WSJT-X/decode6m.f
Joe Taylor 2c17544f3f initial import
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/WSJT/trunk@1 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
2005-12-22 16:40:53 +00:00

162 lines
5.0 KiB
Fortran

subroutine decode6m(data,jz,cfile6,MinSigdB,istart,
+ NFixLen,lcum,f0,lumsg,npkept,yellow)
C Decode a JT6M message. Data must start at the beginning of a
C sync symbol; sync frequency is assumed to be f0.
parameter (NMAX=30*11025)
real data(jz) !Raw data
real s2db(0:43,646) !Spectra of symbols
c real s2(128,646)
real syncsig(646)
real yellow(216)
real ref(0:43)
logical lcum
character*43 pua
character*48 msg
character*6 cfile6
real*8 dpha,twopi
complex*16 z,dz
complex zz
complex ct(0:511)
complex c
common/hcom/c(NMAX)
data pua/'0123456789., /#?$ABCDEFGHIJKLMNOPQRSTUVWXYZ'/
data offset/20.6/
ps(zz)=real(zz)**2 + imag(zz)**2 !Power spectrum function
C Convert data to baseband (complex result) using quadrature LO.
twopi=8*atan(1.d0)
dpha=twopi*f0/11025.d0
dz=cmplx(cos(dpha),-sin(dpha))
z=1.d0/dz
do i=1,jz
z=z*dz
c(i)=data(i)*z
enddo
C Get spectrum for each symbol.
C NB: for decoding pings, could do FFTs first for sync intervals only,
C and then for data symbols only where the sync amplitude is above
C threshold. However, for the average message we want all FFTs computed.
call zero(ref,44)
nh=256
nz=jz/512 - 1
fac=1.0/512.0
do j=1,nz
i0=512*(j-1) + 1
do i=0,511
c fac=1.0/512.0 * abs(i-nh)/float(nh) !Window OK?
ct(i)=fac*c(i0+i)
enddo
call four2a(ct,512,1,-1,1)
C Save PS for each symbol
do i=0,127
xps=ps(ct(i))
if(i.le.43) s2db(i,j)=xps
c s2(i+1,j)=xps
enddo
if(mod(j,3).eq.1) call add(ref,s2db(0,j),ref,44) !Accumulate ref spec
enddo
C Return sync-tone amplitudes for plotting.
iz=nz/3 -1
do i=1,iz
j=3*i-2
yellow(i)=s2db(0,j)-0.5*(s2db(0,j+1)+s2db(0,j+2))
enddo
yellow(216)=iz
fac=3.0/nz
do i=0,43 !Normalize the ref spectrum
ref(i)=fac*ref(i)
enddo
ref(0)=ref(2) !Sync bin uses bin 2 as ref
do j=1,nz !Compute strength of sync
m=mod(j-1,3) !signal at each j.
ja=j-m-3
jb=ja+3
if(ja.lt.1) ja=ja+3
if(jb.gt.nz) jb=jb-3
syncsig(j)=0.5*(s2db(0,ja)+s2db(0,jb))/ref(0)
syncsig(j)=db(syncsig(j)) - offset
do i=0,43 !Normalize s2db
s2db(i,j)=s2db(i,j)/ref(i)
enddo
enddo
C Decode any message of 2 or more consecutive characters bracketed by
C sync-tones above a threshold.
C Use hard-decoding (i.e., pick max bin).
nslim=MinSigdB !Signal limit for decoding
ndf0=nint(f0-1076.77) !Freq offset DF, in Hz
n=0 !Number of decoded characters
j0=0
sbest=-1.e9
do j=2,nz-1,3
if(syncsig(j).ge.float(nslim)) then
C Is it time to write out the results?
if((n.eq.48) .or. (j.ne.j0+3 .and. j0.ne.0)) then
nsig=nint(sbest)
width=(512./11025.)*(1.5*n+1.0)
if(nsig.ge.nslim) then
npkept=npkept+1
write(lumsg,1010) cfile6,tping,width,
+ nsig,ndf0,(msg(k:k),k=1,n)
if(lcum) write(21,1010) cfile6,tping,width,
+ nsig,ndf0,(msg(k:k),k=1,n)
1010 format(a6,2f5.1,i4,i5,6x,48a1) !### 6x was 7x ###
endif
n=0
sbest=-1.e9
endif
j0=j
smax1=-1.e9
do i=1,43 !Pick max bin for 1st char
if(s2db(i,j).gt.smax1) then
smax1=s2db(i,j)
ipk=i
endif
enddo
n=n+1
if(n.eq.1) tping=j*512./11025. + (istart-1)/11025.0 !Start of ping
msg(n:n)=pua(ipk:ipk) !Decoded character
smax2=-1.e9
do i=1,43
if(s2db(i,j+1).gt.smax2) then
smax2=s2db(i,j+1)
ipk=i
endif
enddo
n=n+1
msg(n:n)=pua(ipk:ipk)
sig0=10.0**(0.1*(syncsig(j)+offset))
sig=db(0.5*sig0 + 0.25*(smax1+smax2))-offset
sbest=max(sbest,sig)
endif
enddo
nsig=nint(sbest)
width=(512./11025.)*(1.5*n+1.0)
if(n.ne.0 .and. nsig.ge.nslim) then
npkept=npkept+1
write(lumsg,1010) cfile6,tping,
+ width,nsig,ndf0,(msg(k:k),k=1,n)
if(lcum) write(21,1010) cfile6,tping,
+ width,nsig,ndf0,(msg(k:k),k=1,n)
endif
C Decode average message for the whole record.
call avemsg6m(s2db,nz,nslim,NFixLen,cfile6,lcum,f0,lumsg,npkept)
return
end