WSJT-X/longx.f

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subroutine longx(dat,npts0,ps,DFTolerance,noffset,
+ msg,msglen,bauderr)
C Look for 441-baud modulation, synchronize to it, and decode message.
C Longest allowed data analysis is 1 second.
parameter (NMAX=11025)
parameter (NDMAX=NMAX/25)
real dat(npts0)
real ps(128),psmo(20)
integer DFTolerance
real y1(NMAX)
real y2(NMAX)
real y3(NMAX)
real y4(NMAX)
real wgt(-2:2)
integer dit(NDMAX)
integer n4(0:2)
character msg*40
character c*48
common/acom/a1,a2,a3,a4
data c/' 123456789.,?/# $ABCD FGHIJKLMNOPQRSTUVWXY 0EZ '/
data wgt/1.0,4.0,6.0,4.0,1.0/
NSPD=25 !Change if FSK110 is implemented
LTone=2
NBaud=11025/NSPD
npts=min(NMAX,npts0)
df=11025.0/256.0
smax=0.
C Find the frequency offset of this ping.
C NB: this might be improved by including a bandpass correction to ps.
ia=nint((LTone*NBaud-DFTolerance)/df)
ib=nint((LTone*NBaud+DFTolerance)/df)
do i=ia,ib !Search for correct DF
sum=0.
do j=1,4 !Sum over the 4 tones
m=nint((i*df+(j-1)*NBaud)/df)
do k=-2,2 !Weighted averages over 5 bins
sum=sum+wgt(k)*ps(m+k)
enddo
enddo
k=i-ia+1
psmo(k)=sum
kpk=0
if(sum.gt.smax) then
smax=sum
noffset=nint(i*df-LTone*NBaud)
kpk=k
endif
enddo
if(kpk.gt.1 .and. kpk.lt.20) then
call peakup(psmo(kpk-1),psmo(kpk),psmo(kpk+1),dx)
noffset=nint(noffset+dx*df)
endif
C Do square-law detection in each of four filters.
f1=LTone*NBaud+noffset
f2=(LTone+1)*NBaud+noffset
f3=(LTone+2)*NBaud+noffset
f4=(LTone+3)*NBaud+noffset
call detect(dat,npts,f1,y1)
call detect(dat,npts,f2,y2)
call detect(dat,npts,f3,y3)
call detect(dat,npts,f4,y4)
C Bandpass correction:
npts=npts-(NSPD-1)
do i=1,npts
y1(i)=y1(i)*a1
y2(i)=y2(i)*a2
y3(i)=y3(i)*a3
y4(i)=y4(i)*a4
enddo
call sync(y1,y2,y3,y4,npts,jpk,baud,bauderr)
C Decimate y arrays by NSPD
ndits=npts/NSPD - 1
do i=1,ndits
y1(i)=y1(jpk+(i-1)*NSPD)
y2(i)=y2(jpk+(i-1)*NSPD)
y3(i)=y3(jpk+(i-1)*NSPD)
y4(i)=y4(jpk+(i-1)*NSPD)
enddo
C Now find the mod3 phase that has no tone 3's
n4(0)=0
n4(1)=0
n4(2)=0
do i=1,ndits
ymax=max(y1(i),y2(i),y3(i),y4(i))
if(y1(i).eq.ymax) dit(i)=0
if(y2(i).eq.ymax) dit(i)=1
if(y3(i).eq.ymax) dit(i)=2
if(y4(i).eq.ymax) then
dit(i)=3
k=mod(i,3)
n4(k)=n4(k)+1
endif
enddo
n4min=min(n4(0),n4(1),n4(2))
if(n4min.eq.n4(0)) jsync=3
if(n4min.eq.n4(1)) jsync=1
if(n4min.eq.n4(2)) jsync=2
C Might want to notify if n4min>0 or if one of the others is equal
C to n4min. In both cases, could then decode 2 or 3 times, using
C other starting phases.
C Finally, decode the message.
msg=' '
msglen=ndits/3
msglen=min(msglen,40)
do i=1,msglen
j=(i-1)*3+jsync
nc=16*dit(j) + 4*dit(j+1) +dit(j+2)
msg(i:i)=' '
if(nc.le.47) msg(i:i)=c(nc+1:nc+1)
enddo
return
end