Page 130 - DSP Integrated Circuits

<4

DIGITAL FILTERS

4.1 INTRODUCTION

In this chapter, we will review the basic properties of both FIR and IIR filters as
well as the most commonly used design methods. FIR filters are straightforward to
design by using CAD tools. One of the major drawbacks of FIR filters is that large
amounts of memory and arithmetic processing are needed. This makes them unat-
tractive in many applications.
IIR filters, on the other hand, require much less memory and fewer arithmetic
operations, but they are difficult to design and they suffer from stability problems.
Although the design is much more demanding, the use of an IIR filter may result
in a lower system cost and higher performance. As a main topic we will therefore
present a class of digital IIR filters, namely wave digital filters, which have highly
advantageous properties and are the type of IIR filters we recommend.
A common problem is to transfer signals between two subsystems that operate
with different sample frequencies—for example, between a CD player and a DAT
(digital tape recorder). In other cases a system function (for example, a narrow-band
filter) can be implemented more efficiently by using several sample rates. Such
systems are called multirate systems. We will therefore discuss sample rate con-
verters for changing the sample rate without affecting the information contained
in the signal. Converters for increasing and decreasing the sample frequency are
usually called interpolators and decimators, respectively. We elect, as the topic of
the third design study, to design an interpolator based on wave digital filters.

4.2 FIR FILTERS

Digital filters can be categorized into two classes known as FIR (finite-length
impulse response) and IIR (infinite-length impulse response filters). Advantages of
FIR filters over IIR filters are that they are guaranteed to be stable (see Chapter 5)
and to have a linear-phase response. Linear-phase FIR filters are widely used in dig-
ital communication systems, in speech and image processing systems, in spectral
analysis, and particularly in applications where nonlinear-phase distortion cannot

115