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In Signal Processing , a band-stop filter or '''band-rejection filter''' is a Filter that passes most Frequencies unaltered, but Attenuate s those in a specific 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 ). Notch filters are used in live sound reproduction ( Public Address System s, also known as PA systems) and in Instrument Amplifier (especially amplifiers or preamplifiers for acoustic instruments such as acoustic guitar, mandolin, Bass Instrument Amplifier , etc.) to reduce or prevent feedback, while having little noticeable effect on the rest of the frequency spectrum. Other names include 'band limit filter', 'T-notch filter', 'band-elimination filter', and 'band-rejection filter'. Typically, the width of the stopband is less than 1 to 2 Decade s (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 Semitone s apart. Audio example 1: Anti-hum filter
This means that the filter passes all frequencies, except for the range of 59–61 Hz. This would be used to filter out fundamental noise from a 60 Hz Power Line , though its higher harmonics could still be present. Audio example 2: Anti-presence filter RF example 1: Non-linearities of power amplifiers For instance, when measuring non-linearities of power amplifiers a very narrow notch filter could be very useful to avoid the carrier so maximum input power of e.g. a spectrum analyser used to detect spurious content will not be exceeded. SEE ALSO
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