# Music Education

## Music Technology & Home Recording Glossary O

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OCTAVE - a frequency ratio of 2:1. A musical distance (interval) of 12 semitones. Measure of the distance between one note and another note of the same name, which is eight full notes above or below it on the musical scale. An example of an octave would be "C, D, E, F, G, A, B, C". Another way of looking at it would be to say that a note which is an octave higher on the musical scale will also have a frequency which is twice as high eg if A4 has a frequency of 440 Hz, then A5 (an octave above) will have a frequency of 880 Hz.

OFF-LINE: Process carried out while a recording is not playing. For example, some computer-based processes have to be carried out off-line as the computer isn't fast enough to carry out the process in real time.

OGG Vorbis - open source audio codec designed to compete with MP3. Since it is not licensed like MP3, software using this codec does not have to pay royalties. (This name is thought to have been derived from a character called Nanny Ogg in a the Discworld books by Terry Pratchett.)

Ohm / "Ohm's Law": - Ohm's Law is one of the fundamental laws of electronics, and pertains to the relationship between current and voltage / resistance in an electrical conductor. This relationship states that "Current = Voltage / Resistance". The usual way of expressing this in mathematical terms is "I = V/R", or to make things confusing, you could also say that "V = I / R", or R = V / I. The current is measured in "amperes" or "amps" the voltage is measured in "volts" (duh!) and finally, the resistance is measured in OHM's. So as we can see from above, an "Ohm" is a measurement of the resistance in an electrical conductor. Which can be calculated using the R = V / I equation above. Hence, when someone says that their speakers have an "Impedance" of 8 Ohms, the "impedance" is referring to the factor by which the electrical signal is impeded, hence, a 4 Ohm speaker will offer half the resistance to the electrical current flowing through it than an 8 Ohm one. Having said this impedance varies greatly, and impedance ratings are usually just an average. For more on this, impedance.

OMNI: Meaning all, refers to a microphone that is equally sensitive in all directions, or to the MIDI mode where data on all channels is recognised.

Omnidirectional: - See Polar Pattern. Will pick up sound from all angles equally. An omnidirectional polar pattern should adhere very closely to that of a circle / sphere. There are also Half - Omnidirectional (hemispherical) microphones which do the same thing, only over 180 degrees rather than 360. "PZM" (Pressure Zone Mic) microphones are an example of this. For microphones is means receiving sound evenly from all directions. For speakers this means an even coverage in all directions.

One stop: A wholesale source for music product, primarily used by independent record stores which purchase product from there for retail sale.

OPEN CIRCUIT: A break in an electrical circuit that prevents current from flowing.

OPEN REEL: A tape machine where the tape is wound on spools rather than sealed in a cassette.

OPERATING SYSTEM: The basic software that enables a computer to load and run other programs.

OPTO ELECTRONIC DEVICE: A device where some electrical parameter changes in response to a variation in light intensity. Variable photo resistors are sometimes used as gain control elements in compressors where the side-chain signal modulates the light intensity.

Oscillator - a synthesis module used to create a cyclical waveform. These simple waveforms may then be passed through other modules (LFOs, envelopes, etc.) to add some character. See the Sound Synthesis Guide for more details. An electronic device capable of generating a recurring waveform, or a digital process used by a synthesizer to generate the same.. Electrical circuit designed to generate sonic waveform.

OVERDUB - the ability to record one sound on top of another. Adding something to a previous recording, so that the two (or more) parts may be subsequently played together as a synchronous whole. A part of Multitrack Recording. Enables one or more of previously recorded tracks to be monitored while simultaneously recording one or more signals onto other tracks. This process can be repeated until the song or soundtrack has been built up. If a mistake is made, it is possible to recue the tape to the desired starting point and repeat the process until you have the best take on tape. See Dubbing.

Overload: Distortion which is caused by exceeding the dynamic range of a circuit. To exceed the operating capacity of an electronic or electrical circuit.

Oversampling: A digital filtering technique used in CD components where extra data points are added to the audio read from a disc, creating a signal that is some multiple (usually two, four, or eight times) of the CD format's standard sampling frequency. This process raises the frequency of any false information, which can then be removed by an analog filter. Using the high sample rate, the digital data may be processed with a very steep slope digital filter. As the filter is in the digital domain, unpleasant side-effects such as phase effects are eliminated.

Overtone: A whole-number multiple of the fundamental frequency of a tone. The overtones define the harmonic spectrum of a sound. See Partial.