Equalization (audio)
Equalization, or simply EQ, in sound recording and reproduction is the process of adjusting the volume of different frequency bands within an audio signal. The circuit or equipment used to achieve this is called an equalizer.[1][2]
Most
Equalizers are used in
Terminology
The concept of equalization was first applied in correcting the
Later the concept was applied in
A British EQ or British style equalizer is one with similar properties to those on mixing consoles made in the UK by companies such as Amek, Neve and Soundcraft[4] from the 1950s through to the 1970s. Later on, as other manufacturers started to market their products, these British companies began touting their equalizers as being a cut above the rest. Today, many non-British companies such as Behringer and Mackie[5] advertise British EQ on their equipment. A British style EQ seeks to replicate the qualities of the expensive British mixing consoles.
History
Filtering audio frequencies dates back at least to
The Langevin Model EQ-251A was the first equalizer to use slide controls.[when?] It featured two passive equalization sections, a bass shelving filter, and a pass band filter. Each filter had switchable frequencies and used a 15-position slide switch to adjust cut or boost.[9] The first true graphic equalizer was the type 7080 developed by Art Davis's Cinema Engineering.[when?] It featured 6 bands with a boost or cut range of 8 dB. It used a slide switch to adjust each band in 1 dB steps. Davis's second graphic equalizer was the Altec Lansing Model 9062A EQ. In 1967 Davis developed the first 1/3 octave variable notch filter set, the Altec-Lansing "Acousta-Voice" system.[8]
In 1966, Burgess Macneal and George Massenburg began work on a new recording console. Macneal and Massenburg, who was still a teenager, conceptualized an idea for a sweep-tunable EQ that would avoid inductors and switches. Soon after, Bob Meushaw, a friend of Massenburg, built a three-band, frequency-adjustable, fixed-Q equalizer. When asked who invented the parametric equalizer, Massenburg stated, "Four people could possibly lay claim to the modern concept: Bob Meushaw, Burgess Macneal, Daniel Flickinger, and myself… Our (Bob’s, Burgess’ and my) sweep-tunable EQ was borne, more or less, out of an idea that Burgess and I had around 1966 or 1967 for an EQ… three controls adjusting, independently, the parameters for each of three bands for a recording console… I wrote and delivered the AES paper on Parametrics at the Los Angeles show in 1972… It’s the first mention of 'Parametric' associated with sweep-tunable EQ."[10]
Daniel N. Flickinger introduced the first parametric equalizer in early 1971. His design leveraged a high-performance op-amp of his own design, the 535 series[11][failed verification] to achieve filtering circuits that were before impossible. Flickinger's patent from early in 1971[12] showed the circuit topology that would come to dominate audio equalization until the present day, as well as the theoretical underpinnings of the elegant circuit. Instead of slide potentiometers working on individual bands of frequency, or rotary switches, Flickinger's circuit allowed arbitrary selection of frequency and cut or boost level in three overlapping bands over the entire audio spectrum. Six knobs on his early EQs would control these sweepable filters. Up to six switches were incorporated to select shelving on the high and low bands, and bypassing for any unused band for the purest signal path.
Similar designs appeared soon thereafter from George Massenburg (in 1972) and Burgess McNeal from ITI corp. In May 1972 Massenburg introduced the term parametric equalization in a paper presented at the 42nd convention of the Audio Engineering Society.[13] Most channel equalization on mixing consoles made from 1971 to the present day rely upon the designs of Flickinger, Massenburg and McNeal in either semi or fully-parametric topology.[citation needed] In the late 1990s and in the 2000s, parametric equalizers became increasingly available as digital signal processing (DSP) equipment, usually in the form of plug-ins for various digital audio workstations. Standalone outboard gear versions of DSP parametric equalizers were also quickly introduced after the software versions.
Filter types
Although the range of equalization functions is governed by the theory of linear filters, the adjustment of those functions and the flexibility with which they can be adjusted varies according to the topology of the circuitry and controls presented to the user.
Shelving controls are usually simple first-order filter functions that alter the relative gains between frequencies much higher and much lower than the
A parametric equalizer has one or more sections each of which implements a second-order filter function. This involves three adjustments: selection of the center frequency (in Hz), adjustment of the Q which determines the sharpness of the bandwidth, and the level or gain control which determines how much those frequencies are boosted or cut relative to frequencies much above or below the center frequency selected. In a semi-parametric equalizer the bandwidth is preset by the designer. In a quasi-parametric equalizer, the user is given limited switchable options for bandwidth.[14]
A graphic equalizer also implements second-order filter functions in a more user-friendly manner but with somewhat less flexibility. This equipment is based on a
Since "equalization" in the context of audio reproduction is not used strictly to compensate for the deficiency of equipment and transmission channels, the use of high-pass and low-pass filters may be mentioned. A
A first-order low-pass or high-pass filter has a standard response curve that reduces the unwanted frequencies well above or below the cutoff frequency with a slope of 6 dB per octave. A second-order filter will reduce those frequencies with a slope of 12 dB per octave and moreover may be designed with a higher Q or finite zeros in order to effect an even steeper response around the cutoff frequency. For instance, a second-order low-pass notch filter section only reduces (rather than eliminates) very high frequencies, but has a steep response falling to zero at a specific frequency (the so-called notch frequency). Such a filter might be ideal, for instance, in completely removing the 19 kHz FM stereo subcarrier pilot signal while helping to cut even higher frequency subcarrier components remaining from the stereo demultiplexer.
In addition to adjusting the relative amplitude of frequency bands, an audio equalizer usually alters the relative phases of those frequencies. While the human ear is not as sensitive to the phase of audio frequencies (involving delays of less than 1/30 second), music professionals may favor certain equalizers because of how they affect the timbre of the musical content by way of audible phase artifacts.[15]
High-pass and low-pass filters
A
Shelving filter
While high-pass and low-pass filters are useful for removing unwanted signal above or below a set frequency, shelving filters can be used to reduce or increase signals above or below a set frequency.[16] Shelving filters are used as common tone controls (bass and treble) found in consumer audio equipment such as home stereos, and on guitar amplifiers and bass amplifiers. These implement a first-order response and provide an adjustable boost or cut to frequencies above or below a certain point.
A high shelf or "treble control" will have a frequency response |H(f)| whose square is given by:
where fp and fz are called the pole and zero frequencies, respectively. Turning down the treble control increases fz and decreases fp so that frequences higher than fp are attenuated. Turning up the treble control increases fp and decreases fz so that frequencies higher than fz are boosted. Setting the treble control at the center sets fz = fp so that |H(f)|2 = 1 and the circuit has no effect. At most, the slope of the filter response in the transition region will be 6 dB per octave (thus a doubling of signal voltage and a consequent quadrupling of signal power for every doubling of frequency).
Similarly the response of a low shelf (or "low shelving or "bass control") can be represented as
In this case, the inclusion of the leading factor simply indicates that the response at frequencies much higher than fz or fp is unity and that only bass frequencies are affected.[17]
A high shelving control in which fz is set to infinity, or a low shelving response in which fz is set to zero, implements a first-order low-pass or high-pass filter, respectively. However, the usual tone controls have a more limited range, since their purpose is not to eliminate any frequencies but only to achieve a greater balance when, for instance, the treble is lacking and the sound is not crisp. Since the range of possible responses from shelving filters is so limited, some audio engineers considered shelving controls inadequate for equalization tasks.
On some bass amps and
Graphic equalizer
In the graphic equalizer, the input signal is sent to a bank of filters. Each filter passes the portion of the signal present in its own frequency range or band. The amplitude passed by each filter is adjusted using a slide control to boost or cut frequency components passed by that filter. The vertical position of each slider thus indicates the gain applied to that frequency band, so that the knobs resemble a graph of the equalizer's response plotted versus frequency.
The number of frequency channels (and therefore each one's bandwidth) affects the cost of production and may be matched to the requirements of the intended application. A
Parametric equalizer
Parametric equalizers are multi-band variable equalizers that allow users to control the three primary parameters: amplitude, center frequency and bandwidth. The amplitude of each band can be controlled, and the center frequency can be shifted, and the bandwidth (which is inversely related to "Q") can be widened or narrowed. Parametric equalizers are capable of making much more precise adjustments to the sound than other equalizers, and are commonly used in sound recording and live sound reinforcement. Parametric equalizers are also sold as standalone outboard gear units.
A variant of the parametric equalizer is the semi-parametric equalizer, which is also known as a sweepable filter. It allows users to control the amplitude and frequency, but uses a pre-set bandwidth of the center frequency. In some cases, semi-parametric equalizers allow the user to select between a wide and a narrow preset bandwidth.
Filter functions
The responses of
First-order filters
A first-order filter can alter the response of frequencies above and below a point. In the transition region the filter response will have a slope of up to 6 dB per octave. The bass and treble controls in a hi-fi system are each a first-order filter in which the balance of frequencies above and below a point are varied using a single knob. A special case of first-order filters is a first-order high-pass or low-pass filter in which the 6 dB per octave cut of low or high frequencies extends indefinitely. These are the simplest of all filters to implement individually, requiring only a capacitor and resistor.
Second-order filters
Second-order filters are capable of
where F0 is the
It is also possible to define the Q of a band-pass function as:
where N is the bandwidth in octaves. The reverse mapping is:
A second-order filter response with Q of less than 1/2 can be decomposed into two first-order filter functions, a low-cut and a high-cut (or boost). Of more interest are
The range of second-order filter functions is important because any analog filter function can be decomposed into a (usually small) number of these (plus, perhaps, simpler first-order responses). These are implemented directly by each section of a parametric equalizer where they are explicitly adjusted. And each element of a graphic equalizer based on a filter bank includes one such element whose Q is not adjustable by the user.
Uses
In
Equalization is commonly used to increase the depth of a mix, creating the impression that some sounds in a mono or stereo mix are farther away or closer than others.[3]: 75–76 Equalization is also commonly used to give tracks with similar frequency components complementary spectral contours, known as mirrored equalization. Selected components of parts that would otherwise compete, such as bass guitar and kick drum, are boosted in one part and cut in the other, and vice versa, so that they both stand out.[3]: 76–77
Equalizers can correct problems posed by a room's
During live events where signals from microphones are amplified and sent to
An equalizer can be used to correct or modify the frequency response of a loudspeaker system rather than designing the speaker itself to have the desired response. For instance, the Bose 901 speaker system does not use separate larger and smaller drivers to cover the bass and treble frequencies. Instead it uses nine drivers all of the same four-inch diameter, more akin to what one would find in a table radio.[citation needed] However, this speaker system is sold with an active equalizer. That equalizer must be inserted into the amplifier system so that the amplified signal that is finally sent to the speakers has its response increased at the frequencies where the response of these drivers falls off, and vice versa, producing the response intended by the manufacturer.[19]
A "rumble filter" is a high-pass (low cut) filter with a cutoff typically in the 20 to 40 Hz range; this is the low frequency end of
A
Equalization is used in a reciprocal manner in certain communication channels and recording technologies. The original music is passed through a particular filter to alter its frequency balance, followed by the channel or recording process. At the end of the channel or when the recording is played, a complementary filter is inserted which precisely compensates for the original filter and recovers the original waveform. For instance, FM broadcasting uses a
See also
Citations
- ^ ISBN 9780764577079.
- ^ ISBN 9780295984988.
- ^ ISBN 978-1-4411-5607-5.
- ^ "British EQ". Sweetwater. December 20, 1999. Archived from the original on August 20, 2012. Retrieved November 25, 2013.
- ^ "Extraordinary EQ from Extraordinary Engineers". Mackie. Archived from the original on December 2, 2013. Retrieved November 25, 2013.
- ^ The Telephone and the Multiple Telegraph, retrieved March 3, 2016
- ^ H. Tremaine, Audio Cyclopedia, 2nd. Ed., (H.W. Sams, Indianapolis, 1973)
- ^ a b Dennis Bohn (August 1997). "Operator Adjustable Equalizers: An Overview". Rane Corporation. Archived from the original on April 2, 2014. Retrieved November 25, 2013.
- ^ Rick Chinn. "Langevin EQ-251A Schematic". Retrieved November 25, 2013.
- ^ "A short history of graphic and parametric equalization". February 22, 2016.
- ^ US 3727896
- ^ US 3752928
- ^ George Massenburg (May 1972). "Parametric Equalization" (PDF). Archived from the original (PDF) on July 14, 2011.
{{cite journal}}
: Cite journal requires|journal=
(help) - ^ "What is a Parametric EQ?". Presonus. Retrieved February 17, 2024.
- ^ "Linear Phase EQ, Electronic Musician". Archived from the original on July 16, 2015. Retrieved July 15, 2015.
- ^ "Equalisers Explained". Sound on Sound. July 2001. Archived from the original on December 3, 2013. Retrieved November 25, 2013.
- ISBN 9789812700773.
- ^ Ballou, pp.875-876.
- ^ Stereophile magazine, Bose 901 Loudspeaker Review, 1995.
General sources
- Glen Ballou, "Filters and equalizers", Handbook for Sound Engineers, Fourth edition, Focal Press, 2008 ISBN 0-240-80969-6.
External links
- Playback equalization for 78rpm shellacs and early LPs (EQ curves, index of record labels): Audacity Wiki
- Discriminating EQ frequencies by ear
- Calculator: bandwidth per octave to quality factor and back
- EQ Condensed Overview
- Audio EQ Cookbook[permanent dead link]
- PreSonus Equalizer Terms and Tips
- WikiRecording's Guide to Equalization