ABSTRACT
An audio signal can be transmitted either as a voltage or a current. The construction of the universe is such that almost always the voltage mode is more convenient; consider for a moment an output driving more than one input. When noise is referred to in electronics it means white noise unless it is specifically labelled as something else, because that is the form of noise that most electronic processes generate. Non-white noise is given a colour which corresponds to the visible spectrum – thus red noise has a larger low-frequency content than white noise, while pink is midway between the two. Johnson noise theoretically goes all the way to daylight, but in the real world is ultimately band-limited by the shunt capacitance of the resistor. The noise voltage is of course inseparable from the resistance, so the equivalent circuit is of a voltage source in series with the resistance present.
