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Noise to the max...

"What is Johnson noise?"

It's another name for thermal noise which is a lot more descriptive and after all Johnson is long gone... It's a voltage generated by heat rattling charges around in a resistive circuit. Every little random current from charge movement is turned into voltage by the resistance and the result is a "white noise" voltage. It sounds hissy because each Hz has the same power and there are a lot more of them per octave at high frequencies.

"And why might that be important/unimportant in audio reproduction?"

It establishes the best-case noise floor, and like Tony said, that tends to bite you in applications like Mic. Preamps because the level of intercepted acoustic energy is low. But for a given bandwidth and resistance you are stuck. Well, unless you are willing to record at absolute zero (where I think most rock bands would sound best). On top of that you will be stuck with "excess noise" which simply means noise in excess (natch) of that which would be produced by a "pure" resistor. Notably metal film resistors are usually models of purity compared to carbon-comp resistors which are seriously sullied...

"How is Johnson noise different from shot noise? Or are they the same?"

They are similar but while Johnson noise gets it's energy from heat, shot noise gets it from your power supply. When charge carriers flow through a voltage gradient, say in a transistor or tube, they pick up energy along the way which they then deposit with a pop at the end. While the two sorts of noise differ in some details the upshot is that they add up (RMSly) to form a noise floor that limits the ability of the system to resolve small signals.

There's more too, you have 1/F noise (flicker) going back to the big bang and popcorn noise (named for it's "sound") which is usually caused by process artifacts like regrinds in a record or an inadvertent recombination center in an IC.

When push comes to shove one man's noise is another man's signal so it's personal: Noise is essentially any signal, but traditionally a broad-band one, that you wish wasn't there!

Rick


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  • Noise to the max... - rick_m 09/12/1221:43:54 09/12/12 (0)

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