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Additive Synthesis

A system for generating audio waveforms or sounds by combining basic waveforms or sampled sounds at different pitches, prior to further processing with filters and envelope shapers. The Hammond tonewheel organ was one of the first additive synthesizers, allowing harmonically complex waveforms to be created by combining tones of different pitches using 'harmonic drawbars'.

ADAT Lightpipe

A widely used eight-channel optical digital audio interface developed by Alesis as a bespoke interface for the company's digital eight-track tape machines in the early 1990s (Alesis Digital Audio Tape). The interface transfers up to eight channels of 24-bit digital audio at base sample rates (44.1 or 48 kHz) via a single fibre-optic cable. This 'lightpipe' and its conenctors are physically identical to that used for the TOSlink optical S/PDIF stereo interface found on many digital consumer hi-fi devices. However, while the light-fibre itself can be used interchangeably for either format, the S/PDIF and ADAT interfaces are not compatible in any other way.

Active Loudspeaker or Monitor

A loudspeaker system in which the input signal is passed to a line-level crossover, the suitably filtered outputs of which feed two (or more) power amplifiers, each connected directly to its own drive unit. The line-level crossover and amplifiers are usually (but not always) built in to the loudspeaker cabinet.

Active

The term typically describes an electronic circuit containing transistors, integrated circuits (ICs), tubes or other devices that require power to operate, and which are capable of amplification. Also used to describe loudspeaker monitoring systems which employ separate amplifiers to power each drive unit individually. 

Acoustic Treatment

A general term embracing a range of products or constructions intended to absorb, diffuse or reflect sound waves in a controlled manner, with the intention of bestowing a room with acceptable reverberation times at all frequencies, and a neutral overall sound character.

Acoustic Foam

An open-celled expanded polyurethane or melamine foam that allows sound waves to enter and flow through the foam to dissipate their energy and thus prevent the sound waves from being reflected. The density and depth of the foam affects the frequency range over which it is effective as an acoustic absorber.

AC

Alternating Current (cf. DC). Audio signals are represented in the electrical domain as currents flowing alternately forward and back in the circuits as an analogue of the compression and rarefaction of acoustic air pressure due to the passing of sound waves.

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