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Now looking at definitions starting with letter "o"

open circuit : A circuit through which an electrical current cannot flow, perhaps because a component has failed or a connection has been broken. See also one-legged, short circuit.

open track : On a multitrack tape, any track that has not yet been used, or that may be erased and reused for overdubs.

open-circuit voltage rating : The output voltage of a microphone with no load, i.e., with infinite resistance such as in an open circuit, or when driving a resistive load at least twenty times the microphone’s internal impedance. One of the standard specifications of microphones.

open-loop : An amplifier without feedback is said to be in an open-loop mode, or to be an open-loop amplifier. The feedback around the amplifier closes the loop.

open-reel : A type of tape machine which uses tape wound on spools, rather than tape which is sealed in a cassette.

operating level : The voltage level defined for any audio system at its nominal, 100% modulation level, not including any headroom. Usually defined as 0dBVU for a steady sine wave.

Operating System : the low-level software that provides a computer’s basic features and capabilities. Operating systems can be linked to specific platforms such as Mac OS on PPC or may, like BSD UNIX, be available for a wide variety of CPUs

operator : (1) A term used in Yamaha’s FM synthesizers to refer to the software equivalent of an oscillator, envelope generator, or envelope-controlled amplifier. (2) The general term for a structural component of FM synthesis, analogous to an oscillator/envelope/amplifier in a standard synthesizer parlance.

optical disc (OD) : A very dense type of digital data storage medium. The data are encoded in a spiraling pattern by a laser that carves tiny pits into the surface of the OD master, every change from land to a pit indicating a change from a 1 to a 0, or vice versa. Where no change is recorded, the last-read digit is indicated for each increment of groove length. In the reproduction device, a small laser scans the groove, reading the changes from land to pits, converting this information back into data. The data can be text, video, or digital audio, such as CDs and laserdiscs. See LIMDOW.

optical recording : Sound recording on film. The photographically printed film soundtrack is known as the optical track. See SVA, variable area.

optical sound : The type of sound reproduction on film that employs a photographic printing process of the optical track. As opposed to magnetic film. See optical recording.

optical track : A method of recording an audio signal in the emulsion of a film alongside, and in sync with, the picture frames. See DES. The photographically printed film soundtrack that appears either between one row of perforations and the picture in an answer or release print (in 35mm), or along the edge of the print opposite that with the perforations (in 16mm). The track area on a 35mm print takes up a total width of 100 mils, which, being one-tenth of the space between the sprocket holes, displaces the centerline of the image on the film by 50 mils, hence called the Academy centerline. See DES. (2) The master or original photographic soundtracksound track made directly from the mono mix or 35mm three-track mix. This strip of film has no video image, but is printed along with A- and B-rolls onto the composite answer print or release print. The track can be a negative or positive image, depending on the type of camera and print stock. See also stereo optical print, 50% level.

opticals : (1) Effects on 35mm film that are made in an optical printer, such as dissolves, fades, super-impositions, freeze-frames, matte shots, etc. (2) Loosely, any eaffect used in a film or video production.

opto-electric : A device which uses a variation in light intensity to cause a change in electrical current. Variable photoresistors are sometimes used as gain control elements in compressors where the side-chain signal modulates the light intensity.

opto-isolator : An electronic component that can pass a signal via a light path, avoiding a direct electrical connection between two separate circuits. This will prevent voltage spikes generated in one piece of equipment from damaging another unit in the network, as well as breaking ground loops. The unit consists of a light source (an LED) and a light detector (a phototransistor) enclosed in a sealed box, the whole package looking IC-like. The part of the MIDI specification that deals with hardware requires that all MIDI In connections use an opto-isolator.

ORC : Optical Radiation Corporation. See Cinema Digital Sound.

order : In discussing filters, the number of poles a certain filter possesses is called the order of the filter. Thus a T-section is a third-order filter and an L-section is a second-order filter, etc. The slope, in dB/octavein decibels per octave, of the filter response in its stopband is equal to six times the order.

ORTF : Office de Radiodiffusion-Television Française. A stereo microphone configuration designed by the French national broadcasting system. This method calls for two cardioid microphones to be spaced 17cm (6.7") apart, at an angle of 110˚. The 17cm represents normal ear spacing, and the 110˚ is to simulate the directional pattern of the ears. Recordings made using the ORTF method sound more open and spacious than those produced by the X-Y miking method. ORTF works well with headphone applications, but tends to sound somewhat dry and lacking in warmth due to the directional patterns of the cardioids, which pick up little ambient room sound. However, because of the close spacing of the microphones and the resultant similarity in phase, ORTF does provide mono compatibility, desirable in broadcasting. The Swedish equivalent is NOS, which is the same, except that the angle is 90˚ with a spacing of 30cm (11.8")somewhat wider.

oscillator : (1) An electronic device which generates a periodic signal of a particular frequency, usually a sine wave, but sometimes a square wave or other waveform. In an analog synthesizer, oscillators typically produce regularly repeating fluctuations in voltage--that is, they oscillate. (2) In a digital synthesis, an oscillator more typically plays back a complex waveform by reading the numbers in a wavetable. An oscillator allows a choice of pitch and waveform, the first affects the perceived musical pitch and the second affects the timbre. Additional parameters that are almost always found in the oscillator section of a synthesizer are those that deal with vibrato and pitch-bend.

oscillator sync : A sound synthesis technique whereby one oscillator’s cycle is synchronized to that of a second. This forces the waveform of the slave oscillator to restart its cycle each time the master crosses the zero-point. As a result, the fundamental of the slave is the same as the master, but the waveform is radically changed. The pitch of the controlling oscillator is not normally added into the audio mix, but can be shifted by pitch-bend, envelope, aftertouch or an LFO, producing substantial changes to the harmonic content of the slave oscillator, but without changing the fundamental pitch as does ring modulation. Instead, the higher harmonics around the pitch of the slaved oscillator are emphasized, producing a very hard edge to the tone.

ossia : Italian for "or." Usually used to indicate an alternative version of a musical passage.

ostinato : Italian for "obstinate." A short melodic and/or rhythmic idea which is continually repeated, often in the bass. Known as a riff in popular music.

out-of-phase : A condition where two signals have a phase difference of 180˚, or one-half cycle. It should be called out of polarity, phase being a continuous variable rather than discrete. The same as antiphase. See phase reversal.

out-take : Any take produced in a recording session which is not used in the final master.

outboard : In a recording studio, special equipment such as effects devices and power amplifiers that are not included within the recording console are called outboard devices. Compare with on-board.

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