Page 356 - Analog and Digital Filter Design

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CHAPTER

lNTRODUCTlON TO DIGITAL FILTERS

This chapter outlines the process of digital filtering. Digital filters operate on
digitized analog signals, so the digitization process is important and can be criti-
cal in the system design. Digitization requires the analog signal to be sampled
and then converted into a digital value, based on the amplitude of the sample.
For this reason I will cover the data sampling and digitization operation (under-
sampling, over-sampling, interpolation, and decimation) before considering
digital filters.

The two types of digital filter, finite impulse response (FIR) and infinite impulse
response (IIR), are only described briefly in this chapter. The functions required
to form a digital filter are described, such as multipliers, adders, and delays.
More detail on finding the multiplier coefficients for these types of filters will
be given in Chapters 16 and 17.

DigitaI signal processors (DSPs) are described in terms of how the functions
required in a digital filter are built into the architecture or can be created in soft-
ware. The type of arithmetic DSPs use to handle data during signal processing
is also described. The choice of processing device will determine whether fixed
or floating-point arithmetic is used. Fixed-point arithmetic can affect accuracy
and stability.

Analog-to-Digital Conversion

Analog data cannot be directly input to a digital system; it must be converted
into digital form. Samples of the analog signal are taken at discrete time inter-
vals and then converted into a digital form. This digital form is a binary repre-
sentation of the input voltage at the instant of sampling. Many analog-to-digital
converters produce a data word that is between 8 and 16 bits wide.

In order not to corrupt the data, the sampling frequency must be more than
twice the highest frequency of the input signal. Thus, in telephone systems that