WSJT-X/lib/decode65a.f90

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subroutine decode65a(dd,npts,newdat,nqd,f0,nflip,mode65,sync2,a,dt, &
nbmkv,nhist,decoded)
! Apply AFC corrections to a candidate JT65 signal, then decode it.
parameter (NMAX=60*12000) !Samples per 60 s
real*4 dd(NMAX) !92 MB: raw data from Linrad timf2
complex cx(NMAX/8) !Data at 1378.125 samples/s
complex c5x(NMAX/32) !Data at 344.53125 Hz
complex c5a(512)
real s2(66,126)
real a(5)
logical first
character decoded*22
data first/.true./,jjjmin/1000/,jjjmax/-1000/
data nhz0/-9999999/
save
! Mix sync tone to baseband, low-pass filter, downsample to 1378.125 Hz
dt00=dt
call timer('filbig ',0)
call filbig(dd,npts,f0,newdat,cx,n5,sq0)
call timer('filbig ',1)
! NB: cx has sample rate 12000*77125/672000 = 1378.125 Hz
! Find best DF, f1, f2, and DT. Start by downsampling to 344.53125 Hz
call timer('fil6521 ',0)
! Add some zeros at start of c5 arrays -- empirical fix for negative DT's
nadd=1089
c5x(:nadd)=0.
call fil6521(cx,n5,c5x(nadd+1),n6)
n6=n6+nadd
call timer('fil6521 ',1)
fsample=1378.125/4.
a(5)=dt00
i0=nint((a(5)+0.5)*fsample) - 2 + nadd
if(i0.lt.1) then
i0=1
endif
nz=n6+1-i0
! We're looking only at sync tone here... so why not downsample by another
! factor of 1/8, say? Should be a significant execution speed-up.
call timer('afc65b ',0)
! Best fit for DF, f1, and f2
call afc65b(c5x(i0),nz,fsample,nflip,a,ccfbest,dtbest)
call timer('afc65b ',1)
sync2=3.7e-4*ccfbest/sq0 !Constant is empirical
! Apply AFC corrections to the time-domain signal
! Now we are back to using the 1378.125 Hz sample rate, enough to
! accommodate the full JT65C bandwidth.
call timer('twkfreq ',0)
call twkfreq65(cx,n5,a)
call timer('twkfreq ',1)
! Compute spectrum for each half symbol.
! Adding or subtracting a small number (e.g., 5) to j may make it decode.\
! NB: might want to try computing full-symbol spectra (nfft=512, even for
! submodes B and C).
nsym=126
nfft=512
j=(dt00+dtbest+2.685)*1378.125
if(j.lt.0) j=0
call timer('sh_ffts ',0)
do k=1,nsym
do i=1,nfft
j=j+1
c5a(i)=cx(j)
enddo
call four2a(c5a,nfft,1,1,1)
do i=1,66
jj=i
if(mode65.eq.2) jj=2*i-1
if(mode65.eq.4) jj=4*i-3
s2(i,k)=real(c5a(jj))**2 + aimag(c5a(jj))**2
enddo
enddo
call timer('sh_ffts ',1)
call timer('dec65b ',0)
call decode65b(s2,nflip,mode65,nqd,nbmkv,nhist,decoded)
dt=dt00 + dtbest + 1.7
call timer('dec65b ',1)
return
end subroutine decode65a