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subroutine flat4(s,npts0,nflatten)
! Flatten a spectrum for optimum display
! Input: s(npts) Linear scale in power
! nflatten If nflatten=0, convert to dB but do not flatten
! Output: s(npts) Flattened, with dB scale
implicit real*8 (a-h,o-z)
real*4 s(6827)
real*4 base
real*8 x(1000),y(1000),a(5)
data nseg/10/,npct/10/
npts=min(6827,npts0)
if(s(1).gt.1.e29) go to 900 !Boundary between Rx intervals: do nothing
do i=1,npts
s(i)=10.0*log10(s(i)) !Convert to dB scale
enddo
if(nflatten.gt.0) then
nterms=5
if(nflatten.eq.2) nterms=1
nlen=npts/nseg !Length of test segment
i0=npts/2 !Midpoint
k=0
do n=1,nseg !Skip first segment, likely rolloff here
ib=n*nlen
ia=ib-nlen+1
if(n.eq.nseg) ib=npts
call pctile(s(ia),ib-ia+1,npct,base) !Find lowest npct of points
do i=ia,ib
if(s(i).le.base) then
if (k.lt.1000) k=k+1 !Save these "lower envelope" points
x(k)=i-i0
y(k)=s(i)
endif
enddo
enddo
kz=k
a=0.
call polyfit(x,y,y,kz,nterms,0,a,chisqr) !Fit a low-order polynomial
do i=1,npts
t=i-i0
yfit=a(1)+t*(a(2)+t*(a(3)+t*(a(4)+t*(a(5)))))
s(i)=s(i)-yfit !Subtract the fitted baseline
enddo
endif
900 return
end subroutine flat4
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