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Noise
Noise is a kind of error that occurs when a signal is processed by circuity in a sond card.. Noise is a from of error that is generally unrelated to the input signal.
Noise may either be predictable nature (deterministic) or it may be unpredictable (stochasitic). Unpredictable noise is also called Random Noise and is usually due to thermal noise in components; or is in some sense a noise that is predictable, but has such a complex nature that we can't analyze it and determine how to predict it, yet. Hum and undesired pickup of signals from other equipment or near by broadcast equipment are examples of predictable or determinaistic noise.
Examples of deterministic noise include hum related to electrical power distribution, or intelligable speech or music picked up from a powerful nearby radio station. Deterministic noise is sometimes called Interferring Signal(s).
Stochastic noise typically comes from thermal sources. This is the noise due to the motion of the atoms, molecules, and subatomic particles that make up electronic equipment. Thsi motion is eliminated by cooling equipment to absolute zero, which is of course, impractial.
Long distance communication links may be subject to noise sources that are not part of our planet and celestial in nature.
Some noise may appear to be stochastic, but actually be deterministic. If a large number of varied but predicable noise sources are combined, the result may so complex it appears to be stochastic.
Deterministic noise is reduced by controlling Electromagnetic Interferance (EMI), improving shielding, or improving grounding techniques.
Stochastic noise is reduced by reducing circuit impedance levels or using materials that have less inherent noise.
Modulation noise is a general increase in noise that occurs when a signal is applied. Since this noise is composed of many frequencies, it is classed as a form of nonlinear distortion and noise combined.
Equipment with lower noise and distortion is generally considered to have better quality and be more desirable for the purposes of audio reproduction.
The following chart compares the average person's threshold of hearing to noise levels inherent in properly designed digital systems with different numbers of bits.
From: J. Robert Stuart(Meridian Audio), Digital Audio for the Future, Audio, 3/98 pp 30-37.
This particular chart is based on an extremely loud, even potentially ear-damaging reference level of 120 dB. It can be seen that even for very loud, but more likely, listening levels of 110 dB; a well-designed 16 bit system is unlikely to have audible noise. At more practical peak levels in the 90 dB range, audible noise most certainly will come from other sources.
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