OVERVIEW

There are a variety of methods that can be used to design digital filters as we will see in this chapter and the next. One commonly used method is to use the analog filter approximation functions that have already been developed and simply translate them in a way that will make them usable for discrete-time systems. This method, which will be studied in this chapter, makes use of the large backlog of filter design theory and tables of transfer functions that are readily available. Most of the filters designed using this method will be recursive in nature. That is, the output of the filter will depend on previous values of the output (as well as past and current values of the input). These types of filters can theoretically have impulse responses that continue forever and therefore are commonly referred to as infinite impulse response (IIR) filters.

Another method of designing discrete-time filters will be discussed in the next chapter. That method does not depend on analog filter theory, but rather uses the frequency response of the desired filter to directly determine the digital filter coefficients. The method generally yields nonrecursive filters that have outputs depending only on past and current values of the input. These types of filters generally have an impulse response containing only a finite number of values and thus are commonly called finite impulse response (FIR) filters. As we are about to see, both the IIR and FIR design methods will differ from the analog filter...