DV Info Net - Video Noise

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How does video noise occur? Is underexposure the only factor? I know video noise when I see it, but I don't know how to explain the cause of it in technical terms. Please help.

I've found the answer to my own question.

http://www.cctv-information.co.uk/constant2/sn_ratio.html

Here is a good article on signal-to-noise ratio and it explains the subject very well.

Interesting article, but as far as the under-exposed type of noise you mentioned, are you talking about the red and blue fuzz in the black areas, and he general fuzz everywhere else? That may be caused by your gain level. If you're using a Canon XL or GL series, you've got four choices, +0, +6, +12, or +18. In my opinion, any amount of gain causes unacceptable noise. So I always use the manual setting of 0 gain, and if there's not enough light, I get some light or put away the camera and or go home. The cause, as I understand it, is gain takes what light you have and electonically bumps it up. The processor doesn't know what to do when it bumps up underexposed scenes and fills in the blanks with noise.

So, there's noise due to electronic interference (like the article), and then there's the noise created by the gain.