## Examples of Digital Filters

While any mapping from signals to real numbers can be called a filter, we normally work with filters which have more structure than that. Some of the main structural features are illustrated in the following examples.

The filter analyzed in Chapter 1 was specified by

*difference equation*. This simple filter is a special case of an important class of filters called

*linear time-invariant (LTI) filters*. LTI filters are important in audio engineering because they are the

*only*filters that preserve signal frequencies.

The above example remains a real LTI filter if we scale the input
samples by any real *coefficients*:

If we use complex coefficients, the filter remains LTI, but it becomes
a *complex filter*:

The filter also remains LTI if we use more input samples in a shift-invariant way:

*non-causal*filter example. Causal filters may compute using only

*present and/or past input samples*, , , and so on.

Another class of causal LTI filters involves using *past output
samples* in addition to present and/or past input samples. The
past-output terms are called *feedback*,
and digital filters employing feedback are called
*recursive digital filters*:

An example *multi-input, multi-output* (MIMO)
digital filter is

The simplest *nonlinear* digital filter is

*i.e.*, it squares each sample of the input signal to produce the output signal. This example is also a

*memoryless nonlinearity*because the output at time is not dependent on past inputs or outputs. The nonlinear filter

Another nonlinear filter example is the
*median smoother* of order which assigns the middle value of
input samples centered about time to the output at time .
It is useful for ``outlier'' elimination. For example, it will reject
isolated noise spikes, and preserve steps.

An example of a linear *time-varying* filter is

These examples provide a kind of ``bottom up'' look at some of the
major types of digital filters. We will now take a ``top down''
approach and characterize *all* linear, time-invariant filters
mathematically. This characterization will enable us to specify
frequency-domain analysis tools that work for *any* LTI digital
filter.

**Next Section:**

Linear Filters

**Previous Section:**

Definition of a Filter