The most common type of filter dealt with in practice is a linear,
causal, and time-invariant operator on the vector space consisting of
arbitrary real-valued functions of time. Since we are dealing with
the space of functions of time, we will use the terms *vector,
function,* and *signal* interchangeably. When time is a
continuous variable, the vector space is infinite-dimensional even
when time is restricted to a finite interval. Digital filters are
simpler in many ways theoretically because finite-time digital signals
occupy a finite-dimensional vector space. Furthermore, every linear
operator on the space of digital signals may be represented as a
matrix.^{F.1}If the range of time is restricted to N samples then the arbitrary
linear operator is an N by N matrix. In the discussion that follows,
we will be exclusively concerned with the digital domain. Every
linear filter will be representable as a matrix, and every signal will
be expressible as a column vector.

Linearity implies the superposition principle which is presently indispensible for a general filter response analysis. The superposition principle states that if a signal is represented as a linear combination of signals , then the response of any linear filter may written as the same linear combination of the signals where . More generally,

Causality means that the filter output does not depend on future inputs. This is necessary in analog filters where time is a real entity, but for digital filters causality is highly unnecessary unless the filter must operate in real-time. Requiring a filter to be causal results in a triangular matrix representation.

A time-invariant filter is one whose response does not depend on the
time of excitation. This allows *superposition in time* in
addition to the superposition of component functions given by
linearity. A matrix representing a linear time-invariant filter is
Toeplitz (each diagonal is constant). The chief value of
time-invariance is that it allows a linear filter to represented by
its *impulse response* which, for digital filters, is the response
elicited by the signal
. A deeper consequence of
superposition in time together with superposition of component signal
responses is the fact that every stable linear time invariant filter
emits a sinusoid at frequency in response to an input sinusoid at
frequency after sufficient time for start-up transients to
settle. For this reason sinusoids are called *eigenfunctions* of
linear time-invariant systems. Another way of putting it is that a
linear time-invariant filter can only modify a sinusoidal input by a
constant scaling of its amplitude and a constant offset in its phase.
This is the rationale behind Fourier analysis. The Laplace transform
of the impulse response gives the *transfer function* and the
Fourier transform of the impulse response is the *frequency
response*. It is important to note that relaxing time-invariance only
prevents us from using superposition in time. Consequently, while we
can no longer uniquely characterize a filter in terms of its impulse
response, we may still characterize it in terms of its **basis
function response**.

This will be developed below for the particular basis functions used in the Discrete Fourier Transform (DFT). These basis functions are defined for the N-dimensional discrete-time signal space as

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