Notch filter
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In signal processing, a notch filter (also band-stop filter or band-rejection filter), is a filter that passes most frequencies unaltered, but attenuates those in a narrow range to very low levels. It is the opposite of a band-pass filter.
Other names for the notch filter include 'T-notch filter', 'notched 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 20 times the lowest frequency attenuated). In the audio band, a notch filter uses high and low frequencies that may be only semitones apart.
Example 1: Anti-hum filter
- Low Freq: 59 Hz
- High Freq: 61 Hz
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60HZ_notch_-25db.png
Image:60HZ_notch_-25db.png
- this means that the filter passes all frequencies, except for the range of 59-61Hz. This would be used to filter out any noise from a 60Hz power line.
Example 2: Anti-presence filter
- Low Freq: 1000 Hz
- High Freq: 4000 Hz