WSJT-X/lib/fsk4hf/getfc1.f90

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subroutine getfc1(c,fc1)
parameter (KK=84) !Information bits (72 + CRC12)
parameter (ND=168) !Data symbols: LDPC (168,84), r=1/2
parameter (NS=65) !Sync symbols (2 x 26 + Barker 13)
parameter (NR=3) !Ramp up/down
parameter (NN=NR+NS+ND) !Total symbols (236)
parameter (NSPS=16) !Samples per MSK symbol (16)
parameter (N2=2*NSPS) !Samples per OQPSK symbol (32)
parameter (N13=13*N2) !Samples in central sync vector (416)
parameter (NZ=NSPS*NN) !Samples in baseband waveform (3760)
parameter (NFFT1=4*NSPS,NH1=NFFT1/2)
complex c(0:NZ-1) !Complex waveform
complex c2(0:NFFT1-1) !Short spectra
real s(-NH1+1:NH1) !Coarse spectrum
nspec=NZ/NFFT1
fs=12000.0/72.0
df1=fs/NFFT1
s=0.
do k=1,nspec
ia=(k-1)*N2
ib=ia+N2-1
c2(0:N2-1)=c(ia:ib)
c2(N2:)=0.
call four2a(c2,NFFT1,1,-1,1)
do i=0,NFFT1-1
j=i
if(j.gt.NH1) j=j-NFFT1
s(j)=s(j) + real(c2(i))**2 + aimag(c2(i))**2
enddo
enddo
! call smo121(s,NFFT1)
smax=0.
ipk=0
fc1=0.
ia=nint(40.0/df1)
do i=-ia,ia
f=i*df1
if(s(i).gt.smax) then
smax=s(i)
ipk=i
fc1=f
endif
! write(51,3001) f,s(i),db(s(i))
! 3001 format(f10.3,e12.3,f10.3)
enddo
! The following is for testing SNR calibration:
! sp3n=(s(ipk-1)+s(ipk)+s(ipk+1)) !Sig + 3*noise
! base=(sum(s)-sp3n)/(NFFT1-3.0) !Noise per bin
! psig=sp3n-3*base !Sig only
! pnoise=(2500.0/df1)*base !Noise in 2500 Hz
! xsnrdb=db(psig/pnoise)
return
end subroutine getfc1