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Now looking at definitions starting with letter "a"

AC bias : See bias.

AC coupling : Coupling between electronic circuits that passes only time-varying signals (i.e., alternating current), not direct current.

AC-1 : A form of ADPCM(Adaptive Delta (Differential) Pulse Code Modulation). It was first used in 1985 for digital radio (sound-only) applications and since adopted for other DBS (direct broadcast satellite) services, including soundtrack-with-video, satellite communication networks, and digital cable radio systems. AC-1 has a data rate between 220 kbps and 325 kbps.

AC-2 : A transform encoding/ decoding scheme for audio compression developed by Dolby labs which uses 256-band transform coding at a data rate of 128 kbps or 192 kbps on two channels. Used in the Dolby Fax System and also DP5xx encoding.

AC-3 : A multichannel, digital, split-band, perceptual coding scheme developed by Dolby Labs. It produces a 5.1 channel format, using lossy compression. Designed to be the matrixing format for DVD and surround-sound with HDTV broadcasts. Versatile, in that parameters such as bit-rate and number of channels can be tailored to particular applications, unique in that AC-3’s data bits are distributed dynamically among the filter bands as needed by the particular frequency spectrum or dynamic nature of the program. Data rates vary from 32 kbps for a single mono channel to as high as 640 kbps for 5.1 format. See Dolby Digital.

AC-M : A newly developed codec based on a soft data compression ratio of between 2:1 and 3:1. Used in the Dolby Digital Dubber, it is designed specifically to record eight tracks of 20-bit material on removable media, including Iomega Jaz and MO drives. AC-M is said to be optimized for multiple record/replay generations. Initial tests have reported as many as 14 codec processes being possible with no audio degradation.

ACA : Active Combining Amplifier. See combining amplifier.

Academy centerline : See optical track.

Academy curve/Academy sound : The name of the standard mono optical track that has exsisted since the beginning of sound on film. Standards were codified in 1938, although the standard has changed somewhat through the years. The standard specifies a flat response throughout the range of 100Hz-1.6 kHz and is down 7dB at 40 Hz, 10dB at 5 kHz, and 18 dB at 8 kHz. Also called an N-Curve. See also X-Curve."

Academy leader : The visual countdown that precedes the first program frame of a motion picture. Symbols and numbers on the academy leader are used for aligning the various film reels and the optical track for composite printing, for aligning the workprint and edited soundtracks for mixing, and for timing the change-over from one reel of film to another during projection. Academy leader contains one number per foot following the Picture Start, with 11, 10, etc., leader to three. (As projected, these numbers appear upside-down.) Named after the Academy of Motion Picture Arts and Sciences, which sets all film format standards. See also leader, SMPTE Universal leader, plastic leader, fill leader, LFOP.

Academy Theater : Specifically, the Samuel Goldwyn Theater at the Academy of Motion Pictures Arts and Sciences on Wilshire Boulevard in Los Angeles, considered the best-sounding theater in the world. Academy members screen films at the Academy Theater prior to voting on them for the Oscar awards.

accelerando : An indication that the tempo of a piece of music should gradually be increased.

acceptance angle : The usable working area in front of a microphone is defined by the polar pattern and is called the acceptance angle.

Accidental : In a musical scale, the accidentals are the extra sharp and flat notes that are not part of the diatonic series. For example, in the key of C on the piano, the accidentals are the black keys.

Acmade : The British manufacturer of edgecoding machines.

acoustic baffle : See baffle.

acoustic feedback : A squealing sound when the output of an audio circuit is fed back in phase into the circuit’s input. See feedback.

acoustic intensity : See sound pressure level.

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 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 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.

active : (1) An audio device which requires a power source such as from an AC line or battery, as opposed to passive. Sometimes amplifying components such as transistors or ICs are called active circuit elements. (2) See MIDI patchbay.

active crossover : See crossover network.

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.

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