From a0d471cb0bd5b12a36c777d8eb0f49e6d35a906f Mon Sep 17 00:00:00 2001
From: Steve Franke <s.j.franke@icloud.com>
Date: Wed, 27 Nov 2019 15:58:52 -0600
Subject: [PATCH] Improve FT8 SNR estimates in two ways: (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.
---
CMakeLists.txt | 1 +
lib/ft8/baseline.f90 | 1 +
lib/ft8/get_spectrum_baseline.f90 | 43 +++++++++++++++++++++++++++++++
lib/ft8/sync8.f90 | 3 ++-
4 files changed, 47 insertions(+), 1 deletion(-)
create mode 100644 lib/ft8/get_spectrum_baseline.f90
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f578ac270..4f2a987d5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -487,6 +487,7 @@ set (wsjt_FSRCS
lib/geodist.f90
lib/getlags.f90
lib/getmet4.f90
+ lib/ft8/get_spectrum_baseline.f90
lib/ft2/gfsk_pulse.f90
lib/graycode.f90
lib/graycode65.f90
diff --git a/lib/ft8/baseline.f90 b/lib/ft8/baseline.f90
index 035f9ab4b..496795108 100644
--- a/lib/ft8/baseline.f90
+++ b/lib/ft8/baseline.f90
@@ -37,6 +37,7 @@ subroutine baseline(s,nfa,nfb,sbase)
kz=k
a=0.
call polyfit(x,y,y,kz,nterms,0,a,chisqr) !Fit a low-order polynomial
+ sbase=0.0
do i=ia,ib
t=i-i0
sbase(i)=a(1)+t*(a(2)+t*(a(3)+t*(a(4)+t*(a(5))))) + 0.65
diff --git a/lib/ft8/get_spectrum_baseline.f90 b/lib/ft8/get_spectrum_baseline.f90
new file mode 100644
index 000000000..843e08c2c
--- /dev/null
+++ b/lib/ft8/get_spectrum_baseline.f90
@@ -0,0 +1,43 @@
+subroutine get_spectrum_baseline(dd,nfa,nfb,sbase)
+
+ include 'ft8_params.f90'
+ parameter(NST=NFFT1/2,NF=NMAX/NST-1)
+ real s(NH1,NF)
+ real savg(NH1)
+ real sbase(NH1)
+ real x(NFFT1)
+ real window(NFFT1)
+ complex cx(0:NH1)
+ real dd(NMAX)
+ equivalence (x,cx)
+ logical first
+ data first/.true./
+ save first,window
+
+ if(first) then
+ first=.false.
+ pi=4.0*atan(1.)
+ window=0.
+ call nuttal_window(window,NFFT1)
+ window=window/sum(window)*NSPS*2/300.0
+ endif
+
+! Compute symbol spectra, stepping by NSTEP steps.
+ savg=0.
+ df=12000.0/NFFT1
+ do j=1,NF
+ ia=(j-1)*NST + 1
+ ib=ia+NFFT1-1
+ if(ib.gt.NMAX) exit
+ x=dd(ia:ib)*window
+ call four2a(x,NFFT1,1,-1,0) !r2c FFT
+ s(1:NH1,j)=abs(cx(1:NH1))**2
+ savg=savg + s(1:NH1,j) !Average spectrum
+ enddo
+
+ if(nfa.lt.nint(200.0)) nfa=nint(200.0)
+ if(nfb.gt.nint(4910.0)) nfb=nint(4910.0)
+ call baseline(savg,nfa,nfb,sbase)
+
+return
+end subroutine get_spectrum_baseline
diff --git a/lib/ft8/sync8.f90 b/lib/ft8/sync8.f90
index 219c6ebf1..3d2141c02 100644
--- a/lib/ft8/sync8.f90
+++ b/lib/ft8/sync8.f90
@@ -37,7 +37,8 @@ subroutine sync8(dd,nfa,nfb,syncmin,nfqso,maxcand,s,candidate, &
enddo
savg=savg + s(1:NH1,j) !Average spectrum
enddo
- call baseline(savg,nfa,nfb,sbase)
+! call baseline(savg,nfa,nfb,sbase)
+ call get_spectrum_baseline(dd,nfa,nfb,sbase)
ia=max(1,nint(nfa/df))
ib=nint(nfb/df)