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subroutine decode4(dat,npts,dtx,nfreq,flip,mode4,ndepth,neme,minw, &
mycall,hiscall,hisgrid,decoded,nfano,deepbest,qbest,ichbest)
! Decodes JT4 data, assuming that DT and DF have already been determined.
! Input dat(npts) has already been downsampled by 2: rate = 11025/2.
! ### NB: this initial downsampling should be removed in WSJT-X, since
! it restricts the useful bandwidth to < 2.7 kHz.
use jt4
real dat(npts) !Raw data
character decoded*22,deepmsg*22,deepbest*22
character*12 mycall,hiscall
character*6 hisgrid
real*8 dt,df,phi,f0,dphi,twopi,phi1,dphi1
complex*16 cz,cz1,c0,c1
real*4 sym(207)
twopi=8*atan(1.d0)
dt=2.d0/11025 !Sample interval (2x downsampled data)
df=11025.d0/2520.d0 !Tone separation for JT4A mode
nsym=206
amp=15.0
istart=nint((dtx+0.8)/dt) !Start index for synced FFTs
if(istart.lt.0) istart=0
nchips=0
qbest=0.
qtop=0.
deepmsg=' '
ichbest=-1
c0=0.
k=istart
phi=0.d0
phi1=0.d0
ich1=minw+1
do ich=1,7
if(nch(ich).le.mode4) ich2=ich
enddo
do ich=ich1,ich2
nchips=min(nch(ich),70)
nspchip=1260/nchips
k=istart
phi=0.d0
phi1=0.d0
fac2=1.e-8 * sqrt(float(mode4))
do j=1,nsym+1
if(flip.gt.0.0) then
f0=nfreq + (npr(j))*mode4*df
f1=nfreq + (2+npr(j))*mode4*df
else
f0=nfreq + (1-npr(j))*mode4*df
f1=nfreq + (3-npr(j))*mode4*df
endif
dphi=twopi*dt*f0
dphi1=twopi*dt*f1
sq0=0.
sq1=0.
do nc=1,nchips
phi=0.d0
phi1=0.d0
c0=0.
c1=0.
do i=1,nspchip
k=k+1
phi=phi+dphi
phi1=phi1+dphi1
cz=dcmplx(cos(phi),-sin(phi))
cz1=dcmplx(cos(phi1),-sin(phi1))
if(k.le.npts) then
c0=c0 + dat(k)*cz
c1=c1 + dat(k)*cz1
endif
enddo
sq0=sq0 + real(c0)**2 + aimag(c0)**2
sq1=sq1 + real(c1)**2 + aimag(c1)**2
enddo
sq0=fac2*sq0
sq1=fac2*sq1
rsym=amp*(sq1-sq0)
if(j.ge.1) then
rsymbol(j,ich)=rsym
sym(j)=rsym
endif
enddo
call extract4(sym,ncount,decoded) !Do the convolutional decode
nfano=0
if(ncount.ge.0) then
nfano=1
ichbest=ich
exit
endif
qual=0. !Now try deep search
! if(ndepth.ge.1) then
if(iand(ndepth,32).eq.32) then
call deep4(sym(2),neme,flip,mycall,hiscall,hisgrid,deepmsg,qual)
if(qual.gt.qbest) then
qbest=qual
deepbest=deepmsg
ichbest=ich
endif
endif
enddo
if(qbest.gt.qtop) then
qtop=qbest
endif
qual=qbest
return
end subroutine decode4
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