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Octave

An octave is a frequency ratio of 2:1. An octave band consists of all of the frequencies within an octave. There is one octave between 100 Hz and 200 Hz, and between 1,000 Hz and 2,000 Hz, for example. Octaves are perceived as equal pitch intervals, even though the true bandwidth in Hertz varies with the frequency level of the octave. To our ears, two frequencies an octave apart (whether it might be 100 to 200 Hz or 1,000 to 2,000 Hz) sound like the same note.

It is interesting that our ears obey a precisely logarithmic law when assigning subjective pitches to frequencies. Even though an octave is strictly speaking a subjective judgment, it is so closely equal to frequency doubling (no matter where one is in frequency) that it has been defined as an objective measure.

Off Line

This is one of our industry jargon terms that is sort of self explanatory, yet there is also no commonly agreed upon definition. In a very general sense off line means inactive, or not in use. Equipment that is off line is generally considered to be either powered down or otherwise not connected to the system that is in use. Off line editing, however, is a phrase used in post production to describe editing that occurs apart from the main editing of a project.

Off-Axis

Refers to an audio source that is not directly in front of a transducer, especially a microphone. This results in off-axis coloration; a distortion or change in the frequency response of the reproduced audio signal. Often this coloration is put to good use. For example, many engineers intentionally set up mics on guitar amps so that they are slightly off access to control the amount of high frequencies captured. A microphone will generally produce the "truest" results if it is used on-axis (oriented directly in front of the sound source).

Ohm

Named after its founder (Georg Ohm), the ohm is a measurement of electrical resistance or impedance in a circuit or object. It is symbolized by the Greek letter Omega (¸).

OMF (OMFI)

Abbreviation for Open Media Framework. OMF is a file exchange protocol for digital audio/video workstations that attempts to put all of the important information about a session into a generic format that can be read by other systems. It is analogous to our standard MIDI files in the MIDI world, but much more complex an issue. Lately we are beginning to see it noted as OMFI, where the "I" stands for Interchange, which is more of a semantic than functional difference. In practice OMF allows an audio or video session to be opened and edited in a completely different system. For example: A work in progress on an Avid video system can be opened as a multitrack session in Pro Tools for enhanced audio processing, then imported back to Avid. There are still many limitations and complications with OMF, but because it is so useful there are scores of engineers working with it and sorting out problems. More information on the subject can be found at Avid's Web site in the OMFI section.

Omnidirectional

Literally, from all directions. In audio, microphones are said to be omnidirectional if they can detect sound equally from all directions. Speakers are omnidirectional if they radiate sound in all directions equally; this tends to be the case with subwoofers and low frequency drivers. Low frequencies, in general, tend to be omnidirectional, versus high frequencies which tend to "beam" or be very directional.

Op Amp

hort for Operational Amp, a circuit component used in all sorts of equipment. Though they are technically considered amplifiers they are quite often used in circuits that do not obviously "amplify" signals. Examples would be equalizers, crossovers, compressors, mixers, microphones, keyboards, effects and many, many, many more (the list is endless). Op amps acquired their name from early uses in analog computers (computers perform operations, get it?). They can exhibit very high gain and are extremely easy to build into audio circuits. Nowadays they are available in integrated circuit chips, each of which may have many op amps inside. In some cases they are literally a dime a dozen.

Opto-isolator

An electronic component that contains a light source, usually in the form of an LED, and a light sensing device. When current flows, the LED emits light, which is detected by the light sensor. Opto-isolators are used to replace switches and relays. The big advantage of opto-isolators (also called opto-couplers) is that they have no electrical connection, helping cut down on ground loops.

Orange Book

CD-WO or CD Write Once. This is the spec detailing physical and optical characteristics for recordable or writable CDs, whether audio or CD-ROM.

ORTF

A stereo recording method created by the French national broadcast system to simulate the directional perspective of human ears. Similar in approach to the more conventional X-Y configuration, two microphones are placed in front of a sound source. The mics are spaced 17 cm (about 6 3/4") apart, at an angle of 110 degrees. The ORTF method provides good mono compatibility and stereo imaging, but captures little of the room's ambience (this may or may not be a good thing, depending on the room you are recording!) Try this mic setup the next time you are recording a small ensemble, choir, orchestra, or even a solo acoustic instrument, it works quite well. For those who just HAVE to know what the acronym ORTF stands for, the answer is: "Office de Radiodiffusion-Television Francaise"...

Oscillator

An electronic device which generates a periodic signal of a particular frequency, usually a sine wave, but other waveforms (square, sawtooth, triangle) are often used. Oscillators are common in audio devices such as synthesizers and test signal generators. Early synthesizers used oscillators as the basic component for all of the sounds of the machine. All of the filters and envelopes modified the sound created by the oscillator to produce the desired sound. Nowadays most keyboards produce sounds by playing back samples recorded on chips or by more modern synthesis techniques such as Physical Modeling (see WFTD archive Physical Modeling Synthesis), FM, LA, or any number of other methods that have been employed in the past 10 years.

Oscillator Sync

A phrase used in synthesis to specify a condition where a second oscillator is forced to synchronize its phase with another. This produces the sound that is characteristic of many lead synth patches or pad sounds that are very animated and change over time.

Oversampling

From inSync reader Tom K: "I have heard about oversampling, and that it improves sound quality. But what exactly is going on, and what exactly is improved? Also, when is oversampling used? During recording or during playback? What's the difference between 32x, 64x, 128x, etc. oversampling?"

Tom, this is a fairly deep subject! Here are reasonably brief answers to your questions: Oversampling is used during the analog to digital (A/D) and digital to analog (D/A) conversion processes in a digital recorder, sampler or playback device. Essentially, the sampling rate of the converter is multiplied to a very high rate (i.e. 4x oversampling puts the rate at 176.4 kHz). This accomplishes two things: First, it allows the anti-aliasing and anti-imaging filters on the converters to be much more gentle, which reduces phase distortion. Second, in a 4x oversampled system, it results in a 6 dB drop in noise (other rates result in more or less noise reduction).

Tom continues, "I was talking with a co-worker about digital audio, and I told him that all audio CD's are in 44.1khz 16-bit stereo digital format, and if copied digitally, the audio will be identical when copied back to another CD... but he brought up oversampling, and said that if a CD is recorded with oversampling, a copy of this CD will be worse quality. What's the deal?"

In this case, your co-worker is incorrect (don't rub it in too much!). The data stored on a standard Red Book audio CD is 16-bits, at 44.1 kHz. While oversampling might result in a better A/D conversion when making the CD master, and an oversampling CD player might result in better D/A playback, this is a function of the converters, not the data on the CD. Digitally "cloning" that CD will produce an exact bit-for-bit copy. Since no conversion to analog is used, oversampling does not enter into the picture.

Overtone

Similar in concept to a harmonic. Overtones are tones produced by an instrument (or sound source) that are higher in frequency than the fundamental. They may or may not coincide with the frequencies of a harmonic series (harmonics), but they usually do. The difference is that harmonics are always musically related to the fundamental in that they are integer multiples of it. Overtones of a sound are often exactly the same as its harmonics except the first overtone is considered the second harmonic because the first harmonic is the fundamental. Overtones are also sometimes called partials (more on them later).

Oxide

A three dollar word for rust, and also the material that makes up the business side of recording tape. When metal "oxidizes" (rusts) its chemical/electrical properties change (the actual chemistry lesson is beyond the scope of what we need here), and in some metals becomes suitable for use in recording tape. There are many different tape formulations that are based on different kinds of oxidized metals (chromium, for example), and varying degrees of oxidation, but the bottom line is that, for all practical purposes, we are recording on rust! This oxide material is affixed to some sort of backing (the tape) that can be placed on reels (for recording), platters (for disks), or plastic (for credit cards). This oxide is also the material that sheds from recording tape and accumulates around your tape transport components. It looks like little brown flakes. The more oxide that is shed, the less effective your recording tape becomes.