WSJT-X/lib/fsk4hf/genfsk4hf.f90

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subroutine genfsk4hf(msgbits,f0,id,c)
parameter (KK=84) !Information bits (72 + CRC12)
parameter (ND=84) !Data symbols: LDPC (168,84), r=1/2
parameter (NS=12) !Sync symbols (3 @ 4x4 Costas arrays)
parameter (NR=2) !Ramp up/down
parameter (NN=NR+NS+ND) !Total symbols (98)
parameter (NSPS=2688/84) !Samples per symbol (32)
parameter (NZ=NSPS*NN) !Samples in baseband waveform (3136)
complex c(0:NZ-1) !Complex waveform
integer id0(NN) !2-bit data (values 0-3), all symbols
integer id(ND) !2-bit data (values 0-3), data only
integer*1 msgbits(KK),codeword(2*ND)
integer icos4(4) !4x4 Costas array
data icos4/0,1,3,2/
twopi=8.0*atan(1.0)
fs=12000.0/84.0
dt=1.0/fs
baud=1.0/(NSPS*dt)
call encode168(msgbits,codeword) !Encode the test message
id0(1)=0 !Ramp-up
id0(2:5)=icos4 !First Costas array
id0(48:51)=icos4 !Second
id0(94:97)=icos4 !Third
id0(98)=0 !Ramp down
j=5
do i=1,84 !Data symbols
id(i)=2*codeword(2*i-1) + codeword(2*i)
j=j+1
if(i.eq.43) j=j+4
id0(j)=id(i)
enddo
! Generate the 4-FSK waveform, low tone at f=0
c=0.
phi=0.d0
k=-1
do j=1,NN
dphi=twopi*(f0+id0(j)*baud)*dt
do i=1,NSPS
k=k+1
phi=phi+dphi
if(phi.gt.twopi) phi=phi-twopi
c(k)=cmplx(cos(phi),sin(phi))
enddo
enddo
return
end subroutine genfsk4hf