Equalization

Equalization, (British: equalisation) is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used to achieve equalization is called an equalizer. These devices strengthen (boost) or weaken (cut) the energy of specific frequency bands.

In telecommunications, equalizers are used to render the frequency response—for instance of a telephone line—flat from end-to-end. When a channel has been "equalized" the frequency domain attributes of the signal at the input are faithfully reproduced at the output. Telephones, DSL lines and television cables use equalizers to prepare data signals for transmission.

In the field of audio electronics, the term "equalization" has come to include the adjustment of frequency responses for practical or aesthetic reasons, often resulting in a net response that is not truly equalized. The term EQ specifically refers to this variant of the term.[1] Stereos typically have adjustable equalizers which boost or cut bass or treble frequencies. Broadcast and recording studios use sophisticated equalizers capable of much more detailed adjustments, such as eliminating unwanted sounds or making certain instruments or voices more prominent.

Equalizers are critical to the successful operation of electronic systems such as analog broadcast television. In this application the actual waveform of the transmitted signal must be preserved, not just its frequency content. Equalizing filters must cancel out any group delay and phase delay between different frequency components.

Contents

Uses

Audio and music

Main article: Equalization (audio)

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 which alter the relative gains between frequencies much higher and much lower than the corner frequencies. A low shelf, such as the bass control on most hi-fi equipment, is adjusted to affect the gain of lower frequencies while having no effect well above its corner frequency. A high shelf, such as a treble control adjusts the gain of higher frequencies only. These are coarse adjustments more designed to increase the listener's satisfaction than providing actual equalization in the strict sense of the term.

A parametric equalizer, on the other hand, 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 quasi-parametric or semi-parametric equalizer there is no control for the bandwidth (it is preset by the designer) or is only selected between two presets using a switch.

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 bank of filters covering the audio spectrum in up to 30 frequency bands. Each second-order filter has a fixed center frequency and Q, but an adjustable level. The user can raise or lower each slider in order to visually approximate a "graph" of the intended frequency response.

Variable equalization in audio reproduction was first used by John Volkman working at RCA in the 1920s. That system was used to equalize a motion picture theater sound playback system.[2]

Since "equalization" in the context of audio reproduction isn't used strictly to compensate for the deficiency of equipment and transmission channels, the use of high and low pass filters may be mentioned. A high-pass filter modifies a signal only by eliminating lower frequencies. Thus a low-cut or rumble filter is used to remove infrasonic energy from a program which may consume undue amplifier power and cause excessive excursions in (or even damage to) speakers. A low-pass filter only modifies the audio signal by removing high frequencies. Thus a high-cut or hiss filter may be used to remove annoying white noise at the expense of the crispness of the program material.

A first-order low or high pass filter has a standard response curve which reduces the unwanted frequencies well above or below the corner frequency with a slope of 6 dB per octave. A fancier 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 may alter 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.[3]

Analog telecommunications

Audio lines

Early telephone systems used equalization to correct for the reduced level of high frequencies in long cables, typically using Zobel networks. These kinds of equalizers can also be used to produce a circuit with a wider bandwidth than the standard telephone band of 300 Hz to 3.4 kHz. This was particularly useful for broadcasters who needed "music" quality, not "telephone" quality on landlines carrying program material. It is necessary to remove or cancel any loading coils in the line before equalization can be successful. Equalization was also applied to correct the response of the transducers, for example, a particular microphone might be more sensitive to low frequency sounds than to high frequency sounds, so an equalizer would be used to increase the volume of the higher frequencies (boost), and reduce the volume of the low frequency sounds (cut).

Television lines

A similar approach to audio was taken with television landlines with two important additional complications. The first of these is that the television signal is a wide bandwidth covering many more octaves than an audio signal. A television equalizer consequently typically requires more filter sections than an audio equalizer. To keep this manageable, television equalizer sections were often combined into a single network using ladder topology to form a Cauer equalizer.

The second issue is that phase equalization is essential for an analog television signal. Without it dispersion causes the loss of integrity of the original waveshape and is seen as smearing of what were originally sharp edges in the picture.

Digital telecommunications

Main article: Equalizer (communications)

Modern digital telephone systems have less trouble in the voice frequency range as only the local line to the subscriber now remains in analog format, but DSL circuits operating in the MHz range on those same wires may suffer severe attenuation distortion, which is dealt with by automatic equalization or by abandoning the worst frequencies. Picturephone circuits also had equalizers.

See also

References

  1. ^ Ballou, pp.875-876.
  2. ^ H. Tremaine, Audio Cyclopedia, 2nd. Ed., (H.W. Sams, Indianapolis, 1973)
  3. ^ Linear Phase EQ, Electronic Musician
  • Glen Ballou, "Filters and equalizers", Handbook for Sound Engineers, Fourth edition, Focal Press, 2008 ISBN 0240809696.

External links