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asynchronous : Not according to a fixed rate of repetition. An asynchronous signal can occur at intervals which do not necessarily coincide with a fixed-rate system or master clock pulse.

antinode : A place of minimum sound pressure level in a standing wave, as opposed to a node, which is a maximum level.

anode : The anode in any electronic component, such as a silicon diode or a vacuum tube, is the electrode normally connected to the positive voltage.

ambient noise : Ambient sound which is environmental in nature, such as traffic noise coming through walls, heating or air conditioning, or other extraneous sounds which cannot be turned off or removed.

All-Notes-Off : A MIDI command, recognized by some but not all synthesizers and sound modules, that causes any notes that are currently sounding to be shut off. The panic button on a synth or sequencer usually transmits All-Notes-Off messages on all 16 MIDI channels.

adjustable turnover : A variable tone control in a preamplifier which allows the adjustment of the boost/cut and the frequency below or above which the gain/attenuation is applied (turnover), but not the rolloff slope of the shelving equalizer. If it were possible to adjust the rolloff slope, the result would be a fully parametric tone control.

biased noise : A tape loop of audio silence, processed by a recorder with some kind of noise-reduction enabled, such as Dolby-SR (where it is called SR noise) or dbx or dB. Used to check equipment for ground loops or other problems in recordings and/or masters. Sometimes called an alignment recording. See Dolby noise.

bridged mono : A method of combining both channels of stereo power amplifiers to create a doubly powerful single-channel (monaural) amplifier. See bridge(2).

Cannon connector : See XLR.

clock noise : An artifact of digital-to-analog conversion that creates staircase-like changes in voltage produced by the converter. Most clock noise is caused by shifts in the zero-crossing times. See quantization noise, reconstruction filter.

correlated noise : See distortion.

electrostatic noise : A field of random electrical charges that can affect an audio line. Electrostatic noise can be generated by neon or fluorescent lighting, electrical motors, and other broad-spectrum emissions sources. Electrostatic noise is the electrical field which is generated by EMI.

equivalent input noise (EIN) : EIN is becomming a common method for specifying noise in audio equipment. This is a computed figure equal to the noise measued at some gain setting, minus the gain. For example, if a microphone preamplifier puts out -85dBV noise when set for 40dB of gain, the EIN is -125dBV. Note that, while -125dBV seems better than -85dBV, both figures represent the same amount of noise."

noise reduction Two technologies for noise reduction have become standard in the consumer and professional recording industry: dbx and Dolby. All two-ended noise reduction systems are a type of compander, i.e., they operate by encoding the signal at the record end, and decoding the signal, restoring the dynamic range and frequency spectrum, upon playback.

Single-ended noise reduction systems need no encoding or decoding. The NR is applied to noisy instruments or microphones and works either by dynamic filtering or downward expan-sion. Dynamic filtering works will with noisy synthesizer sounds, but can cause pumping and breathing. A downward expander attenuates any signal below the threshold. It works when applied to cut the buzz on a noisy guitar amplifier, but it can also cut off quiet signals like reverb tails if the threshold is set too high. See dynamic filter, spectral recording.

MIDI Note Number : The decimal number, from 0-128, which represents the equal temperament scale of about eight octaves, where 60 represents Middle-C, having a frequency of 261.63Hz. The MIDI note number 36, for example, corresponds to the 4th key on a piano, referred to as C1, with a frequency of 32.7Hz. Middle-C is sometimes called C3 or C4, depending on the author. Commonly, modern instruments are tuned to A440, that is A3/A4, MIDI note number 69.

noise floor The noise floor is the intrinsic noise of any audio device or other electronic system, generally measured in dBm. Sometimes the noise floor is measured in terms of RMS voltage rather than power, and this makes sense in the case of devices such as voltage amplifiers or tape recorders. Includes Johnson noise and flicker noise. See quiescent noise. To calculate the intrinsic noise level of a device, expressed in watts: if one took one 600Ω resistor on the input of a (noiseless) microphone preamp with a 60dB gain, the output would be about -100dBm. This is the lowest possible noise floor:


Shannons channel capacity the :The formula, DRmax=W log 2 (1+sn) , where DR is data rate, W is channel bandwidth, and SN is the S/N ratio. It expresses the relationship between these three variables in digital audio.

flicker noise : At high frequencies, intrinsic noise is dominated by Johnson noise. At lower frequencies, there exists a critical frequency, , at which noise rises proportional to 1/. Below this critical frequency, the noise is called flicker noise. Sometimes referred to as flicker effect.

high-output low-noise (HOLN) : A type of magnetic recording tape with very high sensitivity to applied magnetic fields, and with a very high S/N ratio, commonly used in professional audio applications.

intrinsic noise : The intrinsic noise of any (electronic) system is the noise which is a function of the real part of the input impedance (Z) impedance and the temperature, i.e., noise which is inherent in the system itself, as opposed to being induced by any source external to the system or device. See Johnson noise.

isochronous : Strictly speaking, isochronous means, "having a periodic nature." Sine waves are isochronous. In audio-speak, however, isochronous usually refers to two signals which, while produced at the same sampling rate, are not synchronized. In this case, due to small variations in the clocks which drove the sampling frequency, the two signals will eventually drift out of synchronization, even if started at precisely the same time. Isochronous signals are a problem when attempting to play back audio, video, and/or control signals which require timecode-accurate alignment for the very accurate long-term synchronization of sound and vision. See also FireWire, coherence, incoherent.

Johnson noise : See noise floor. Johnson noise is the broadband white noise power associated with electrical resistance at temperatures above absolute zero. The Johnson noise level is the limiting minimum noise any circuit can attain. Also called thermal noise.

just noticeable difference : A psychoacoustic term which refers to the smallest timing difference the human aural system is capable of detecting between two sound sources, approximately 6s. This is just an artifact of human hearing and is not related to the Haas effect.

light metronome : A metronome which silently marks beats by flashing a light on and off, as opposed to audible clicks, to mark the tempo.

microphonic noise : Noise generated within an audio cable, caused by changes in capacitance between the inner conductors in the cable and/or its shield. Microphonic noise can result from unstable dielectric (insulating) material that allows the conductors and/or shield to move in relation to one another.

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