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asynchronous : Not according to a fixed rate of repetition. An asynchronous signal can occur at intervals which do not necessarily coincide with a fixed-rate system or master clock pulse.
articulation : The way of characterizing notes (usually in a melody) by the precise control of their individual lengths to produce or eliminate gaps between them. The terms staccato and legato reflect the two extremes of articulation. It is one of the most important ways by which music can be shaped into phrases.
Apple (�) menu : The main menu on a Mac, used to access system utilities (such as Keycaps), applications, files, and control panels. This is the equivalent of the Start menu on a PC-type system.
aperture time errors : In an A/D converter, the sample-and-hold circuit would ideally take zero time to determine the level of the signal waveform and to hold this level until the next sample is called for. However, it takes a finite time to charge the holding capacitor in the sample-and-hold circuit, and this is called the aperture time. Because the time required to establish a new value of charge depends on the amount of change in the signal level from one sample to the next, the aperture time will vary with the rate of change in signal level, increasing for high-level, high-frequency signals. The starting time of the sampling aperture is also slightly uncertain, and this is called jitter. In other words, lack of precision in the sampling time leads to amplitude errors in rapidly changing signals. The errors involve aperture time, uncertainty in aperture time, and jitter. The result is distortion of the sampled signal which rises with frequency.
analog sequencer : A sequencer into which sounds for storage and playback are fed as analog signals, via analog potentiometers.
AM suppression : The ability of an FM tuner or receiver to reject amplitude modulation of the received signal and be sensitive only to frequency modulation. Much of the interference and noise in broadcasting appears as amplitude modulation, so a tuner with good AM suppression will have less distortion and noise than a tuner with poorer suppression. Also called AM rejection.
anacrusis : See beat.
amp/speaker simulator : A filter circuit that mimics the amplifier and loudspeaker voicing of an electric guitar and amplifier system.
Amplitude Modulation (AM) : The instantaneous amplitude modulation of one signal by another. This results in the formation of sidebands which contain the same information as the original signals, but translated upwards and downwards in frequency. In AM radio transmission, the audio signal is combined with a very high-frequency sine wave, called a carrier, in such a way that the amplitude of the carrier is varied in exact response to the amplitude and frequency of the signal. This is called the amplitude modulation of the carrier. The modulated carrier is transmitted at high power where it is received by radio sets that are tuned to the carrier frequency. The modulated carrier is then demodulated by a process called detection, recovering the original signal. In radio, a circuit that does amplitude modulation is also called a heterodyne.
amplitude errors : See frequency response errors, jitter.
amplitude : The relative strength (amount) of a signal, without regard to its frequency content. Amplitude is measured by determining the amount of fluctuation in air pressure (of a sound), voltage (of an electrical signal), or numerical data (in a digital application). When the signal is in the audio range, amplitude is perceived as loudness. Amplitude is the measurement of how much energy the sound has, i.e., the total change in air pressure during a single cycle of the sound wave. Amplitude, or sound pressure, is measured in a scale called decibels (dB). An increase of 3dB is equal to a doubling of a soundís pressure. Amplitude can be expressed as either a negative or positive number, depending on the signals being compared. See also magnitude, SPL.
ambient sound : Sounds such as reverberation, room tone, walla and atmospherics that form a background to the main sound, usually in the context of a film soundtrack of a motion picture, taking place at any given moment. The lack of ambient sound is noticeable because the human hearing system expects it. See also ambient noise.
alternating current (AC) : An electrical current that periodically changes in direction. The rate of alternation is called the frequency and is measured in cycles per second, or Hertz. Audio signals are always alternating, the frequency corresponding to the pitches of the sounds the signals represent. See Appendix B.
air suspension : An acoustic suspension loudspeaker.
aftertouch : A type of MIDI controller data, generated by pressing down on one or more keys after they have reached and are resting on the keybed. Also called pressure. See channel pressure, poly pressure.
AES/EBU null clock : See null clock.
AES/EBU : Audio Engineering Society/European Broadcast Union. A standard for encoding multiple channels of digital audio along a serial cable, officially named AES3-1985. The standard specifiesUsually uses 3-pin XLR jacks and balanced line cables, usually running at +4dBm. Originally designed as a self-clocking system, a subsequent addendum to the specification permitted master clock systems. Two channels of digital audio data are multiplexed on a single conductor within the cable, with a maximum bit depth of 24 bits. Data are transmitted at 64 times the sample rate, allowing the possibility of sending two channels of 24-bit audio (plus ECC) to play the resulting stereo signal in real-time. AES/EBU does not carry the SCMS copy code, and is a self-clocking protocol. It has been adopted by the EIAJ, which calls it the CP-340, Type 1. See also S/PDIF.
adjustable turnover : A variable tone control in a preamplifier which allows the adjustment of the boost/cut and the frequency below or above which the gain/attenuation is applied (turnover), but not the rolloff slope of the shelving equalizer. If it were possible to adjust the rolloff slope, the result would be a fully parametric tone control.
active equalizer : An equalizer that employs active components such as transistors or ICs in its processing circuits. A pre-amplifying circuit generally follows each stage of actual equalization, boosting the signal level to restore unity gain. See also passive equalizer.
acoustic suspension : A loudspeaker designed for, or used in, a sealed enclosure. Typically, a low-frequency loudspeaker baffle where most of the damping of the cone is the result of the elasticity of the air in the sealed cabinet.
acoustics : The science or study of sound and its interaction with the human hearing mechanism.
acoustic lens : A device placed in front of a high-frequency speaker that disperses or directs the sound in a desired pattern. Normally used to increase the angle of dispersion, either horizontally, vertically, or both.
acoustic labyrinth : (1) A type of design for the housing of highly directional microphones that enhances the rejection of off-axis sources. Two or more concentric tubes in front of (and sometimes around) the capsule create a compact series of folded pathways through which all sounds approach the diaphragm. Those arriving on-axis reach the capsule via these paths in phase coherence. Off-axis sounds, due to the different lengths of the passways, reach the diaphragm and are partially or fully removed due to phase cancellation. (2) A type of speaker enclosure in which sound waves emanating from the rear of the woofer cone travel through a long, folded interior path before coupling with the outside. This extends bass response considerably."
acoustic feedback : A squealing sound when the output of an audio circuit is fed back in phase into the circuitís input. See feedback.
acoustic intensity : See sound pressure level.
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