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Ideal Filters

Module by: Melissa Selik, Richard Baraniuk. E-mail the authors

Summary: Describes five ideal filters.

Note: You are viewing an old version of this document. The latest version is available here.

There are four fundamental filters. They are

  • Lowpass -- blocks high frequencies, allowing low frequencies through
  • Highpass -- blocks low frequencies, allowing high frequencies through
  • Bandpass -- blocks all frequencies excepth those within a certain range
  • Bandstop -- blocks only the frequencies within a certain range, allowing all others to pass through

Figure 1: Ideal frequency domain representations of the four fundamental filters.
(a) Lowpass (b) Highpass (c) Bandpass (d) Bandstop
Figure 1(a) (idealfilters1.jpg) Figure 1(b) (idealfilters2.jpg) Figure 1(c) (idealfilters3.jpg) Figure 1(d) (idealfilters4.jpg)

Another term one may come across in the study of filters is an "allpass" filter. This is one that allows all frequencies through. The only meaningful effect an allpass filter can have is on the phase of the signal.

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