Stray signals and noise

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The physical and human spects dealing with audio, video, and images

Vision and color perception: objectively describing color · the eyes and the brain · physics, numbers, and (non)linearity · color spaces · references, links, and unsorted stuff

Audio physics and physiology: Basic sound physics · Human hearing, psychoacoustics · Descriptions used for sound and music

Digital sound and processing: capture, storage, reproduction · programming and codescs · some glossary · Audio and signal processing - unsorted stuff

Image: file formats · image processing

Video: format notes · encoding notes

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Unsorted: Signal analysis, modeling, processing (some audio, some more generic)

For more, see Category:Audio, video, images

Noise in general

Noise can mean anything you didn't want in your signal.

It can come

from the environment (e.g. capacitive and inductive coupled signals from other equipment, or just physical noise when trying to record sound)
from recording equipment,
from the sampling itself,
from sensor irregularities,
from distortion caused by inductive/capacitive effects
the medium you store on,
voltage transients,
and many more places.

To mathematicians and mostly to sound engineers,

noise tends to indicate broad-spectrum, non-periodic signals,
and regularly is more or less uniformly present in all the sound/data,
because most of the mentioned effects relate to general setup and don't change very quickly.
Describing noise frequency content is somewhat funny.
Say, white noise cannot be accurately modeled as a function of time,
but can be approximated well enough in frequency content.

Note that some noise has a uniform and randomly-distributed nature, others comes in fixed patterns. Fixed-pattern noise, such as an image sensor with misbehaving lines, columns, and pixels, or mains hum, tends to be a lot more noticeable than wide-spectrum noise of the same magnitude.

The term noise floor refers to a basic level of noise present in a system or medium, usually the sum of all the specific sources that happen to be more or less uniform over time, and often mostly unavoidable due to it combing from the physical reality of electronics, wiring, environment, or whatnot.

In various devices it tends to be largely thermal noise(verify).

The noise floor settles the smallest measurement that a system can deal with certainty/consistency (which is one reason this concept of "the combination of a bunch of noise" gets a name, and relates to dynamic range).

See also

And many more, including perhaps: