oblique mode. A room mode created by a sound wave reflecting across all six surfaces (four walls, floor, and ceiling) in a room. Oblique modes are roughly half as strong as tangential modes, and one-fourth as strong as axial modes.
octave. 1. A doubling or halving of frequency. 2. A musical interval of eight diatonic steps or twelve equal half-steps.
odd-order harmonic distortion. A type of distortion that results primarily in additional third and fifth harmonics being added to a signal. Odd-order harmonic distortion is often produced when digital devices clip; it tends to be unpleasant-sounding to most listeners.
OEM. Original Equipment Manufacturer. A company that sells products made by another manufacturer under its own name or that uses parts or sub-assemblies from another company to make its own products. The term may also be used to refer to a company that makes products that another company will sell as its own products.
OFC. Oxygen-Free Copper. The more pure, or oxygen-free, copper is, the better it conducts electricity. Some audiophiles and engineers feel this makes OFC a superior choice for audio cables, though it can be substantially more expensive.
off-axis. 1. Positioning a source somewhere other than directly in front of the diaphragm of a directional microphone. Depending on the microphone’s polar pattern, an off-axis position may result in attenuated levels and/or coloration. 2. A position that is not directly in front of a speaker. Depending on the position in the speaker’s dispersion pattern, off-axis positioning may result in sonic coloration. See Figure O.1.
Figure O.1. A sound entering a directional microphone from off-axis (any direction other than directly in the front) will have some degree of coloration.
off line. 1. Inactive. 2. Editing or processing that takes place outside of real time. 3. Not connected to or accessible by a network.
offset. A slight shift in relation to time code that allows devices to synchronize correctly under certain conditions. An offset may move a device’s synchronization either earlier or later in time (positive or negative offset).
Ogg Vorbis. An open source, unpatented audio file format named for the Vorbis data compression scheme used to create the files. (The name “Vorbis” is taken from a name used in Terry Pratchett’s Discworld series of books, while “Ogg” comes from “ogging,” which is computer game jargon.) Ogg Vorbis is part of the Ogg project, whose goal is to create a completely open source multimedia system.
ohm. The unit of measurement for electrical resistance and impedance. The ohm was named for German physicist Georg Ohm.
Ohm’s law. An electrical interrelationship, discovered by German physicist Georg Ohm around 1827, between voltage, current, and resistance. Ohm’s law states that voltage equals current times resistance (V = I × R). The law can be rearranged to find resistance (R = V/I) or current (I = V/R). Basically, this means that as, for example, voltage changes, either resistance or current (or both) must also change, and so on.
OLED. Organic Light Emitting Diode, a.k.a. LEP (Light-Emitting Polymer) or OEL (Organic Electroluminescent). An LED whose luminescent layer is composed of organic material. The big advantage OLEDs have over LCDs (Liquid Crystal Displays) is that they don’t require a backlight, and therefore do not draw as much power and can be made much thinner. The biggest disadvantage is the limited lifespan of the organic materials—about 25% of the lifespan of LCD or LED technologies.
OMF. Open Media Framework, a.k.a. OMFI (Open Media Framework Interchange). A standard file format for exchanging audio and video files between different editing and production programs.
omnidirectional. Literally, in all directions at once. Omnidirectional microphones pick up sound in a spherical pattern, equally well from all directions. Low-frequency speakers tend to be omnidirectional in their dispersion pattern. See Figure O.2.
omni mode. A MIDI channel mode in which a device will respond to messages and data received on any or all MIDI channels.
OMS. Opcode MIDI System, later changed to Open Music System. A Macintosh-based MIDI environment developed by Opcode in 1990. OMS facilitated MIDI communication between hardware devices, computers, and software applications, such as sequencers and editor/librarians. OMS and FreeMIDI, a competing system developed by MOTU, were made obsolete by Core MIDI in Apple’s OS X.
on-axis. 1. Positioning a source directly in front of the diaphragm of a microphone, at 0 degrees in the polar pattern. 2. A position directly in front of a speaker, in the middle of the dispersion pattern.
onboard. Synonym for “built-in,” as in “onboard effects.”
one-off. A single copy of something or copies that are made one at a time. For example, a CD-R (or CD-Rs) burned one at a time from an audio project, as opposed to a mass-produced, commercially replicated CD.
one-shot sample. A digital sample that does not loop. A one-shot sample is triggered, plays back, and stops. Examples would include drum hits, orchestral stabs, short musical phrases, sound effects, and others.
op amp. Short for “operational amplifier.” A type of electronic integrated circuit chip that amplifies the level of signals. Op amps are used in many types of audio equipment.
opcode. 1. Short for “operation code.” A part of machine language that tells the computer what operation is to be performed. 2. Opcode was a MIDI software/hardware company founded in 1985 and acquired by Gibson in 1998. Opcode created popular MIDI software including Vision (sequencer), Studio-Vision (among the first sequencers to add audio recording and editing), Galaxy (universal editor/librarian), OMS (Opcode MIDI System, later Open Music System), and others. By 2000, opcode was defunct.
open. 1. An incomplete electronic circuit with a switch that is not closed or a break at some point. 2. A patch bay that has no normalling between two patch points.
open-air (a.k.a. open back). A type of headphones that do not seal off the wearer’s ears from external sound or prevent internal sound from bleeding to the outside world. Open-air headphones can be more comfortable for long periods of time, but they are not suitable for tracking where isolation is required.
open mic. A microphone that is on and capturing, or available to capture, sound.
OpenSound Control (a.k.a. OSC). A network protocol designed for real-time communication among synthesizers, computers, and multimedia devices and software. OpenSound Control was intended to supplant MIDI; it operates at broadband speeds and allows communication via Ethernet connections or over the Internet.
open source software (a.k.a. OSS). Software that has been created and released under an open source license, which allows users to not only use, but modify and redistribute, the software. Open source software is often programmed by a public group of collaborators and distributed for free via download over the Internet.
operating system. A collection of basic instructions that define the operation of all digital devices, including computers, synthesizers, samplers, cell phones, iPods, and so on. The operating system is the underlying program on top of which all other software runs. Examples for computers include Windows, OS X, Linux, and others.
operator. The equivalent of an oscillator—a module that generates an output signal representing a cyclical waveform—in an FM (frequency modulation) synthesizer.
operator error (a.k.a. PEBKAC). A common cause of equipment maladies. Generally curable by reading the manual.
optical compressor. A compressor design based around an electro-optical element (a combination light source and photocell light sensor). The intensity of the light source is proportional to the input signal level; the photocell detects the light and controls the level of an amplifier to reduce the output level. Threshold, ratio, and other settings are used to specifically set the response of the compressor.
optical media. A disc media, such as CD or DVD, that stores digital information in a format that can be read (and/or written) by an optical device, such as a laser. Optical media has several advantages over similar removable magnetic devices (such as floppy disks and others), including large storage capacities and durability.
option anxiety. A situation faced by many modern technology-savvy musicians and engineers, where the plethora of available options makes reaching a decision very difficult. Whether it is choosing among buying one of 200 different kinds of microphones, selecting which of a multitude of plug-ins to use for a task, or figuring out which DAW to choose as a platform (among many other similar decisions, small and large), the range of workable options is so large that deciding which is the best solution is a serious problem.
opto-isolator (a.k.a. optocoupler, photocoupler). An electronic component that combines an LED light source and a light sensor to pass signal. When current is present, the LED turns on and light is detected by the sensor. Since opto-isolators use light instead of electrical contact, they can isolate a circuit and help prevent ground loops and damage from voltage spikes. According to the MIDI Specification, MIDI inputs must be opto-isolated.
Orange Book. One of a set of “Rainbow Books” with colored covers containing the Sony/Philips-developed specifications for different optical compact disc formats. The Orange Book contains the specifications for CD-R and CD-RW.
organ stop (a.k.a. stop). Technically, the length of the air column in an organ pipe. The organ stop determines the pitch produced by that pipe; there is one pipe per pitch the organ can produce. Today, “organ stop” is used to refer to the control on the organ that calls up a particular setting. The term carried over to tonewheel and other types of organs as well.
ORTF. Office de Radiodiffusion Television Français. A near-coincident stereo microphone technique in which two cardioid microphones are placed 17 centimeters (slightly less than 6-3/4 inches) apart, at an angle of 110 degrees. ORTF provides good mono compatibility and localization and imaging, but does not pick up much room ambience. See Figure O.3.
Figure O.3. The ORTF stereo miking technique uses two cardioid microphones placed 17 cm apart at a 110-degree angle.
OS. 
See operating system.
OSC. See OpenSound Control.
oscillator. An analog circuit or digital algorithm that generates an output signal representing a cyclical waveform.
oscillator sync. A synthesizer function in which a slave oscillator synchronizes the period of its output signal with the period of the output signal from another master oscillator. There are two types: hard sync, in which the slave waveform cycle is retriggered every time the master waveform begins a new cycle, and soft sync, in which the slave waveform is inverted (its direction is reversed) every time the master waveform begins a new cycle.
oscilloscope. A diagnostic device used to display signal waveforms. Oscilloscopes are useful for a variety of measurements as well as for simply viewing and verifying a waveform shape.
OSS disc. Optimal Stereo Sound disc. See Jecklin Disc.
ostinato. A repeating musical pattern. A repeating loop or a repeating pattern played by a synth’s arpeggiator could be considered an ostinato.
OS X. Version 10 of the Macintosh operating system. OS X was a complete break from the technologies used for previous Mac operating systems because it was the first Mac system based on UNIX.
outboard gear. Hardware processing equipment used to supplement the capabilities in a mixer or DAW. Outboard gear can include microphone preamps, compressors, limiters, EQs, summing boxes, effects processors, and more.
out of phase. A relationship in time of two sound waves of the same frequency, where the peaks and troughs in the waveforms don’t line up with one another perfectly. If two identical signals are 180 degrees out of phase, the highest peaks in one signal exactly line up with the lowest troughs in the other signal, and the two will cancel each other out completely, resulting in silence. More often two signals are less than 180 degrees out of phase, resulting in partial cancellation and a filtered/hollow tonality. See Figure O.4.
Figure O.4. If two identical signals arrive at slightly different times, there will be phase cancellation problems. Note how different the bottom waveform, which was created by summing the top and middle waveforms, is from the original two waveforms—this is caused by the middle waveform being very slightly out of phase with the top waveform.
output. A connection or jack used to send signal out of a device.
output transformer. A transformer used to match the output of a device to the device it will be feeding. See also transformer.
over. An over is a signal that exceeds 0 dBFS for a certain number of samples in a row. Since a digital system cannot represent a signal higher than 0, the waveform will not be accurately represented. An over is the digital equivalent of clipping an analog signal. A few overs may not be audible or may not create a problem in a track during production, but overs are a problem if they become audible as distortion. Even a single over can be problematic in a final mastered track that is being submitted for duplication to CD.
overdrive. A distortion effect created by increasing the level of a signal until it clips or distorts.
overdub. 1. The process of recording additional tracks over existing basic tracks in order to finish a production. For example, the basic tracks might consist of the rhythm section recorded at one session, all in one pass. Vocals, solos, and other tracks might be overdubbed one at a time at a later session. 2. An additional track that has been added to existing tracks.
over easy. A trademarked term used by dbx for soft-knee compression.
overhead microphones. See overheads.
overheads. One, two, or more microphones positioned over a drum kit. Overheads are intended to capture cymbals as well as a complete picture of the drum kit along with a certain amount of room ambience. Overheads are also often used with small ensembles, such as string quartets.
overload. A condition in which the maximum level a device can handle has been exceeded. Overload usually results in distortion. See also clipping.
oversampling. A sampling and signal-processing technique in which the sample rate is multiplied to a very high rate (anywhere from 4× to 256× or even higher). The biggest reason to oversample is to make it easier to design anti-aliasing filters. Once sampled, the signal is digitally filtered and downsampled to the desired rate. In some cases, certain types of noise may also be reduced.
overtone. Tones within a sound produced by a source that are higher than, and accompany, the fundamental tone. Overtones may or may not be part of the harmonic series of the fundamental, depending on their mathematical relationship with the fundamental. (Overtones must be integer multiples of the fundamental to qualify as harmonics.)
overview. A window or view in a DAW, sequencer, or audio program that displays the entire track or project. This is useful for finding a location within a track.
overwrite. To store new data that erases old data. In destructive recording and editing, new files are created that replace existing files. See also destructive editing, destructive recording.
Oxford 911 Bridge (a.k.a. OXFW911). A high-performance FireWire “bridge” chip that allows an IDE drive to communicate with a FireWire 400 bus at high speeds. The Oxford chip is part of the electronics in an external FireWire drive enclosure.
Oxford 922 Bridge (a.k.a. OXFW922). A high-performance FireWire “bridge” chip for use with a FireWire 800 bus. The Oxford chip is part of the electronics in an external FireWire drive enclosure.
oxide. An oxide is a compound created when a metal oxidizes or combines with oxygen—in common language, it rusts. Different formulations of oxide are used as magnetic materials to coat recording tapes and hard disk platters.