what is noise??

[eie]

Switch with 2 select inputs have noise??
do IC have noise??

[audioguru]

Hi Eie,
If you switch between two inputs that have a different DC voltage, you will hear a "pop" noise, or see a flash on a video screen.
Even if you switch between two audio sources that are capacitively-coupled and are grounded by a resistor, if one signal is positive at the moment when you switch to another one that is negative, you will hear a "pop" noise. Digital switches have a "zero-crossing" circuit to eliminate the pop.
If you use a switch that is designed to switch a high mains current and has silver contacts that corrode, to switch low-level audio or video, then you will either get nothing or a lot of crackling noise and distortion. Switches with gold contacts (at very little extra cost) are designed for that application. Switches are also made with bifurcated contacts or with a wiping action (self-cleaning) for low-noise.

Of course ICs have noise. If you use an old general purpose one like a 741 as a low-level microphone high-gain preamp, its output will have a constant "ssssssssssshhhhhhh" noise. If you use an IC that is designed for low-noise low-level audio or video, the noise will be so small you won't notice it.