WSJT-X/flat2.f

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subroutine flat2(ss,n,nsum)
real ss(2048)
real ref(2048)
real tmp(2048)
nsmo=20
base=50*(float(nsum)**1.5)
ia=nsmo+1
ib=n-nsmo-1
do i=ia,ib
call pctile(ss(i-nsmo),tmp,2*nsmo+1,50,ref(i))
enddo
call pctile(ref(ia),tmp,ib-ia+1,68,base2)
C Don't flatten if signal is extremely low (e.g., RX is off).
if(base2.gt.0.05*base) then
do i=ia,ib
ss(i)=base*ss(i)/ref(i)
enddo
else
do i=1,n
ss(i)=0.
enddo
endif
return
end