In
signal processingSignal processing is an area of electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time to perform useful operations on those signals...
, a
band-stop filter or
band-rejection filter is a
filterIn signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
that passes most
frequenciesFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a
band-pass filterA band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit...
. A
notch filter is a band-stop filter with a narrow
stopbandA stopband is a band of frequencies, between specified limits, in which a circuit, such as a filter or telephone circuit, does not let signals through, or the attenuation is above the required stopband attenuation level...
(high
Q factorIn physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
). Notch filters are used in live sound reproduction (Public Address systems, also known as PA systems) and in
instrument amplifierAn instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...
(especially amplifiers or preamplifiers for acoustic instruments such as acoustic guitar, mandolin,
bass instrument amplifierThe double bass, also called the upright bass or contrabass, is the largest and lowest-pitched bowed string instrument in the modern symphony orchestra. The name, "double bass," derives from the early use of the instrument to double—an octave lower where possible—the bass part written...
, etc.) to reduce or prevent
feedbackAudio feedback is a special kind of feedback which occurs when a sound loop exists between an audio input and an audio output...
, while having little noticeable effect on the rest of the frequency spectrum.
In
signal processingSignal processing is an area of electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time to perform useful operations on those signals...
, a
band-stop filter or
band-rejection filter is a
filterIn signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
that passes most
frequenciesFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a
band-pass filterA band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit...
. A
notch filter is a band-stop filter with a narrow
stopbandA stopband is a band of frequencies, between specified limits, in which a circuit, such as a filter or telephone circuit, does not let signals through, or the attenuation is above the required stopband attenuation level...
(high
Q factorIn physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
). Notch filters are used in live sound reproduction (Public Address systems, also known as PA systems) and in
instrument amplifierAn instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...
(especially amplifiers or preamplifiers for acoustic instruments such as acoustic guitar, mandolin,
bass instrument amplifierThe double bass, also called the upright bass or contrabass, is the largest and lowest-pitched bowed string instrument in the modern symphony orchestra. The name, "double bass," derives from the early use of the instrument to double—an octave lower where possible—the bass part written...
, etc.) to reduce or prevent
feedbackAudio feedback is a special kind of feedback which occurs when a sound loop exists between an audio input and an audio output...
, 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-reject filter'.
Typically, the width of the stopband is less than 1 to 2
decadeOne decade is a factor of 10 difference between two numbers measured on a logarithmic scale. It is especially useful when referring to frequencies and when describing frequency response of electronic systems, such as audio amplifiers and filters.-Calculations:The factor-of-ten in a decade can be...
s (that is, the highest frequency attenuated is less than 10 to 100 times the lowest frequency attenuated). In the
audioAn audio frequency , or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human. While the range of frequencies that any individual can hear is largely related to environmental factors, the generally accepted standard range of audible frequencies...
band, a notch filter uses high and low frequencies that may be only
semitoneA semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music, and it is considered the most dissonant. The most commonly written form of this interval is the minor second, notated using two adjacent letter names A semitone, also called a...
s apart.
Audio example 1: Anti-hum filter
- Low Freq: 59 Hz
The hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
- High Freq: 61 Hz
The hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
This means that the filter passes all frequencies, except for the range of 59–61 Hz. This would be used to filter out the
mains humElectric hum, mains hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local electric utility configuration...
from a 60 Hz
power lineElectric power transmission is the bulk transfer of electrical energy, a process in the delivery of electricity to consumers. A power transmission network typically connects power plants to multiple substations near a populated area...
, though its higher harmonics could still be present. The common European version of the filter would have a 49–51 Hz range.
Audio example 2: Anti-presence filter
- Low Freq: 1 kHz
- High Freq: 4 kHz
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
- Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit...
- High-pass filter
A high-pass filter is an LTI filter that passes high frequencies well but attenuates frequencies lower than the cutoff frequency. The actual amount of attenuation for each frequency is a design parameter of the filter...
- Low-pass filter
A low-pass filter is a filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter when used in...
- Mains hum
Electric hum, mains hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local electric utility configuration...
- Parametric equalizer
- Stopband
A stopband is a band of frequencies, between specified limits, in which a circuit, such as a filter or telephone circuit, does not let signals through, or the attenuation is above the required stopband attenuation level...
- Bass instrument amplification
Bass instrument amplification for the bass guitar, double bass and similar instruments is distinct from other types of amplification systems due to the particular challenges associated with low-frequency sound reproduction. This distinction affects the design of the loudspeakers, the cabinet, and...
(discusses use of notch filter to prevent feedback)