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Encyclopedia > Equalization filter

An equalization (EQ) filter is a filter, usually adjustable, chiefly meant to compensate for the unequal frequency response of some other signal processing circuit or system. In electronics and signal processing, a filter is a device or process that modifies a signal. ... Frequency response is the measure of any systems response to frequency, but is usually used in connection with electronic amplifiers and similar systems, particularly in relation to audio signals. ... Signal processing is the processing, amplification and interpretation of signals and deals with the analysis and manipulation of signals. ...


An EQ filter typically allows the user to adjust one or more parameters that determine the overall shape of the filter's transfer function. It is generally used to improve the fidelity of sound, to emphasize certain instruments, to remove undesired noises, or to create completely new and different sounds. A transfer function is a mathematical representation of the relation between the input and output of a linear time-invariant system. ...


Equalizers may be designed with peaking filters, shelving filters, or high-pass and low-pass filters.


There are three primary types of equalizers with peaking filters:

  • parametric equalizers
  • graphic equalizers
  • notch filters

All equalizers with peaking filters have three variables:

  • Frequency - All equalizers built on peaking filters use a bell curve which allows the equalizer to operate smoothly across a range of frequencies. The center frequency occurs at the top of the bell curve and is the frequency most affected by equalization. It is often notated as fc and is measured in Hz.
  • Q - This is a variable (quality factor) which refers to the width of the bell curve. The higher the Q, the narrower the bandwidth. A high Q means that only a few frequencies are affected, whereas an low Q affects many frequencies.
The formula for conversion from bandwidth in octaves to Q is:
, where n is the bandwidth in octaves.
  • Boost/Cut - Also called gain, this determines how much of the selected frequencies should be present. A boost means that those frequencies will be louder after being equalized, whereas a cut will soften them. The amount of boost/cut or gain is measured decibels, such as +3 dB or -6 dB. A boost or gain of +6 dB generally amounts to the sound being twice as loud after equalization, and most sounds are EQ'd with in the +/- 6 dB range.

A parametric equalizer uses independent parameters for Q, frequency, and boost/cut. Any frequency or range of frequencies can be selected and then processed. This is the most powerful EQ because it allows control over all three variables. This EQ is predominantly used in recording and mixing. Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... The graph of the probability density function of the normal distribution is sometimes called the bell curve or the bell-shaped curve; see normal distribution. ... The decibel (dB) is a measure of the ratio between two quantities, and is used in a wide variety of measurements in acoustics, physics and electronics. ...


A graphic equalizer uses predetermined Q and frequencies which are equally spaced according to the musical intervals, such as the octave (12-band graphic EQ) or one third of an octave (36-band graphic EQ). These frequencies can then be independently boost or cut. This type of EQ is often used for live applications, such as concerts. In music, an octave (sometimes abbreviated 8ve or 8va) is the interval between one musical note and another with half or double the frequency. ...


A notch filter is an EQ with a very high fixed Q. The frequency and boost/cut remain variable. This kind of EQ is useful in multimedia applications and in audio mastering.


Shelving filters

Shelving filters, unlike those described above, boost or cut from a determined frequency until they reach a preset level which is applied to the rest of the frequency spectrum. This kind of filter is usually found on the treble and bass controls of home audio units. These filters are also used in audio mastering. Familiar concepts associated with a frequency are colors, musical notes, radio/TV channels, and even the regular rotation of the earth. ...


High pass and low pass filters boost or cut frequencies above or below a selected frequency, called the cutoff frequency. A high pass filter allows only frequencies above the cutoff frequency to pass through unaffected. Frequencies below the cutoff frequency are then attenuated at a constant rate per octave. Low pass filters operate similarly, except that only frequencies below the cutoff are allowed to pass through. Common attenuation rates are 6, 12, and 18 dB per octave. These filters are used to reduce noise and hiss, eliminate pops, and remove rumble. It is common to use a high pass filter (at about 60 - 80 Hz) when recording vocals to eliminate rumble.


Almost all filters (both analog and digital) induce phase shift on the outgoing audio signal, which can cause a problem in mixing. The higher the value of Q, the more this phase shifting occurs. Therefore, EQ is often used sparingly, unless a particular effect is desired. Waves with the same phase Waves with different phases The phase of a wave relates the position of a feature, typically a peak or a trough of the waveform, to that same feature in another part of the waveform (or, which amounts to the same, on a second waveform). ...


See also

In audio processing, equalization (EQ) is the process of modifying the frequency envelope of a sound. ...

External links

  • Calculator: bandwidth in octaves N to quality factor Q and back

  Results from FactBites:
 
Equalization filter - Wikipedia, the free encyclopedia (683 words)
An equalization (EQ) filter is a filter, usually adjustable, chiefly meant to compensate for the unequal frequency response of some other signal processing circuit or system.
A graphic equalizer uses predetermined Q and frequencies which are equally spaced according to the musical intervals, such as the octave (12-band graphic EQ) or one third of an octave (36-band graphic EQ).
Shelving filters, unlike those described above, boost or cut from a determined frequency until they reach a preset level which is applied to the rest of the frequency spectrum.
Active Filters (2352 words)
The equalizer frequency response is shown below, correcting for a woofer with peaked response (Qp = 1.21) and early roll-off (Fp = 55 Hz), to obtain a response that is 6 dB down at 19 Hz and with Q = 0.5.
The equalizer action is difficult to visualize in the time domain, because the driver output waveform is the convolution of the input signal s(t) with the impulse response of the equalizer h
The equalizer output signal is convolved with the impulse response h
  More results at FactBites »

 
 

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