Multistage Filters
Why Are Multistage Filters Needed?
Typically used with multirate filters, multistage filters can
bring efficiency to a particular filter implementation. Multistage
filters are composed of several filters. These different parts of
the mulitstage filter, called stages, are connected
in a cascade or in parallel. However such a design can conserve resources
in many cases. There are many different uses for a multistage filter.
One of these is a filter requirement that includes a very narrow transition
width. For example, you need to design a lowpass filter where the
difference between the pass frequency and the stop frequency is .01
(normalized). For such a requirement it is possible to design a single
filter, but it will be very long (containing many coefficients) and
very costly (having many multiplications and additions per input sample).
Thus, this single filter may be so costly and require so much memory,
that it may be impractical to implement in certain applications where
there are strict hardware requirements. In such cases, a multistage
filter is a great solution. Another application of a multistage filter
is for a mulitrate system, where there is a decimator or an interpolator
with a large factor. In these cases, it is usually wise to break up
the filter into several multirate stages, each comprising a multiple
of the total decimation/interpolation factor.
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Optimal Multistage Filters in Filter Design Toolbox Software
As described in the previous section, within a multirate filter
each interconnected filter is called a stage. While
it is possible to design a multistage filter manually, it is also
possible to perform automatic optimization of a multistage filter
automatically. When designing a filter manually it can be difficult
to guess how many stages would provide an optimal design, optimize
each stage, and then optimize all the stages together. Filter Design Toolbox software
enables you to create a Specifications Object, and then design a filter
using multistage as an option. The rest of the work is done automatically.
Not only does Filter Design Toolbox software determine the optimal
number of stages, but it also optimizes the total filter solution.
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 | Multirate Filters | | Example Case for Multirate/Multistage Filters |  |
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