Digital noise is a series of (more or less) random low/high pulses. Analog noise is an analog signal without any (apparent) order. There exist several types of noise, the most important ones being “white” noise, “pink” noise and “brown” noise.
White noise (which is not the same as Gaussian noise) is characterized by a flat spectrum, i.e. the signal has equal power in any frequency band (up to some bandwidth). Futhermore, it must have zero autocorrelation. For digital noise, this means that the the sequence of LOWs and HIGHs is statistically uncorrelated. (Especially this means that there is no period which, of course, is not true for simple digital noise sources as the one presented here.)
There exists a fairly simple way to create digital white noise using a shift register with n bits. Each clock pulse shifts the content one bit to the right and while the rightmost bit gets lost, the leftmost is filled up with an XOR combination of two other bits (a and b) from the shift register. The period and pulse distribution characteristics of the noise source are determined by which two bits are chosen.
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