Definition of noise at

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Canon XL H Series HDV Camcorders
Canon XL H1S (with SDI), Canon XL H1A (without SDI). Also XL H1.

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Old September 13th, 2006, 09:29 AM   #1
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Definition of noise

It's a very basic question but when people are referring to noise in video material what exactly do they mean?

Also what techniques are used to control this?

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Old September 13th, 2006, 09:57 AM   #2
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Definitions vary. The broadest is "anything which is not the signal of interest". Narrower ones exclude "interference" and "distortion" which leaves the perturbations to a signal caused by the fact that the atoms in the wires and semiconducters of a device at finite temperature are in constant random motion. This kind of noise is called, therefore, thermal noise and is completely random and totally independent of the signal. In video it appears, mostly in dark areas of the picture because it is small in amplitude, as bits of colored confetti fluttering about through the image.

Probably as much research has been expended in controlling noise as in trying to keep people looking young. It is one of the engineers' most difficult tasks. The simplest method is to simply make sure the signal is greater than the noise by amplifying the signal but that doesn't always work as the sensors themselves produce noise and amplification boosts both the signal and the noise. Operation of the camera in light bright enough that the sensor's self noise is masked isn't always possible as we want low light sensitivity. Given the random nature of noise it "averages out". This means, in video, that if I add several frames together that the picture parts will add up while the noise parts won't which makes the "signal to noise ratio" greater. This is the technique used by many cameras such as the XL-H1 to improve low light performance. The obvious down side comes when something moves between the averaged frames. Blurs and trails result.

Coring is another noise reduction trick which assumes that anything that changes rapidly over a small distance (as noise does because it is random from pixel to pixel) is noise. The high frequency variations are separated and limited before being recombined with the rest of the signal. This does work for noise but also smooths textured surfaces (texture represents rapid change over small distance.
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Old September 13th, 2006, 12:01 PM   #3
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AJ's answer is bang on. 99% of the time when videographers complain about noise it is basically "grain" caused by shooting in low light. The solution is to add more light when shooting instead of cranking up the gain. A post-production fix (of sorts) is to use a gaussian blur on a low setting or one of the specialized plug-ins designeed for the purpose - they tend to be expensive and render-intensive.
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