### Nested Allpass Filters

Another common method for increasing the density of an allpass impulse
response is to *nest* two or more allpass filters, as described
in §2.8.2 and shown in Fig.2.32 on page . In
general, a nested allpass filter is created when one or more of its
delay elements is replaced by another allpass filter. As we saw in
§2.8.2, first-order nested allpass filters are equivalent to
*lattice filters*. This equivalence implies that any order
transfer function (any poles and zeros)
may be obtained from a linear combination of the
delay elements of nested first-order allpass filters, since this is a
known property of the lattice filter [297].

In general, any delay-element or delay-line inside a stable allpass-filter can be replaced with any stable allpass-filter, and the result will be a stable allpass.

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Example Schroeder Reverberators

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Schroeder Allpass Sections