Tweak decoder for better decodes under poor conditions

lib/ft8/ft8_downsample.f90

@@ -5,15 +5,23 @@ subroutine ft8_downsample(dd,newdat,f0,c1)
   parameter (NMAX=15*12000,NSPS=1920)
   parameter (NFFT1=192000,NFFT2=3200)      !192000/60 = 3200
   
-  logical newdat
+  logical newdat,first
   complex c1(0:NFFT2-1)
   complex cx(0:NFFT1/2)
-  real dd(NMAX),x(NFFT1)
+  real dd(NMAX),x(NFFT1),taper(0:100)
   equivalence (x,cx)
-  save cx
+  data first/.true./
+  save cx,first,taper
 
+  if(first) then
+     pi=4.0*atan(1.0)
+     do i=0,100
+       taper(i)=0.5*(1.0+cos(i*pi/100))
+     enddo
+     first=.false.
+  endif
   if(newdat) then
-! Data in dd have changed, recompute the long FFT     
+! Data in dd have changed, recompute the long FFT
      x(1:NMAX)=dd
      x(NMAX+1:NFFT1)=0.                       !Zero-pad the x array
      call four2a(cx,NFFT1,1,-1,0)             !r2c FFT to freq domain
@@ -23,9 +31,9 @@ subroutine ft8_downsample(dd,newdat,f0,c1)
   df=12000.0/NFFT1
   baud=12000.0/NSPS
   i0=nint(f0/df)
-  ft=f0+8.0*baud
+  ft=f0+8.5*baud
   it=min(nint(ft/df),NFFT1/2)
-  fb=f0-1.0*baud
+  fb=f0-1.5*baud
   ib=max(1,nint(fb/df))
   k=0
   c1=0.
@@ -33,6 +41,8 @@ subroutine ft8_downsample(dd,newdat,f0,c1)
    c1(k)=cx(i)
    k=k+1
   enddo
+  c1(0:100)=c1(0:100)*taper(100:0:-1)
+  c1(k-1-100:k-1)=c1(k-1-100:k-1)*taper
   c1=cshift(c1,i0-ib)
   call four2a(c1,NFFT2,1,1,1)            !c2c FFT back to time domain
   fac=1.0/sqrt(float(NFFT1)*NFFT2)