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Noise, in terms of analog video and TV, is the random dot pattern that becomes superimposed on the screen as a result of radiated electromagnetic noise picked up by the TV's antenna or digital receiver. White noise, or snow as it is called occasionally, is seen usually on poor analog reception sets, VHS tapes or when digital TV owners try to access an input that is not connected.
When the input is not connected, the "no signal" noise is due to luminance signal, thermal noise from the TV or stray electromagnetic fields from around the living area.
The snow is also less random than would be seen on CRT and analog television sets.