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https://github.com/saitohirga/WSJT-X.git
synced 2025-11-03 13:30:52 -05:00
a0d471cb0b
(i) SNR no longer saturates at +20 dB (ii) a large signal in the passband no longer causes the SNR of weaker signals to be biased low.
50 lines
1.3 KiB
Fortran
50 lines
1.3 KiB
Fortran
subroutine baseline(s,nfa,nfb,sbase)
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! Fit baseline to spectrum (for FT8)
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! Input: s(npts) Linear scale in power
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! Output: sbase(npts) Baseline
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implicit real*8 (a-h,o-z)
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real*4 s(1920)
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real*4 sbase(1920)
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real*4 base
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real*8 x(1000),y(1000),a(5)
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data nseg/10/,npct/10/
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df=12000.0/3840.0 !3.125 Hz
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ia=max(1,nint(nfa/df))
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ib=nint(nfb/df)
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do i=ia,ib
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s(i)=10.0*log10(s(i)) !Convert to dB scale
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enddo
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nterms=5
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nlen=(ib-ia+1)/nseg !Length of test segment
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i0=(ib-ia+1)/2 !Midpoint
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k=0
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do n=1,nseg !Loop over all segments
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ja=ia + (n-1)*nlen
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jb=ja+nlen-1
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call pctile(s(ja),nlen,npct,base) !Find lowest npct of points
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do i=ja,jb
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if(s(i).le.base) then
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if (k.lt.1000) k=k+1 !Save all "lower envelope" points
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x(k)=i-i0
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y(k)=s(i)
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endif
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enddo
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enddo
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kz=k
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a=0.
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call polyfit(x,y,y,kz,nterms,0,a,chisqr) !Fit a low-order polynomial
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sbase=0.0
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do i=ia,ib
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t=i-i0
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sbase(i)=a(1)+t*(a(2)+t*(a(3)+t*(a(4)+t*(a(5))))) + 0.65
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! write(51,3051) i*df,s(i),sbase(i)
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!3051 format(3f12.3)
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enddo
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return
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end subroutine baseline
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