WSJT-X/lib/fsk4hf/subtractft8.f90
Steven Franke d8f9f385b4 Use quarter-symbol steps for time sync. Lower sync threshold. Implement subtraction and two pass decoding. Use osd2 only near nfqso.
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@7861 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
2017-07-12 21:49:39 +00:00

62 lines
1.5 KiB
Fortran

subroutine subtractft8(dd,itone,f0,dt)
! Subtract an ft8 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=15*12000,NFRAME=1920*79)
parameter (NFFT=NMAX,NFILT=1400)
real*4 dd(NMAX), window(-NFILT/2:NFILT/2)
complex cref,camp,cfilt,cw
integer itone(79)
logical first
data first/.true./
common/heap8/cref(NFRAME),camp(NMAX),cfilt(NMAX),cw(NMAX)
save first
nstart=dt*12000+1
call genft8refsig(itone,cref,f0)
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 subtractft8