Glossary of Audio Terminology

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Index | References



AC (Alternating Current) Current in a conductor that changes direction over time.

AC-3 (audio coding 3) Dolby's digital audio data compression algorithm adopted for HDTV transmission and used in laserdiscs and CDs for 5.1 multichannel home theater use. See: Dolby Digital. Competes with DTS Zeta Digital. The terms AC-1 and AC-2 are other versions developed by Dolby for different applications.

acquisition time The time required for a sample-and-hold (S/H) circuit to capture an input analog value; specifically, the time for the S/H output to approximately equal its input.

acronym A word formed from the first letters of a name, such as laser for light amplification by stimulated emission of radiation, or by combining initial letters or parts of a series of words, such as radar for radio detecting and ranging. The requirement of forming a word is what distinguishes an acronym from an abbreviation (or initialism as the Canadian academicians say). Thus modem [modulator-demodulator] is an acronym, and AES [Audio Engineering Society] is an abbreviation or initialism. [Unsubstantiated rumor has it that the word "acronym" itself is an acronym, created from the phrase "abbreviating by cropping remainders off names to yield meaning" -- but it has never been confirmed.] (Thanks MR!)

active crossover A loudspeaker crossover requiring power to operate. Usually rack-mounted as a separate unit, active crossovers require individual power amplifiers for each output frequency band. Available in configurations known as stereo 2-way, mono 3-way, and so on. A stereo 2-way crossover is a two channel unit that divides the incoming signal into two segments, labeled Low and High outputs. A mono 3-way unit is a single channel device with three outputs, labeled Low, Mid and High. In this case, the user sets two frequencies: the Low-to-Mid, and the Mid-to-High crossover points. Up to stereo 5-way configurations exist for very elaborate systems. See: passive crossover

active equalizer A variable equalizer requiring power to operate. Available in many different configurations and designs. Favored for low cost, small size, light weight, loading indifference, good isolation (high input and low output impedances), gain availability (signal boosting possible), and line-driving ability. Disadvantages include, increased noise performance, limited dynamic range, reduced reliability, and RFI susceptibility; however, various manufactures continue to develop technologies improve on these limitations.

adaptive delta modulation (ADM) A variation of delta modulation in which the step size may vary from sample to sample.

ADAT (Alesis Digital Audio Tape) Digital tape recording system developed by Alesis, and since licensed to Fostex & Panasonic, putting 8-tracks of 16-bit, 44.1kHz digital audio on S-VHS tape.

ADAT ODI (optical digital interface) See ADAT Optical.

ADAT Optical Alesis's proprietary multichannel optical (fiber optic) digital interface specification for their family of ADAT modular digital multitrack recorders. This standard describes transmission of 8-channels of digital audio data through a single fiber optic cable.

A/D (analog-to-digital converter) The electronic component which converts the instantaneous value of an analog input signal to a digital word (represented as a binary number) for digital signal processing. The A/D is the first link in the digital chain of signal processing.

ADPCM (adaptive differential pulse code modulation) A very fast data compression algorithm based on the differences occurring between two samples.

ADSL (Asymmetrical digital subscriber Line) A communication technology used to transmit digital data over telephone wires. It is expected to transmit up to six megabits per second and be used for on demand services such as video conferences.

AES (Audio Engineering Society) Founded in 1949, the largest professional organization for electronic engineers and all others actively involved in audio engineering. Primarily concerned with education and standardization.

AES/EBU interface The serial transmission format standardized for professional digital audio signals (AES3-1992 AES Recommended Practice for Digital Audio Engineering - Serial Transmission Format for Two-Channel Linearly Represented Digital Audio Data) A specification using time division multiplex for data, and balanced line drivers to transmit two channels of digital audio data on a single twisted-pair cable using 3-pin (XLR) connectors. Issued as ANSI S4.40-1985 by the American National Standards Institute. In addition, information documents are being prepared describing the transmission of AES3 formatted data by unbalanced coaxial cable and by fiber optic cable.

AF AF (audio frequency) (also abbreviated af or a.f.) refers to alternating current (AC) having a frequency such that, if applied to a transducer such as a loudspeaker or headset, will produce acoustic waves within the range of human hearing. The AF range is generally considered to be from 20 Hz to 20 kHz ( 20,000 Hz.)

AFL Abbreviation for after fade listen, a term used on recording consoles and mixers, referring to a signal taken after the main channel fader; hence this sampling point tracks the main fader level. Also referred to as post fade solo, but since PFL already meant pre fade, AFL was adopted to prevent confusion. Compare with PFL.

algorithm A structured set of specifications that define methods and procedures tailored to accomplish a signal processing or transmitting task. For example, a fast Fourier transform (FFT), or a finite impulse response (FIR) filter are common DSP algorithms.

aliasing The problem of unwanted frequencies created when sampling a signal of a frequency higher than half the sampling rate. See: Nyquist frequency.

all-pass filter A filter that provides only phase shift or phase delay without appreciably changing the magnitude characteristic.

AM ( Amplitude Modulation )  A method of transmitting data or audio signals using a radio frequency (RF) carrier by varying the total power ( amplitude ) of the carrier's radiated electromagnet (EM) wave.  The original signal is encoded in the carrier by mixing the audio or data signal with the RF carrier in a modulator ( essentially a mixer ).  The output of the modulator is filtered, amplified and fed to the antenna.   Broadcast stations in the US "AM" band use this modulation technique.  AM is susceptible to static from electrical sources such as power lines, thunderstorms and electrical motors.

ampere Abbr. I, also A. 1. A unit of electric current in the International standard meter-kilogram-second (mks) system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2E-7 newtons per meter of length. 2. A unit in the International System specified as one International coulomb per second and equal to 0.999835 ampere. [After André Marie Ampère.]

Ampère, André Marie (1775-1836) French physicist and mathematician who formulated Ampère's law, a mathematical description of the magnetic field produced by a current-carrying conductor.

Amplifier  A device that changes a small movement into a larger movement. However, in general the most common and useful amplifiers actually use a small amount of electrical force ( voltage ) to control a larger amount of electrical force. Most useful amplifiers are said to be linear amplifiers, that is the amplified output is always related to the input in a linear relationship. This relationship ( or the ration of  output / input ) is known as the gain of the amplifier. The process of amplifying a signal invariably introduces some noise and distortion into the signal, and the process cannot be 100% efficient - amplifiers will always produce some waste heat.

amplifier classes Audio power amplifiers are classified according to the relationship between the output voltage swing and the input voltage swing, thus it is primarily the design of the output stage that defines each class. Classification is based on the amount of time the output devices operate during one complete cycle of signal swing. This is also defined in terms of output bias current [the amount of current flowing in the output devices with no applied signal]. For discussion purposes (with the exception of class A), assume a simple output stage consisting of two complementary devices (one positive polarity and one negative polarity) -- tubes (valves) or any type of transistor (bipolar, MOSFET, JFET, IGFET, IGBT, etc.).

  • Class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers in reality are not complementary designs. They are single-ended designs with only one type polarity output devices. They may have "bottom side" transistors but these are operated as fixed current sources, not amplifying devices. Consequently class A is the most inefficient of all power amplifier designs, averaging only around 20% (meaning you draw about 5 times as much power from the source as you deliver to the load!) Thus class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power. The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion. [Much mystique and confusion surrounds the term class A. Many mistakenly think it means circuitry comprised of discrete components (as opposed to integrated circuits). Such is not the case. A great many integrated circuits incorporate class A designs, while just as many discrete component circuits do not use class A designs.]
  • Class B operation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio.
  • Class AB operation is the intermediate case. Here both devices are allowed to be on at the same time (like in class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle but less than the entire cycle. That is, only a small amount of current is allowed to flow through both devices, unlike the complete load current of class A designs, but enough to keep each device operating so they respond instantly to input voltage demand s. Thus the inherent non-linearity of class B designs is eliminated, without the gross inefficiencies of the class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes class AB the most popular audio amplifier design.
  • Class AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.
  • Class C use is restricted to the broadcast industry for radio frequency (RF) transmission. Its operation is characterized by turning on one device at a time for less than one half cycle. In essence, each output device is pulsed-on for some percentage of the half cycle, instead of operating continuously for the entire half cycle. This makes for an extremely efficient design capable of enormous output power. It is the magic of RF tuned circuits (flywheel effect) that overcomes the distortion create d by class C pulsed operation.
  • Class D operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle (Sampling Theorem). Theoretically since the output devices are either completely on or completely off they do not dissipate any power. If a device is on there is a large amount of current flowing through it, but all the voltage is across the load, so the power dissipated by the dev ice is zero (found by multiplying the voltage across the device [zero] times the current flowing through the device [big], so 0 x big = 0); and when the device is off, the voltage is large, but the current is zero so you get the same answer. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%. [Historical note: the original use of the term "Class D" referred to switching amplifiers that employed a resonant circuit at the output to remove the harmonics of the switching frequency. Today's use is much closer to the original "Class S" designs.
  • Class E operation involves amplifiers designed for rectangular input pulses, not sinusoidal audio waveforms. The output load is a tuned circuit, with the output voltage resembling a damped single pulse. Normally Class E employs a single transistor driven to act as a switch.


The following terms, while generally agreed upon, are not considered "official" classifications
  • Class F Also known by such terms as "biharmonic," "polyharmonic," "Class DC," "single-ended Class D," "High-efficiency Class C," and "multiresonator." Another example of a tuned power amplifier, whereby the load is a tuned resonant circuit. One of the differences here is the circuit is tuned for one or more harmonic frequencies as well as the carrier frequency. See References: Krauss, et al. for complete details.
  • Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks [ thus the nickname rail-switcher]. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is becoming common for pro audio designs. [Historical note: Hitachi is credited with pioneering class G designs with their 1977 Dynaharmony HMA 8300 power amplifier.]
  • Class H operation takes the class G design one step further and actually modulates the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices [thus the nickname rail-tracker]. The efficiency of class H is comparable to class G designs. [Historical note: Soundcraftsmen is credited with pioneering class H designs with their 1977 Vari-proportional MA5002 power amplifier.]
  • Class S First invented in 1932, this technique is used for both amplification and amplitude modulation. Similar to Class D except the rectangular PWM voltage waveform is applied to a low-pass filter that allows only the slowly varying dc or average voltage component to appear across the load. Essentially this is what is termed "Class D" today. See References: Krauss for details.

amplitude 1. Greatness of size; magnitude. 2. Physics. The maximum absolute value of a periodically varying quantity. 3. Mathematics. a. The maximum absolute value of a periodic curve measured along its vertical axis. b. The angle made with the positive horizontal axis by the vector representation of a complex number. 4. Electronics. The maximum absolute value reached by a voltage or current waveform.

analog A real world physical quantity or signal characterized by being a continuously variable representation of the original event (rather than discrete jumps in signal level representing binary numbers that recording the value of the event at discrete moments).

Annex D Graphics An addendum to the H.320 videoconferencing protocol used for still image transfer between dissimilar video conferencing systems.

ANSI (pronounced "ann-see") (American National Standards Institute) An organization that develops and publishes standards for voluntary use in the U.S.A.

Antenna a specialized transducer that converts radio-frequency (RF) fields into alternating current (AC) or vice-versa. There are two basic types: the receiving antenna, which intercepts RF energy and delivers AC to electronic receiving equipment, and the transmitting antenna, which is fed with AC from electronic transmitting equipment and generates an RF field.

anti-aliasing filter A low-pass filter used at the input of digital audio converters to attenuate frequencies above the half-sampling frequency to prevent aliasing.

anti-imaging filter A low-pass filter used at the output of digital audio converters to attenuate frequencies above the half-sampling frequency to eliminate image spectra present at multiples of the sampling frequency.

Application A computer program designed for a particular use, such as a word processor or spreadsheet.

ASA (Acoustical Society of America) Founded in 1929, the oldest organization for scientist and professional acousticians and others engaged in acoustical design, research and education.

ASCII (pronounced "ask-ee") (American Standard Code for Information Interchange) An ANSI standard data transmission code consisting of seven information bits, used to code 128 letters, numbers, and special characters. Many systems now use an 8-bit binary code, called ASCII-8, in which 256 symbols are represented (for example, IBM's "extended ASCII").

ASIC (application-specific integrated circuit) A large-scale integrated circuit whose function is determined by the final mask layer for a particular application or group of applications; for example, an IC that does all the functions of a modem.

ASPEC (adaptive spectral perceptual entropy coding) A bit rate reduction standard for high quality audio. Jointly developed by AT&T Bell Labs, Thomson, the Fraunhofer Society and CNET. Characterized by high degrees of compression to allow audio transmission on ISDN.

asymmetrical (non-reciprocal) response Term used to describe the comparative shapes of the boost/cut curves for variable equalizers. The cut curves do not mirror the boost curves, but instead are quite narrow, intended to act as notch filters.

asynchronous A transmission process where the signal is transmitted without any fixed timing relationship between one word and the next (and the timing relationship is recovered from the data stream).

ATM (asynchronous transfer mode) networking An extremely fast networking technology. ATM specifies the protocol (i.e., the order and sequence) of the digital information on the network, but not the physical means of transmission (e.g., fiber optic, twisted-pair, etc.).

atmospheric pressure Pressure caused by the weight of the atmosphere. At sea level it has a mean value of one atmosphere but reduces with increasing altitude.

Audio 1. Of or relating to humanly audible sound, i.e., audio is all the sounds that humans hear. 2. a. Of or relating to the broadcasting or reception of sound. b. Of or relating to high-fidelity sound reproduction. [Audio traveling through air is vibrations, or cycles of alternating pressure zones that travel through the air as waves. Each wave can be view as a zone of Rarefaction followed by a cycle of compression.]

audio compression See: digital audio data compression


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