2005-12-22 16:40:53 +00:00
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subroutine decode6m(data,jz,cfile6,MinSigdB,istart,
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+ NFixLen,lcum,f0,lumsg,npkept,yellow)
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C Decode a JT6M message. Data must start at the beginning of a
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C sync symbol; sync frequency is assumed to be f0.
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parameter (NMAX=30*11025)
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real data(jz) !Raw data
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real s2db(0:43,646) !Spectra of symbols
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c real s2(128,646)
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real syncsig(646)
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real yellow(216)
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real ref(0:43)
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logical lcum
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character*43 pua
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character*48 msg
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character*6 cfile6
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real*8 dpha,twopi
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complex*16 z,dz
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complex zz
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complex ct(0:511)
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complex c
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common/hcom/c(NMAX)
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data pua/'0123456789., /#?$ABCDEFGHIJKLMNOPQRSTUVWXYZ'/
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data offset/20.6/
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2006-04-05 20:07:32 +00:00
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ps(zz)=real(zz)**2 + aimag(zz)**2 !Power spectrum function
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2005-12-22 16:40:53 +00:00
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C Convert data to baseband (complex result) using quadrature LO.
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twopi=8*atan(1.d0)
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dpha=twopi*f0/11025.d0
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dz=cmplx(cos(dpha),-sin(dpha))
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z=1.d0/dz
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do i=1,jz
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z=z*dz
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c(i)=data(i)*z
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enddo
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C Get spectrum for each symbol.
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C NB: for decoding pings, could do FFTs first for sync intervals only,
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C and then for data symbols only where the sync amplitude is above
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C threshold. However, for the average message we want all FFTs computed.
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call zero(ref,44)
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nz=jz/512 - 1
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fac=1.0/512.0
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do j=1,nz
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i0=512*(j-1) + 1
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do i=0,511
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ct(i)=fac*c(i0+i)
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enddo
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call four2a(ct,512,1,-1,1)
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C Save PS for each symbol
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do i=0,127
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xps=ps(ct(i))
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if(i.le.43) s2db(i,j)=xps
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c s2(i+1,j)=xps
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enddo
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if(mod(j,3).eq.1) call add(ref,s2db(0,j),ref,44) !Accumulate ref spec
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enddo
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C Return sync-tone amplitudes for plotting.
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iz=nz/3 -1
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do i=1,iz
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j=3*i-2
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yellow(i)=s2db(0,j)-0.5*(s2db(0,j+1)+s2db(0,j+2))
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enddo
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yellow(216)=iz
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fac=3.0/nz
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do i=0,43 !Normalize the ref spectrum
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ref(i)=fac*ref(i)
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enddo
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ref(0)=ref(2) !Sync bin uses bin 2 as ref
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do j=1,nz !Compute strength of sync
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m=mod(j-1,3) !signal at each j.
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ja=j-m-3
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jb=ja+3
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if(ja.lt.1) ja=ja+3
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if(jb.gt.nz) jb=jb-3
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syncsig(j)=0.5*(s2db(0,ja)+s2db(0,jb))/ref(0)
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syncsig(j)=db(syncsig(j)) - offset
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do i=0,43 !Normalize s2db
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s2db(i,j)=s2db(i,j)/ref(i)
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enddo
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enddo
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C Decode any message of 2 or more consecutive characters bracketed by
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C sync-tones above a threshold.
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C Use hard-decoding (i.e., pick max bin).
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nslim=MinSigdB !Signal limit for decoding
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ndf0=nint(f0-1076.77) !Freq offset DF, in Hz
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n=0 !Number of decoded characters
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j0=0
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sbest=-1.e9
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do j=2,nz-1,3
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if(syncsig(j).ge.float(nslim)) then
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C Is it time to write out the results?
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if((n.eq.48) .or. (j.ne.j0+3 .and. j0.ne.0)) then
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nsig=nint(sbest)
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width=(512./11025.)*(1.5*n+1.0)
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if(nsig.ge.nslim) then
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npkept=npkept+1
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write(lumsg,1010) cfile6,tping,width,
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+ nsig,ndf0,(msg(k:k),k=1,n)
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if(lcum) write(21,1010) cfile6,tping,width,
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+ nsig,ndf0,(msg(k:k),k=1,n)
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1010 format(a6,2f5.1,i4,i5,6x,48a1) !### 6x was 7x ###
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endif
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n=0
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sbest=-1.e9
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endif
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j0=j
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smax1=-1.e9
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do i=1,43 !Pick max bin for 1st char
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if(s2db(i,j).gt.smax1) then
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smax1=s2db(i,j)
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ipk=i
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endif
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enddo
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n=n+1
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if(n.eq.1) tping=j*512./11025. + (istart-1)/11025.0 !Start of ping
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msg(n:n)=pua(ipk:ipk) !Decoded character
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smax2=-1.e9
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do i=1,43
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if(s2db(i,j+1).gt.smax2) then
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smax2=s2db(i,j+1)
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ipk=i
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endif
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enddo
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n=n+1
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msg(n:n)=pua(ipk:ipk)
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sig0=10.0**(0.1*(syncsig(j)+offset))
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sig=db(0.5*sig0 + 0.25*(smax1+smax2))-offset
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sbest=max(sbest,sig)
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endif
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enddo
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nsig=nint(sbest)
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width=(512./11025.)*(1.5*n+1.0)
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if(n.ne.0 .and. nsig.ge.nslim) then
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npkept=npkept+1
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write(lumsg,1010) cfile6,tping,
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+ width,nsig,ndf0,(msg(k:k),k=1,n)
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if(lcum) write(21,1010) cfile6,tping,
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+ width,nsig,ndf0,(msg(k:k),k=1,n)
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endif
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C Decode average message for the whole record.
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call avemsg6m(s2db,nz,nslim,NFixLen,cfile6,lcum,f0,lumsg,npkept)
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return
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end
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