilt(1:nfft)=cfilt(1:nfft)*cw(1:nfft)
call four2a(cfilt,nfft,1,1,1)
! Subtract the reconstructed signal
do i=1,nframe
j=nstart+i-1
if(j.ge.1 .and. j.le.NMAX) dd(j)=dd(j)-2*REAL(cfilt(i)*cref(i))
enddo
return
end subroutine subtractft4