Band-stop filter

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Image:Ideal Band Stop Filter Transfer Function.PNG

In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a range to very low levels. It is the opposite of a band-pass filter. A notch filter is a band-stop filter with a narrow stopband (high Q factor).

Other names include 'T-notch filter', 'band-elimination filter', and 'band-rejection filter'.

Typically, the width of the stopband is less than 1 to 2 decades (that is, the highest frequency attenuated is less than 10 to 100 times the lowest frequency attenuated). In the audio band, a notch filter uses high and low frequencies that may be only semitones apart.

Audio example 1: Anti-hum filter

Image:60HZ notch -25db.png

Low Freq: 59 Hz
High Freq: 61 Hz

This means that the filter passes all frequencies, except for the range of 59–61 Hz. This would be used to filter out any noise from a 60 Hz power line.

Image:Bandstop filter.jpg

Audio example 2: Anti-presence filter