## Digital Waveguide Filters

A *Digital Waveguide Filter* (DWF) can be defined as any digital
filter built by stringing together digital waveguides.^{C.7} In the case of a cascade
combination of unit-length waveguide sections, they are essentially
the same as *microwave filters* [372] and *unit element
filters* [136] (Appendix F). This section shows how
DWFs constructed as a cascade chain of waveguide sections, and
reflectively terminated, can be transformed via elementary
manipulations to well known *ladder* and *lattice* digital
filters, such as those used in speech modeling
[297,363].

DWFs may be derived conceptually by *sampling* the unidirectional
traveling waves in a system of ideal, lossless waveguides. Sampling
is across time and space. Thus, variables in a DWF structure are
equal *exactly* (at the sampling times and positions, to within
numerical precision) to variables propagating in the physical medium
in an interconnection of uniform 1D waveguides.

### Ladder Waveguide Filters

An important class of DWFs can be constructed as a linear cascade
chain of digital waveguide sections, as shown in Fig.C.24 (a
slightly abstracted version of Fig.C.19). Each block labeled
stands for a state-less Kelly-Lochbaum scattering junction
(§C.8.4) interfacing between wave impedances
and . We call this a *ladder waveguide filter* structure.
It is an exact bandlimited discrete-time model of a sequence of wave
impedances , as used in the Kelly-Lochbaum piecewise-cylindrical
acoustic-tube model for the vocal tract in the context of voice
synthesis (Fig.6.2). The delays between scattering
junctions along both the top and bottom signal paths make it possible
to compute all of the scattering junctions in parallel--a property
that is becoming increasingly valuable in signal-processing
architectures.

To transform the DWF of Fig.C.24 to a conventional *ladder
digital filter* structure, as used in speech modeling
[297,363], we need to (1) terminate on the right
with a pure reflection and (2) eliminate the delays along the top
signal path. We will do this in stages so as to point out a valuable
intermediate case.

### Reflectively Terminated Waveguide Filters

A reflecting termination occurs when the wave impedance of the next section is either zero or infinity (§C.8.1). Since Fig.C.24 is labeled for force waves, an infinite terminating impedance on the right results in

### Half-Rate Ladder Waveguide Filters

The delays preceding the two inputs to each scattering junction can be
``pushed'' into the junction and then ``pulled'' out to the outputs
and combine with the delays there. (This is easy to show using the
Kelly-Lochbaum scattering junction derived in §C.8.4.)
By performing this operation on every other section in the DWF chain,
the *half-rate ladder waveguide filter* shown in Fig.C.25 is
obtained [432].

This ``half-rate'' ladder DWF is so-called because its sampling rate
can be cut in half due to each delay being two-samples long. It has
advantages worth considering, which will become clear after we have
derived conventional ladder digital filters below. Note that now
*pairs* of scattering junctions can be computed in parallel, and
an exact physical interpretation remains. That is, it can still be
used as a general purpose physical modeling building block in this
more efficient (half-rate) form.

Note that when the sampling rate is halved, the physical wave variables (computed by summing two traveling-wave components) are at the same time only every other spatial sample. In particular, the physical transverse forces on the right side of scattering junctions and in Fig.C.25 are

respectively. In the half-rate case, adjacent spatial samples are separated in time by half a temporal sample . If physical variables are needed only for even-numbered (or odd-numbered) spatial samples, then there is no relative time skew, but more generally, things like collision detection, such as for slap-bass string-models (§9.1.6), can be affected. In summary, the half-rate ladder waveguide filter has an alternating half-sample time skew from section to section when used as a physical modeling building block.

### Conventional Ladder Filters

Given a reflecting termination on the right, the half-rate DWF chain of Fig.C.25 can be reduced further to the conventional ladder/lattice filter structure shown in Fig.C.26.

To make a standard ladder/lattice filter, the sampling rate is cut in
half (*i.e.*, replace by ), and the scattering junctions are
typically implemented in one-multiply form (§C.8.5) or
normalized form (§C.8.6), etc. Conventionally, if the
graph of the scattering junction is nonplanar, as it is for the
one-multiply junction, the filter is called a *lattice filter*;
it is called a *ladder filter* when the graph is planar, as it is
for normalized and Kelly-Lochbaum scattering junctions. For all-pole
transfer functions
, the *Durbin
recursion* can be used to compute the reflection coefficients
from the desired transfer-function denominator polynomial coefficients
[449]. To implement arbitrary transfer-function zeros, a
linear combination of delay-element outputs is formed using weights
that are called ``tap parameters'' [173,297].

To create Fig.C.26 from Fig.C.24, all delays along the top rail are pushed to the right until they have all been worked around to the bottom rail. In the end, each bottom-rail delay becomes seconds instead of seconds. Such an operation is possible because of the termination at the right by an infinite (or zero) wave impedance. Note that there is a progressive one-sample time advance from section to section. The time skews for the right-going (or left-going) traveling waves can be determined simply by considering how many missing (or extra) delays there are between that signal and the unshifted signals at the far left.

Due to the reflecting termination, conventional lattice filters cannot be extended to the right in any physically meaningful way. Also, creating network topologies more complex than a simple linear cascade (or acyclic tree) of waveguide sections is not immediately possible because of the delay-free path along the top rail. In particular, the output cannot be fed back to the input . Nevertheless, as we have derived, there is an exact physical interpretation (with time skew) for the conventional ladder/lattice digital filter.

### Power-Normalized Waveguide Filters

Above, we adopted the convention that the time variation of the wave impedance did not alter the traveling force waves . In this case, the power represented by a traveling force wave is modulated by the changing wave impedance as it propagates. The signal power becomes inversely proportional to wave impedance:

In [432,433], three methods are discussed for making signal
power *invariant* with respect to time-varying branch impedances:

- The
*normalized waveguide*scheme compensates for power modulation by scaling the signals leaving the delays so as to give them the same power coming out as they had going in. It requires two additional scaling multipliers per waveguide junction. - The
*normalized wave*approach [297] propagates*rms-normalized waves*in the waveguide. In this case, each delay-line contains and . In this case, the power stored in the delays does not change when the wave impedance changes. This is the basis of the*normalized ladder filter*(NLF) [174,297]. Unfortunately, four multiplications are obtained at each scattering junction. - The
*transformer-normalized waveguide*approach changes the wave impedance at the output of the delay back to what it was at the time it entered the delay using a ``transformer'' (defined in §C.16).

The transformer-normalized DWF junction is shown in Fig.C.27 [432]. As derived in §C.16, the transformer ``turns ratio'' is given by

So, as in the normalized waveguide case, for the price of two extra multiplies per section, we can implement time-varying digital filters which do not modulate stored signal energy. Moreover, transformers enable the scattering junctions to be varied independently, without having to propagate time-varying impedance ratios throughout the waveguide network.

It can be shown [433] that cascade waveguide chains built using
transformer-normalized waveguides are *equivalent* to those
using normalized-wave junctions. Thus, the transformer-normalized DWF
in Fig.C.27 and the wave-normalized DWF in Fig.C.22 are
equivalent. One simple proof is to start with a transformer
(§C.16) and a Kelly-Lochbaum junction (§C.8.4),
move the transformer scale factors inside the junction, combine terms,
and arrive at Fig.C.22. One practical benefit of this
equivalence is that the normalized ladder filter (NLF) can be
implemented using only three multiplies and three additions instead of
the usual four multiplies and two additions.

The transformer-normalized scattering junction is also the basis of
the *digital waveguide oscillator* (§C.17).

**Next Section:**

``Traveling Waves'' in Lumped Systems

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Scattering at Impedance Changes