From 91e3dbdf20b9ece72d945aeca8db7bae60a71c4f Mon Sep 17 00:00:00 2001
From: Steve Franke <s.j.franke@icloud.com>
Date: Sat, 2 Feb 2019 16:29:17 -0600
Subject: [PATCH] New scheme for finding candidates. Speed up sync. Improved
 SNR calibration.

---
 lib/ft4/ft4_decode.f90     | 15 ++++++++++-----
 lib/ft4/ft4_params.f90     |  4 ++--
 lib/ft4/getcandidates4.f90 | 39 +++++++++++++++++++++++++-------------
 3 files changed, 38 insertions(+), 20 deletions(-)

diff --git a/lib/ft4/ft4_decode.f90 b/lib/ft4/ft4_decode.f90
index f5a3174ad..7b4edcf79 100644
--- a/lib/ft4/ft4_decode.f90
+++ b/lib/ft4/ft4_decode.f90
@@ -78,7 +78,7 @@ subroutine ft4_decode(cdatetime0,tbuf,nfa,nfb,nfqso,iwave,ndecodes,mycall,    &
    ndecodes=0
    do icand=1,ncand
       f0=candidate(1,icand)
-      xsnr=10*log10(candidate(3,icand))-18.0
+      snr=candidate(3,icand)-1.0
       if( f0.le.375.0 .or. f0.ge.(5000.0-375.0) ) cycle
       call clockit('ft4_down',0)
       call ft4_downsample(iwave,f0,cd2)  !Downsample from 512 to 32 Sa/Symbol
@@ -89,15 +89,15 @@ subroutine ft4_decode(cdatetime0,tbuf,nfa,nfb,nfqso,iwave,ndecodes,mycall,    &
 ! Sample rate is now 12000/16 = 750 samples/second
       do isync=1,2
          if(isync.eq.1) then
-            idfmin=-50
-            idfmax=50
+            idfmin=-12
+            idfmax=12
             idfstp=3
             ibmin=0
             ibmax=374
             ibstp=4
          else
-            idfmin=idfbest-5
-            idfmax=idfbest+5
+            idfmin=idfbest-4
+            idfmax=idfbest+4
             idfstp=1
             ibmin=max(0,ibest-5)
             ibmax=min(ibest+5,NMAX/NDOWN-1)
@@ -265,6 +265,11 @@ subroutine ft4_decode(cdatetime0,tbuf,nfa,nfb,nfqso,iwave,ndecodes,mycall,    &
             if(idupe.eq.1) exit
             ndecodes=ndecodes+1
             decodes(ndecodes)=message
+            if(snr.gt.0.0) then
+               xsnr=10*log10(snr)-14.0
+            else
+               xsnr=-18.0
+            endif
             nsnr=nint(xsnr)
             freq=f0
             tsig=mod(tbuf + ibest/750.0,100.0)
diff --git a/lib/ft4/ft4_params.f90 b/lib/ft4/ft4_params.f90
index bcc7f7404..02d9de655 100644
--- a/lib/ft4/ft4_params.f90
+++ b/lib/ft4/ft4_params.f90
@@ -11,6 +11,6 @@ parameter (NZ=NSPS*NN)                !Sync and Data samples (52736)
 parameter (NZ2=NSPS*NN2)              !Total samples in shaped waveform (53760)
 parameter (NMAX=5*12000)              !Samples in iwave (60,000)
 parameter (NFFT1=2048, NH1=NFFT1/2)   !Length of FFTs for symbol spectra
-parameter (NSTEP=NSPS/4)              !Coarse time-sync step size
-parameter (NHSYM=NMAX/NSTEP-3)        !Number of symbol spectra (1/4-sym steps)
+parameter (NSTEP=NSPS)                !Coarse time-sync step size
+parameter (NHSYM=(NMAX-NFFT1)/NSTEP)  !Number of symbol spectra (1/4-sym steps)
 parameter (NDOWN=16)                  !Downsample factor
diff --git a/lib/ft4/getcandidates4.f90 b/lib/ft4/getcandidates4.f90
index 402116a27..47b08d426 100644
--- a/lib/ft4/getcandidates4.f90
+++ b/lib/ft4/getcandidates4.f90
@@ -56,19 +56,32 @@ subroutine getcandidates4(id,fa,fb,syncmin,nfqso,maxcand,savg,candidate,   &
 
   ncand=0
   f_offset = -1.5*12000/512
-  do i=1,maxcand
-     ipk=maxloc(savsm(nfa:nfb))
-     ip=nfa-1+ipk(1)
-     xmax=savsm(ip)
-     savsm(max(1,ip-8):min(NH1,ip+8))=0.0
-     if(xmax.ge.syncmin) then
-        ncand=ncand+1
-        candidate(1,ncand)=ip*df+f_offset 
-        candidate(2,ncand)=-99.9
-        candidate(3,ncand)=xmax
-     else
-        exit
-     endif
+  do i=nfa+1,nfb-1
+    if(savsm(i).ge.savsm(i-1) .and. savsm(i).ge.savsm(i+1) .and. savsm(i).ge.syncmin) then
+      del=0.5*(savsm(i-1)-savsm(i+1))/(savsm(i-1)-2*savsm(i)+savsm(i+1))
+      fpeak=(i+del)*df+f_offset
+      speak=savsm(i) - 0.25*(savsm(i-1)-savsm(i+1))*del
+      ncand=ncand+1
+      if(ncand.gt.maxcand) exit
+      candidate(1,ncand)=fpeak
+      candidate(2,ncand)=-99.99
+      candidate(3,ncand)=speak
+    endif
   enddo
+
+!  do i=1,maxcand
+!     ipk=maxloc(savsm(nfa:nfb))
+!     ip=nfa-1+ipk(1)
+!     xmax=savsm(ip)
+!     savsm(max(1,ip-8):min(NH1,ip+8))=0.0
+!     if(xmax.ge.syncmin) then
+!        ncand=ncand+1
+!        candidate(1,ncand)=ip*df+f_offset 
+!        candidate(2,ncand)=-99.9
+!        candidate(3,ncand)=xmax
+!     else
+!        exit
+!     endif
+!  enddo
 return
 end subroutine getcandidates4