a cappella. 1. Vocal music without instrumental accompaniment. 2. In remixing, the raw lead vocal track from the original multitrack recording that is used as the basis for the new arrangement.

A/D. See analog-to-digital converter.

A/D/A. Analog-to-digital-to-analog conversion.

A-440. The note A, tuned to 440 Hz. A-440 is the standard tuning pitch for most Western music.

AAC (a.k.a. MPEG-2 Part 7 and MPEG-4 Part 3). Advanced Audio Coding, a lossy digital audio data compression and encoding system developed by the MPEG group, as the basis of MPEG-4 and other Internet, wireless, and digital broadcast systems, and intended as the successor to MP3. Claimed improvements over MP3 include more (and higher) sample frequencies, up to 48 channels, increased coding efficiency, improved high-frequency response over 16 kHz, and more. A wide variety of hardware and software devices support AAC, most notably Apple’s iTunes and iPods.

A-B comparison. Switching between two audio sources or pieces of equipment to compare a parameter setting, component, or sound quality. The best A-B comparisons are “blind,” where the listener does not know the identity of the sources until after the test is concluded. The test results are better if the two sources can be switched instantly.

A-B repeat. Function on hardware or software audio recorders/players that cycles or loops a section of a track.

A-B stereo. A stereo microphone technique that uses two identical omnidirectional microphones placed some distance apart. A-B stereo miking is often used when the sound source is large and/or when the distance from the microphones to the source is great. (Omnidirectional microphones are used to maintain consistent low-frequency response regardless of distance.) Careful mic placement will result in a wide stereo field with a good balance of room ambience and direct source signal. See also spaced omni.

ABS. See absolute time code.

absolute phase. More accurately termed absolute polarity, absolute phase describes a situation in which the phase remains constant throughout a signal path compared to a reference. In practice, absolute phase means that the + and – polarity of a system are maintained from beginning to end (see Figure A.1). For example, the leading edge of a sound wave usually causes a positive voltage in a microphone, and this voltage remains positive through all the connections and components in the audio system until it reaches the loudspeaker, where it produces a positive (forward) motion in the speaker diaphragm. Some listeners feel that absolute phase is essential for the best sound quality.

Figure A.1. Positive and negative polarity are maintained throughout a system with absolute phase.

A-B-X test. A method of comparing a change in a component or sound quality. The listener is given three sources. A and B are the original source and the original source with some modification to the signal path or sound quality. The third source, X, is the same as either A or B. An assistant selects among the sources while the listener attempts to determine which two are the same and which one is different. The test is repeated enough times to ensure non-random results.

A-weighting. Using a filter to reduce certain frequencies when measuring sound in order to obtain results that match better with the frequency response of our ears. Though A-weighting has many legitimate uses, some manufacturers use A-weighting when making measurements for the specifications for their gear in order to disguise poor performance.

absolute time code (a.k.a ATC). A specific type of time code that is automatically encoded into the subcode area of a digital tape machine. The machine uses this time code for auto-locating and for inter-machine synchronization. Many digital tape machines allow the ATC to be converted to SMPTE for other applications. Unlike SMPTE and other types of time code, ATC always starts at zero at the beginning of the tape.

absorber. Acoustical device that reduces the level of sound waves by converting sound energy into heat.

absorption. In acoustics, using “soft” material to reduce the intensity of sound waves. Absorptive materials convert sonic energy into heat in order to reduce sound energy and level.

accent. Emphasis placed on a specific musical note. Repeated accents at regular rhythmic intervals are a component of “groove” or “feel.”

access time. The amount of time it takes for a hard disk or optical disc to reach and read a requested data sector. There are several factors that determine access time, including rotational delay, transfer time, and seek time. Lower access times are always better, especially for audio and video applications in which a large amount of data must be accessed rapidly.

accurate. Uncolored and undistorted reproduction, meaning the output signal for a piece of gear is a faithful version of the input.

acetate. A test disc pressing used to determine how a mastered recording will sound when transferred to a vinyl phonograph record.

acoustic coefficient. Rating for the performance of an acoustical material at a particular frequency. Values range from 0 (totally reflective) to 1 (totally absorptive).

acoustic foam. A special type of open-cell foam designed to absorb sound waves.

acoustic suspension. In a sealed-cabinet loudspeaker design, the air trapped in the enclosure serves as acoustic suspension—as a “spring” that returns the speaker driver to its neutral position.

acoustic treatment. Acoustic devices and materials installed in a space to control the behavior of sound. There are three types: absorbers, bass traps, and diffusors.

acoustics. The study of sound, or the behavior of sound, within an enclosed space.

action. The playability of a keyboard or other musical instrument. For keyboards, action refers to how a keyboard works mechanically and how much pressure it takes to press a key. Some types include:

active. The opposite of passive. An active device has its own power source and can amplify the signal. The advantage is that signal loss due to processing or other factors can be corrected. The disadvantage is that the amplifier components will add noise, distortion, and coloration to the signal to some degree compared to a passive device.

active monitor. Type of self-contained studio speaker with an amplifier and other electronics built into the speaker cabinet itself (see Figure A.2). Active monitors have some advantages over passive designs: The electronics and amplification can be specifically tailored to the speakers and cabinet used. The speaker-level cables are of minimal length. Because no external amps, crossovers, and other electronics are required, active monitors can be compact, portable, and easy to place, and they generally are more convenient. However, some listeners prefer to match their own electronics to the speaker, and others feel that performance may be compromised in order to fit everything within the space constraints of the cabinet. Others feel that the amplifier and its performance may suffer from the constant vibration due to proximity to the speaker drivers.

Figure A.2. Active monitors feature built-in electronics including amplifiers, crossovers, and sometimes tone controls.

active sensing. A MIDI system “safety” message automatically sent to indicate that a device is online. Active sensing messages are sent by a piece of gear when it has been sitting idle for 300 ms or so, to let connected devices know that it is still there and that there is no danger of stuck notes due to a broken MIDI connection.

adaptive noise reduction. Technology designed to “intelligently” detect and reduce or remove noise from a signal or environment.

ADAT. Alesis Digital Audio Tape. A groundbreaking modular multitrack digital audio tape/recorder format developed by Alesis in the early 1990s. Arguably responsible in large part for the price versus technology revolution that led to the rise of affordable high-quality home and project studios. ADAT tape machines could record eight tracks with 16-bit resolution (later generations supported 20-bit resolution) to S-VHS videotapes. The native sample rate for ADATs was 48 kHz, though varispeed could be used to achieve a 44.1-kHz sample rate. Multiple ADAT machines could be linked together for increased track count, resulting in the MDM (modular digital multitrack) system.

ADAT Optical. Format developed by Alesis for transferring eight tracks of digital audio over a single optical cable using TOSLink connections that has become an industry-standard I/O connection. Companies such as Apogee have developed extensions to the basic ADAT optical format that support higher resolutions and sample rates (though at the expense of reduced track counts). See also lightpipe.

ADB. Apple Desktop Bus, an older format and connection type used for connecting peripherals, such as a mouse, keyboard, and trackball, to Macintosh computers. ADB used a four-pin cable that was similar to an S-Video cable. ADB has been superseded by USB.

ADC. See analog-to-digital converter.

additive synthesis. Type of synthesis based on the work of French mathematician Joseph Fourier, whose efforts showed that a sound wave could be broken down into a series of component partials or overtones. Additive synthesis creates a sound by combining sine waves with different frequencies, envelopes, and amplitudes to construct a more complex waveform. Additive synths are typically digital, though pipe organs and tonewheel organs, which have different stops to represent harmonics, could also be considered additive.

ADR. Automatic Dialog Replacement or Additional Dialog Recording. A process where an actor over-dubs new lines to replace the voice recorded during filming or videotaping.

ADSR. An abbreviation for the attack/decay/sustain/release parameters used in synthesizer or sampler envelope generators (see Figure A.3). An ADSR module is a four-stage processor that can be used to control the volume envelope of a note or to modulate another processor, such as a filter. The attack, decay, and release parameters are rate or time controls, while sustain sets volume level.

Figure A.3. An ADSR has four stages—attack, decay, sustain, and release—that can be used to control synthesizer or sampler parameters, such as a sound’s volume envelope.

ADT. See automatic double tracking.

AES. Audio Engineering Society. An international professional society dedicated to audio technology. Its committees have been responsible for developing several important industry standards. www.aes.org.

AES/EBU. An abbreviation for Audio Engineering Society/European Broadcast Union. AES/EBU is a protocol jointly developed by the two organizations for routing two-channel digital audio signals between various pieces of equipment. Most typically, AES/EBU signals are carried on 110-ohm shielded twisted-pair balanced cables with XLR connectors that resemble standard microphone cables. (Mic cables may be used to carry AES/EBU, but purists prefer true digital cables.) AES/EBU can be run over distances up to 100 meters with resolution up to 24 bits.

AES3. See AES/EBU.

AFL. After Fade Listen. A switchable function on mixing consoles used to monitor a channel’s signal after the volume fader. (The pan control is also sometimes included.) This allows the engineer to listen to the signal by itself, without hearing any of the other channels. See also solo, solo in place.

AFM. American Federation of Musicians. A union representing professional musicians. www.afm.org.

aftertouch. Pressure changes applied to a keyboard’s key after the note has been struck, while the note it plays is sustaining. Aftertouch is carried as a MIDI channel message and has a value ranging from 0 (no aftertouch) to 127 (full aftertouch). Aftertouch messages may be routed to control any parameter that a receiving device allows. Common destination parameters include vibrato amount and volume level (allowing, for example, horn or string patches to swell in volume as a note is held). See also monophonic after-touch, polyphonic aftertouch.

AFTRA. American Federation of Television and Radio Artists. A union representing artists involved in news and broadcast, entertainment programming, recording, and other media. www.aftra.org.

AGC. Automatic Gain Control. A circuit that automatically adjusts the level of audio at the input of a device. AGC functions in similar fashion to a compressor or limiter (and may be known as limiting in certain devices). AGC is most often included on inexpensive recorders or on devices used for easy remote recording.

aggregate device. A feature of Apple’s Mac OS X that allows multiple audio interfaces connected to the computer to appear as one large audio interface to software applications. The interfaces can include any combination of USB, FireWire, and internal devices.

AHDSFR. Attack/hold/decay/sustain/fade/release. An envelope generator that adds a hold time stage between the attack and decay stages and a fade time stage that occurs between the sustain and release stages (see Figure A.4). See also ADSR, AHDSR, envelope generator.

Figure A.4. An AHDSFR adds hold and fade stages to the standard ADSR envelope.

AHDSR. Attack/hold/decay/sustain/release. An envelope generator that adds a hold stage between the attack and decay stages that maintains the peak attack level for a specified amount of time (see Figure A.5). See also ADSR, envelope generator.

Figure A.5. An AHDSR adds a fifth hold stage to the standard ADSR envelope.

AIFF. Audio Interchange File Format. A digital audio file specification created in 1985 that allows different applications and platforms to share audio files. Most digital audio software, as will some hardware recorders and synthesizers, will create, save, export, and import AIFF files.

AIT. Advanced Intelligent Tape. A special eight-millimeter tape format developed by Sony for large-capacity, high-speed data storage and backup.

algorithm. A mathematical procedure for calculating a specified result. Algorithms are the basis for all processing and operations in audio software.

alias. 1. In the Mac operating system, an icon that points to another file. (Known as a “shortcut” in the Windows operating system.) Double-clicking an alias finds the file the alias points at and launches it. Aliases are convenient for organizing files and folders. 2. A false frequency created when the frequency of audio being sampled exceeds the Nyquist frequency. See also aliasing.

aliasing. Distortion that results from audio frequencies exceeding the Nyquist frequency during sampling (see Figure A.6). A frequency higher than the Nyquist frequency is folded over into the audible range. For example, if a frequency of 25 kHz is sampled by a system with a Nyquist frequency of 20 kHz, the 25-kHz signal will “fold over” around the Nyquist frequency, resulting in a 15-kHz alias frequency. Most samplers have anti-aliasing filters to prevent this problem, though aliasing can also be a problem when a sample is stretched too far from its base pitch. (For a visual equivalent, think of how a wagon or car’s wheels seem to spin backward in a movie. The film’s frame rate isn’t high enough to capture the speed at which the wheels spin.)

Figure A.6. Aliasing results when frequencies higher than the Nyquist frequency are sampled digitally. The frequency above the Nyquist frequency “folds over” back into the audio range. When the original frequency (top) is sampled at too low of a rate (middle), the result will be an “artificial” lower frequency that does not exist in the original signal.

all-button mode. An undocumented “feature” in UREI (later Universal Audio) 1176 compressor/limiters. The 1176 has four preset buttons selecting ratios at 4:1, 8:1, 12:1, and 20:1. When all four buttons are pressed simultaneously, the ratio changes to between 12:1 and 20:1, and ultra-fast attack and release times create a punchy, overdriven tone that results in intense, in-your-face sounds (see Figure A.7).

Figure A.7. Pressing all four ratio preset buttons on an 1176-style compressor/limiter, such as this plug-in re-creation, results in a unique, punchy, overdriven sound.

all notes off. 1. A MIDI system exclusive command that tells a receiving device to turn off all notes. 2. A “panic button” command found in some synths and MIDI software that sends a note-off message for each of the 128 notes on the 16 MIDI channels.

all tube. A circuit that uses tubes as signal path components wherever possible.

alnico. A magnetic alloy created by combining aluminum, nickel, and cobalt. Various formulations of alnico are used by electric guitar manufacturers for pickups, and alnico has been used in speaker construction for many decades.

alternate MIDI controller. Any MIDI controller that isn’t a keyboard. Examples include MIDI guitar, electronic drums, wind controllers, and others.

alternating current. Electric current that switches directions. In the United States, our homes run on AC that changes polarity 60 times per second (60 Hz). Audio signals are also a form of alternating current, with the positive/negative alternation corresponding to the frequency of the sound waves.

AM. See amplitude modulation.

ambiance. See ambience.

ambience. 1. Sense of space. 2. The acoustical character of a space.

Amen break. An oft-sampled four-bar drum loop taken from the song “Amen, Brother,” recorded by The Winstons in 1965.

amp. 1. Short for ampere, named for André-Marie Ampère, a French physicist and mathematician who explored the link between electricity and magnetism. A unit of electrical current flow. One amp equals 6.24 × 10¹⁸ electrons per second. 2. Short for amplifier.

amperage. A measure of the number of electrons flowing past a certain point in a second.

amplifier. 1. Electronic device used to increase input signals to a higher amplitude for driving a speaker or other application. Although the most recognizable amplifiers are the common power amplifier and instrument (guitar, bass, keyboard) amplifier, there are amplifiers contained within many other devices, such as synthesizers and samplers. See also Class A, Class A/B, Class D. 2. An electronic component that increases the gain of a signal—for example, “op amp” (operational amplifier).

amplitude. 1. Size or magnitude. 2. The strength or sound pressure level of a signal measured in decibels.

amplitude modulation. A synthesis technique where a carrier wave with fixed frequency has its level (amplitude) modulated by a second wave. If the modulating wave has its frequency in the lower audio range, the modulation will be heard as tremolo (pulsating volume level). At higher modulator frequencies, the result will be sum and difference sidebands in addition to the carrier wave. If only the sum and difference tones are output, the process is called ring modulation, and it produces non-harmonic, metallic sounds, or when used to process a vocal as the carrier, it produces “robotic” un-pitched voices. (A great example is the “robot” voices in Battlestar Galactica.)

analog modeling. Using digital algorithms to re-create the characteristics of analog devices.

analog sequencer. A module in a synthesizer that plays back a series of voltages, usually set manually using sliders or controlled knobs. This sequence of voltage settings can be used to control the pitch of a VCO (voltage-controlled oscillator), the cutoff frequency of a VCF (voltage-controlled filter), or any other voltage-controllable parameter.

analog synthesizer. A type of synthesizer that creates and controls sounds using electrical voltages. An analog oscillator is used to generate one of several available waveforms; sine, triangle, sawtooth, and square waves are common options. Filters are used to remove various frequencies from these waveforms. There are a variety of types of modules or stages used in analog synthesizers for creating, processing, and controlling voltages: voltage-controlled oscillators, noise generators, voltage-controlled filters, voltage-controlled amplifiers, low-frequency oscillators, envelope and transient generators, analog sequencers, mixers, ring modulators, and more. Later generations of analog synths added digital/microprocessor control capability, which allowed for saving and recalling presets and MIDI control.

analog. A representation of a signal or information with another continuously varying medium. For example, in audio, using voltages to represent changes in sound pressure. Other analogs include the grooves in a vinyl record and the changes in magnetism on a magnetic tape.

analog-to-digital converter. An electronic device that measures the analog voltage representing an audio signal and converts it into a digital representation of the signal.

anechoic. 1. Totally dead acoustically, literally without echoes. 2. A space that supports no reflection of sound waves. The closest thing to an anechoic situation occurring in earthbound nature is a large, open meadow, but even there, the ground below could create some reflection.

anechoic chamber. A specially treated room designed to be totally absorptive at all frequencies. Anechoic chambers are used for testing and measurement of specifications, not for recording or listening to music.

ANSI. American National Standards Institute. A nonprofit organization that promotes standards and facilitates development of new standards.

anti-aliasing filter. A filter in an analog-to-digital converter that removes frequencies above the Nyquist frequency to prevent aliasing. In some devices, this may be a two-stage process, with an analog filter operating on the oversampled signal, then a digital filter operating after the signal has been downsampled to the actual sample rate.

anti-imaging filter (a.k.a. reconstruction filter). A filter in a digital-to-analog converter used to remove unwanted high frequencies and noise generated by the stair-step waveform resulting from converting a digital representation of a signal into an analog representation.

anti-node. A position along a sound wave at which there is maximum motion, or in a standing wave where there is maximum amplitude. See also node.

aperiodic. A non-repetitive event happening at irregular intervals. An aperiodic waveform would not have a pitch, since it doesn’t vibrate in a periodic fashion.

API. Application Programming Interface. An application or tools or templates that help programmers create programs that work in a particular operating system. There are also APIs for creating software that runs within another application—for example, the VST plug-in API published by Steinberg.

application. A computer program.

APRS. Association of Professional Recording Services. A British trade association for the sound recording, sound for picture, and music industries. www.aprs.co.uk.

architecture. The components and configuration that make up a piece of hardware or software. The architecture determines what a device is capable of doing, how signals get in and out, and so on.

archive. 1. To back up data for long-term storage. 2. A collection of data that is no longer actively used, but is being stored in case of future need. 3. A data-compressed version of a file, such as a ZIP file.

arpeggiator. A module or control processor provided in some synthesizers that creates an arpeggio based on the chord the user is playing, using either digital control or analog voltages. Arpeggiators can create arpeggios ranging in complexity from basic ascending or descending arpeggiated chord notes to very complex note patterns.

articulation. A musical nuance or expression, such as a slur or legato or staccato performance. Sample libraries attempt to provide examples of as many articulations as possible. For example, a library might include string articulations, such as marcato, spiccato, tremolo bowing, and pizzicato.

artifacts. Extraneous frequencies, noises, or distortions added to a sound by recording, editing, or processing.

ASCAP. American Society of Composers, Authors, and Publishers. A nonprofit organization that licenses copyrighted works and collects and distributes royalties for composers, songwriters, and music publishers. www.ascap.com.

ASCII. American Standard Code for Information Interchange. A standard developed by ANSI that defines how computers deal with text data.

ASIC. Application-Specific Integrated Circuit. A custom integrated circuit chip designed to provide a function not available using “generic” ICs.

ASIO. Audio Stream Input/Output. A driver protocol developed by Steinberg that provides audio software applications with low-latency multichannel access to audio interface I/O, as well as synchronization between audio, MIDI, and video. ASIO supports both the Mac (OS 9 or earlier) and Windows platforms. ASIO bypasses the operating system audio functions to provide direct, high-speed communication.

asperity. An imperfection or roughness in the surface of a magnetic tape. Enough asperities will result in asperity noise, a low-frequency rumble.

asynchronous. Not synchronized, or not happening at regular intervals. In computer communications, this means that a start and stop bit are used between messages, rather than specific synchronized timing.

ATA. Advanced Technology Attachment (a.k.a. PATA, Parallel Advanced Technology Attachment). A computer protocol introduced in 1986 that specifies how the motherboard interfaces with the disk controller and hard drives. ATAPI (Advanced Technology Attachment with Packet Interface) was later developed, which allowed ATA to support other types of devices. The ATA bus is a parallel protocol that supports master and slave devices. There are several different variations on the ATA protocol:

ATAPI. Advanced Technology Attachment Packet Interface. A computer protocol similar to IDE that provides control over optical and tape drives.

ATC. See absolute time code.

ATRAC. Adaptive Transform Acoustic Coding. An audio data compression system developed by Sony for reducing the size of audio files, which works on the premise that low-level audio frequency components will be masked by louder frequencies, and therefore can be removed from the signal.

attack. 1. The beginning portion of a sound. 2. In a compressor, the amount of time between the instant the signal crosses the threshold and when compression begins to occur. With the proper attack setting, the compressor can ignore attack transients while still compressing the rest of the signal. 3. In a noise gate, the amount of time it takes for the gate to open after the signal has crossed the threshold. Very fast settings (10–50 microseconds) allow transients to pass; slow settings (up to one second) can be used to change the attack portion of a signal’s envelope. 4. In a synthesizer or sampler, the time it takes for a note to reach full volume. An instrument such as a piano, snare drum, or guitar has a fast attack, where a string section might have a slow, swelling attack. See Figure A.8.

Figure A.8. Attack refers to the beginning of an event, such as the start of a sound wave or synthesizer note, or where a change in a processor’s response occurs.

attack time. The time it takes for a signal or sound to go from silence to maximum level. The character and amount of any subsequent room ambience or reverb can be influenced by the attack time of a signal.

attenuate. To reduce the level of a signal.

attenuation. Reduction in signal level.

AU. See Audio Units.

audio. 1. Sound. 2. Electrical signal representing sound.

audio interface. A device that provides audio input and output for a computer. Its features may include analog inputs and/or outputs (with or without analog-to-digital and/or digital-to-analog conversion), digital inputs and outputs, monitor connections and control, built-in effects or dynamics processing, built-in DSP for reducing or eliminating monitor latency, and clock and/or synchronization inputs and outputs. Audio interfaces are available in a variety of formats, including expansion cards that mount in internal computer slots, external boxes that connect to the computer via USB or FireWire, or combinations of expansion cards mounted in the computer that connect to external input/output boxes.

audio restoration. The process of removing hiss, hum, clicks, pops, and other unwanted noises from an audio recording to restore it to its original sound quality or to enhance it to modern standards.

audio taper pot. A potentiometer that uses a logarithmic curve when attenuating signal. The logarithmic curve more accurately corresponds to how our ears respond to sound level changes, so the volume change using an audio taper pot is perceived as linear by our ears.

Audio Units (a.k.a. AU). A real-time native plug-in format developed by Apple Computer and used in Mac-based Core Audio–compatible host audio software operating under OS X, such as Apple Logic Studio, Ableton Live, BIAS Peak, MOTU Digital Performer, GarageBand, and others. The Audio Units format supports both processing and virtual instrument plug-ins.

audiology. The study of human hearing and perception.

AudioSuite. A non-real-time native plug-in format developed for Pro Tools by Digidesign. Since AudioSuite plug-ins don’t operate in real time, they place no load on the computer during playback or recording.

Audiowire. A proprietary protocol developed by MOTU for connecting the company’s external audio interfaces to an expansion card mounted in a computer using FireWire-style cable.

author. 1. To assemble and program a CD or DVD. 2. The person who writes a book, such as a music technology glossary.

authorization. A code that allows for the legal use of a piece of software.

authorize. To enable a piece of software for legal use.

auto accompaniment. Software or hardware that can automatically play backing tracks for a song or piece of music. Auto accompaniment can range from basic drum beats in a home organ to full arrangements played by a keyboard, standalone device, or piece of software. In some cases, auto accompaniment can even respond “intelligently” to what the user is playing.

auto input monitoring. A feature on recorders and DAW software that automatically switches a record-enabled track to monitor the audio coming into the input when the recorder is put into record mode, a punch-in takes place, or the transport is stopped. When the software or hardware is playing back, the audio recorded on the track is heard.

auto-locate. To recall a specific time point in a recording and automatically rewind or fast-forward the recorder to that point.

auto-locator. A hardware device that provides remote control over various functions of a recorder, including storing time locations within the recording. These locations can be recalled later, causing the recording to rewind or fast-forward to that spot.

auto punch. A feature of some recorders and DAW software that automatically drops record-enabled tracks in and out of record mode when certain time locations are reached. The user sets in and out points that cause the system to begin and stop recording. This is much easier to do than on older systems, where the recording engineer had to manually “punch” the Record button at the appropriate time, then punch the Play button again at the appropriate time to take the recorder out of record mode.

auto save. A feature of some software that automatically saves the open file to disk at regular time intervals.

automatic double tracking. (a.k.a. artificial double tracking). A system, said to be developed for The Beatles for the Revolver album, that was used to duplicate the sound of double tracking a vocal—overdubbing a second pass of a vocal or instrument part. In modern studios, an analog or digital processor is used to delay the original signal by a few milliseconds; the delayed version is then mixed with the dry signal. Since the delay is so short, the effect resembles the sound of a vocalist singing along with himself in near unison.

automatic mixer. A type of audio mixer that can detect signal and automatically turn off channels when no signal is passing through them. Automatic mixers are used to help prevent feedback and background noise pickup in churches, audio for video, and broadcast applications.

automation. Using computer control to record changes to switch positions, fader moves, and other control statuses, and to play them back as an audio recording is played. Automation allows accurate, repeatable, and recallable control over a mixer, processor, or performance.

automation curve. A string of automation data entered into a track to control a parameter, such as volume, pan, and others (see Figure A.9).

Figure A.9. An automation curve represents the automation data recorded or entered into a track. Here we see a curve that will control the track’s volume fader.

automation modes. Different ways that automation data can be entered or modified in real time, using a control surface or moving fader system. Typical automation modes include:

autopan. A type of effect that automatically moves a signal back and forth across the stereo field under control of an LFO.

auto-wah (a.k.a. envelope filter). A type of filter effect used with guitars, basses, and other instruments that resembles a wah-wah pedal, except that the cutoff frequency of the bandpass filter is controlled by the level of the incoming signal rather than a control pedal. In some cases, the filter cutoff frequency may be controlled by an LFO.

aux. See aux send.

aux bus. See aux send.

aux send. Short for auxiliary send. A bus in a mixer that creates a separate mix, independent of the main signal path and mix. Aux sends are often used to send a mix to an effect such as reverb or to headphones or a monitor system. See Figure A.10.

Figure A.10. An aux send splits off signal from a mixer’s main signal path and routes it to another destination, such as headphones or an effect processor.

aux track. A track that is used as a destination for an aux send or as an aux return for internal or external effects processors in a DAW. Aux tracks are also used as virtual instrument tracks in some DAWs.

auxiliary send. See aux send.

average level. The overall average volume level of a signal at a given point in time. The average will fluctuate in response to peaks and dips in the level. One function of compressors and other dynamics processors is to reduce the peaks in a signal so that the average level can be increased.

AVI. Audio Video Interleaved. A Microsoft-developed format for computer multimedia files containing interleaved audio and video data.

AWG. American Wire Gauge. A standard for indicating wire diameter.

AWM. Advanced Wave Memory. A type of synthesis developed by Yamaha that uses sampled sounds as the raw material, which is then processed by filters, modulation, envelopes, and other tools.

ax/axe. Slang for a musical instrument.

axial mode. A room mode caused by sound reflecting between two parallel surfaces (see Figure A.11).

Figure A.11. A sound reflecting between two parallel room surfaces creates an axial mode.

axial room mode. See axial mode.