WSJT-X/lib/degrade_snr.f90

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subroutine degrade_snr(d2,npts,db,bw)
! Degrade S/N by specified number of dB.
integer*2 d2(npts)
p0=0.
do i=1,npts
x=d2(i)
p0=p0 + x*x
enddo
p0=p0/npts
if(bw.gt.0.0) p0=p0*6000.0/bw
s=sqrt(p0*(10.0**(0.1*db) - 1.0))
fac=sqrt(p0/(p0+s*s))
do i=1,npts
d2(i)=nint(fac*(d2(i) + s*gran()))
enddo
return
end subroutine degrade_snr