This formula can be found from a patent related to parallel combs structure. The
formula places the first echoes coming out of parallel combs to uniformly distributed
sequence. If T_ ,...,T_n are the delay lines in increasing order, the formula can be
derived by setting T_(k-1)/T_k = Constant and T_n/(2*T_1) = Constant, where 2*T_1 is the
echo coming just after the echo T_n.
I figured this out myself as it is not told in the patent. The formula is not the best
which one can come up. I use a search method to find echo sequences which are uniform
enough for long enough time. The formula is uniform for a short time only.
The formula doesn't work good for series allpass and FDN structures, for which a
similar formula can be derived with the same idea. The search method works for these
structures as well.