An electronic filter that modifies the frequency response (amplitude and phase versus frequency) of a system for a specific purpose. Equalizers typically realize a more complicated frequency response in which the amplitude response varies continuously with frequency, amplifying some frequencies and attenuating others. An equalizer may have a response fixed in time or may be automatically and continuously adjusted. However, its frequency response is usually matched to some external physical medium, such as an acoustic path or communication channel, and thus inherently needs to be adjustable.
Equalizers can be used in many applications. In music and sound reproduction, equalizers can compensate for artifacts of the electrical-to-sound conversion or for unwanted characteristics of the acoustic environment such as sound reflections or absorption. Sound-recording and sonar systems can use equalizers to reduce unwanted interference. Most analog recording and playback devices, such as audio and video tape recorders, incorporate equalizers to compensate for the undesirable aspects of the recording medium, such as high-frequency roll-off, as well as to reduce noise and maximize dynamic range. See also Electrical interference; Electrical noise; Sonar; Sound recording.
Equalization is also used to enhance the performance of systems that communicate or record digital signals (streams of bits). All communications and recording systems utilize a physical medium, such as wires; coaxial cables; radio, acoustic, or optical-fiber waveguides; or magnetic and optical recording media. These media cause distortion; that is, the output signal is different from the input signal. For example, on radio or acoustic channels there are often multiple paths from transmitter to receiver, each having a slightly different delay and superimposed at the receiver. An equalizer is an electrical device that compensates for this distortion, reversing the effect of the channel and returning a waveform approximating the input signal. The channel output signal in response to a particular input signal (…,0, 0, 1, 0, 0…) may differ from the input, but the equalizer output reproduces the channel input, at least to close approximation (see illustration). See also Distortion (electronic circuits).
Communications channel with an equalizer placed at the output. The equalizer recovers an accurate replica of the channel input signal.
If the characteristics of the channel are well known, the equalizer can be fixed, or nonadaptive. More commonly, the detailed characteristics of a channel are not known in advance. For example, an equalizer may be required to compensate for any length of wire, from very short up to a maximum. In other cases, the channel may be varying with time, as is characteristic of the radio channel from a fixed transmitter to a moving vehicle. An adaptive equalizer is able to adjust itself to compensate. Adaptive equalizers are important for achieving high bit rates in telephone computer modems, and also for digital communications over radio channels. See also Data communications; Modem.
