Glossary of Audio Terminology

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Index | References



back-emf (back-electromotive force) Literally, back-voltage, is a phenomena found in all moving-coil electromagnetic systems, but for audio is most often used with respect to loudspeaker operation. This term describes the action where, after the signal stops, the speaker cone continues moving, causing the voice coil to move through the magnetic field (now acting like a electrical generator or dynamic microphone), creating a new voltage that tries to drive the cable back to the power amplifier's output. If the loudspeaker is allowed to do this, the cone flops around like a dying fish. It does not sound good! The only way to stop back-emf is to make the loudspeaker "see" a almost dead short, i.e., zero ohms looking backward, or as close to it as possible. See: damping factor

background music Music played at a lower volume used as an alternative to silence in public and commercial spaces. Intelligibility is now an issue as this music is not intended to compete with human speech.  Often background music is preformed without lyrics and not by the original artist. Contrast with foreground music.

balance control A control found most commonly on professional and consumer stereo preamplifiers, used to change the relative loudness (power) between the left and right channels. One channel is made (apparently) louder by attenuating the opposite channel. Although many circuits are used by different manufactures the ideal balance control should attenuated both channels by 3 to 6 dBs at the center position.  Rotating it away from the center position causes one channel to be attenuated, while having no effect on the other channel, and vice-versa. At each extreme of rotation one channel is shorted to ground and therefore completely off while the other channel is unattenuated.  Contrast with pan and crossfade controls.

balanced line The preferred method (for hum free) interconnecting of sound systems using a shielded twisted-pair cable. Because of its superior noise immunity, balanced lines also find use in interconnecting data signals, e.g., RS-422, and digital audio, e.g., AES/EBU. The principal behind balanced lines is that the signal is transmitted over a complete path down one wire and back on the another ( think of a close line around two pullies ).  The shield does not carry any information, thus it is free to function as a true shield, but must be earth grounded at one end to be successful.

[Long Answer: To understand why balanced lines are so successful, first examine a balanced, or differential (equivalent term) output stage, and then an input stage: A differential output stage simultaneously drives two lines, one positive and one negative. The voltage difference between these two wires is the audio signal.  Note that the audio signal exists uniquely between these two lines — not between them and ground. The complete circuit path travels down on the positive line and back on the negative line. Ground is not needed to transmit the signal — this is the essence and power of balanced lines. Ground is used only for shielding and safety purposes. Conversely, an unbalanced line is one that transmits the audio signal between one wire and ground. The circuit path is down the wire and back through the shield cable connected to ground. Ground is the return path; the circuit does not work without it. A balanced (or differential) input stage extracts the difference between the two input lines, and that, of course, is the desired audio signal. It receives the difference signal sent down the cable by the differential output. This circuit’s primary advantage is its great noise rejection ability. It has what is called common-mode rejection. The concept here relies on induced (received form radiated signals) noise showing up equally (or common) on each wire. It is mainly due to EMI (electromagnetic interference: from near by magnetic fields), RFI (radio frequency interference: from strong broadcast radio signals), electrical noise present in the ground references, or a combination of all three. Balanced line designs have equal impedance from each line relative to ground, guaranteeing equal noise susceptibility. Since the balanced input stage amplifies only the difference between the lines, it rejects everything else (noise) that is common ( or equal ) in both lines.]

bandpass filter A filter that has a limited passband between two frequencies.  Neither of the cutoff frequencies being zero or infinite. The bandpass frequencies are normally defined as the  frequencies that are attenuated  -3 dB from the passband.

band In telecommunication, a band - sometimes called a frequency band - is a specific range of frequencies in the radio frequency (RF) spectrum.  Each band has a defined upper and lower frequency limit.

bandwidth Abbr. BW In a general sense, this term describes the information-carrying capacity of a given transmission medium. It is a measurement of how much information can be carried in a given time period (usually a second).  It can apply to analog telephone (POTS), Ethernet networks, digital computer system buses, radio frequency signals, and VGA video signals used to connect projectors and monitors. See our TechNote About Bandwidth for more information.

baud rate (pronounced "bawd"; after Baudot Code named for the French telegrapher Emile Baudot, 1845-1903) The transmitted signaling speed, or keying rate of a modem. Often confused with bit rate. Bit rate and baud rat are NOT synonymous and shall not be interchanged in usage. For example, one baud equals one half dot cycle per second in Morse code, one bit per second in a train of binary signals, and one 3-bit value per second in a train of signals each of which can assume one of 8 different states, and so on - all brought to you by the magic of advanced coding techniques that allow more than one bit per baud. Preferred usage is bit rate, with baud used only when the details of a modem are specified.

Baxandall tone controls The most common form of active bass and treble tone control circuit based upon British engineer P.J. Baxandall's paper "Negative Feedback tone Control -- Independent Variation of Bass and Treble Without Switches," Wireless World, vol. 58, no. 10, October 1952, p. 402. The Baxandall design is distinguished by having very low harmonic distortion due to the use of negative feedback.

BCD 1. (binary-coded decimal) Pertains to a number system where each decimal digit is separately represented by a 4-bit binary code; for example, the decimal number 23 is represented as 0010 0011 (2 = 0010 and 3 = 0011, grouped together as shown), while in straight binary notation, 23 is represented as 10111.

bel Abbr. b, B Ten decibels. [After Alexander Graham Bell.] The Bel was the amount a signal dropped in level over a one-mile distance of telephone wire. See: decibel

Bell, Alexander Graham (1847-1922) Scottish-born American inventor of the telephone. The first demonstration of electrical transmission of speech by his apparatus took place in 1876. Bell also invented the audiometer, an early hearing aid, and improved the phonograph.

Bessel crossover A type of crossover utilizing low-pass filter design characterized by having a linear phase response (or maximally flat phase response), but also a monotonically decreasing passband amplitude response (which means it starts rolling off at DC and continues throughout the passband). Linear phase response (e.g., a linear plot of phase shift vs. frequency produces a straight line) results in constant time-delay (all frequencies within the passband are delayed the same amount). Consequently the value of linear phase is it reproduces a near-perfect step response, i.e., there is no overshoot or ringing resulting from a sudden transition between signal levels. The drawback is a sluggish roll-off rate. For example, for the same circuit complexity a Butterworth response rolls off nearly three times as fast. This circuit is based upon Bessel polynomials; however, the filters whose network functions use these polynomials are correctly called Thompson filters [W.E. Thomson, "Delay Networks Having Maximally Flat Frequency Characteristics," Proc. IEEE, part 3, vol. 96. Nov 1949, pp. 487-490].

binary A condition in which there are two possible states; for example, the binary number system (base-2) using the digits 0 and 1.

bit Abbreviation for binary unit or binary digit. The smallest amount of digital information. A bit can store or represent only two states, 0 and 1. [The orginal term binary unit was coined by John Tukey of Bell Laboratories to represent the basic unit of information as defined by Shannon as a message representing one of two states.]

bit clock The synchronizing signal that indicates the rate of individual data bits over a digital audio interface.

bit error rate The number of bits processed before an erroneous bit is found.

bit rate The rate or frequency at which bits appear in a bit stream. The bit rate of raw data from a CD, for example, is 4.3218 MHz.

bit stream A binary signal without regard to grouping.

bit-mapped display A display in which each pixel's color and intensity data are stored in a separate memory location.

Bluetooth  Early misunderstanding led many people to believe Bluetooth was a wide-area networking solution.  Bluetooth is actually a short-run cable replacement, intended originally to connect a cell phone to a headset and microphone. Bluetooth is a trademark of L M Ericsson in Sweden.  WPAN (Wide Personal Area Network) is a trademark of the IEEE for the same protocol.

BNC (bayonet Neill Concelman, or baby N-connector, or bayonet connector, or bayonet Navy connector) A bayonet-locking connector for slim coaxial cables. What "BNC" truly stands for is still debated: 1) There is an "N-connector," similar to, but larger than the BNC-type, used for fat coaxial cables; 2) Neill Concelman is credited with inventing this connector; 3) It is a bayonet-style; 4) and many claim it was named for the Navy who developed the connector for war-time communication during WWII -- so, take your pick.

Boole, George (1815-1864) British mathematician who devised a new form of algebra that represented logical expressions in a mathematical form now known as Boolean Algebra. [See Maxfield in References]

boost/cut equalizer The most common graphic equalizer. Available with 10 to 31 bands, on 1-octave to 1/3-octave spacing. The flat (0 dB) position locates all sliders at the center of the front panel. Comprised of bandpass filters, all controls start at their center 0 dB position and boost (amplify or make larger) signals by raising the sliders, or cut (attenuate or make smaller) the signal by lowering the sliders on a band-by-band basis. Commonly provide a center-detent feature identifying the 0 dB position. Proponents of boosting in permanent sound systems argue that cut-only use requires adding make-up gain which runs the same risk of reducing system headroom as boosting.

Bps (Bits Per Second) A measure of bandwidth (the amount of data that can flow in a given time).  Bps is defined as the number of bits per second that can be transmitted over the specified data transmission medium.

BRI (Basic Rate Interface) An ISDN access or subscriber line, consisting of two 64Kbps B (bearer) channels and a 16 Kbps D channel. See ISDN

broadband digital link with a broad bandwidth - that is, a broadband link - generally, one that is capable of caring enough information to sustain the succession of images in a video transmission (see Bandwidth). Technically a channel in which a wide band of frequencies is available to transmit information.

buffer In data transmission, a temporary storage location for information being sent or received.

bus An electrical conductor used for transmitting signal or power from one or more sources to one or more destinations.

Butterworth crossover A type of crossover circuit utilizing low-pass filter design characterized by having a maximally flat magnitude response, i.e., no amplitude ripple in the passband. This circuit is based upon Butterworth functions (or Butterworth polynomials).  Named after S. Butterworth, a British engineer who first described this response in his paper "On the Theory of Filter Amplifiers," Wireless Engineer, vol. 7, 1930, pp. 536-541. Eleven years later, V.D. Landon coined the phrase maximally flat in his paper "Cascade Amplifiers with Maximal Flatness," RCA Review, vol. 5, 1941, pp. 347-362.]

byte A group of eight digital bits (a word) operating together to represent one value.


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