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170 Chapter 4 Digital Filters
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Using Equations (4.8) through (4.10), with 81 « 0.0288 and §2 « 10~ , the
required filter length is estimated to N = 24. Note that the filter only needs to have
a large attenuation in frequency bands where unwanted images appear. The fre-
quency bands between these image bands are called don't-care bands. We get
Band 1 Band 2 Band 3
The magnitude response is shown in Figure 4.62. Note the don't-care band
between the two stopbands.
Figure 4.62 Magnitude response of the interpolating FIR filter
