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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.

amplifier gain : The amount of amplification that an amplifier provides is called its gain. The gain is a ratio of the input signal level to the output signal level and is simply a number. Commonly expressed in dBindB, one should not express the voltage gain of an amplifier in dBindB unless the input and output impedances are matched as the gain of a typical amplifier is not related to its power output capability. For instance, if an amplifier has a voltage gain of 10, it might be said that it has a gain of 20dB because it actually would raise the power level of a signal by 20dB if the input and output impedances were matched. In practice, however, this is very seldom the case, and the true power gain is usually very much different from what would be predicted by the voltage gain. See impedance matching.

amplifier : An electrical circuit or device designed to increase the current, voltage, or power of an applied signal. An amplifier is an active device and, strictly speaking, should always increase the power of a signal; some amplifiers, such as certain distribution amplifiers, may only reduce the impedance level of the signal for the purpose of driving long lines.

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.

all-pass filter : See all-pass network.

all-pass network : An all-pass network, also called an all-pass filter, is an electrical circuit with a uniform amplitude response versus frequency response, but with a phase-shift which does not vary in a linear relationship with frequency. (A pure time-delay device such as a digital delay line will have a phase-shift which is directly proportional to frequency, i.e., its phase-shift increases at a constant rate with frequency.) Complex filters often have significant phase distortion because they are not phase linear, and an all-pass network can be designed to correct phase anomalies without affecting the amplitude response.

allegro : A lively to reasonably fast tempo: 116-150 bpm. Allegretto is a slightly slower tempo than allegro.

All-Notes-Off : A MIDI command, recognized by some but not all synthesizers and sound modules, that causes any notes that are currently sounding to be shut off. The panic button on a synth or sequencer usually transmits All-Notes-Off messages on all 16 MIDI channels.

alignment recording : See biased noise.

alla breve : A term historically related to mediśval note lengths, in which the breve was one of the shortest notes. In modern usage, the term is usually used to denote 2/2 (cut-time). In commercial and popular music, it is frequently used to mean half-time, i.e., play twice as fast. See time signature.

alignment : (1) In tape recording, the process of adjusting all parameters of the position and orientaton of the tape heads and guides with respect to the tape path. See azimuth. (2) The adjustment or calibration of any parameter of an electronic circuit or device, e.g., program level, bias level, to bring this parameter into conformance with an industry standard. (3) The process of matching mixer and recorder meters so that only one meter needs to be watched during recording. When the mixer and (analog) recorder are both peaking about 0VU, this minimizes the noise and distortion in both units. Ideally, both units would be matched with a steady tone (the C or B two octaves above middle-C, or about 2kHz, for example.) See line-up tone.

aliasing : Distortion that is produced when higher harmonic components of the input audio signal sampled by a digital recording device, or generated within a digital sound source, lie above the Nyquist frequency. This happens when the sampling rate is less than twice the frequency of the signal being sampled. The effects of aliasing differ from some other types of distortion in that its pitch changes radically when the pitch of the intended sound changes. Also called foldover. See anti-aliasing filter.

airline version : A remixed and possibly re-edited version of a film that has any objectionable material removed. The airline film standard is more stringent even than those of the broadcast networks, and is often used as a benchmark for TV viewing.

algorithmic composition : A type of composition in which the large outlines of the piece, or the procedures to be used in generating it, are determined by the human composer while some details, such as pitches or rhythms, are created by a computer program.

alias : A file on a Mac that serves as a pointer to another file. The most common use for an alias is to sit on a desktop or in a top-level folder, where the real document or application file is nested deep within the file system. This is similar to a shortcut file on a PC-type computer.

after-fader listen : (AFL) On many recording and mixing consoles, there is an option labeled AFL, or after-fader listen. This allows the listener to hear the audio after the channel fader has effected the audio signal. The AFL is also known as post-fader listen. The oposite of AFL is pre-fader listen.

AES/EBU null clock : See null clock.

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 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 baffle : See baffle.

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

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