Certain systems described in these standards employ feedback shift registers to modify sequences or patterns of bits in a predetermined manner or to restore such modified bit patterns to their original sequence. With outputs of suitably selected stages added modulo-2 and applied to its feedback loop, an n-stage feedback shift register will generate a bit sequence or pattern (2^n - 1) bits long before repeating. Because such repeating sequences exhibit many of the statistical properties of uniformly distributed random number sequences (e.g., their probability density and autocorrelation functions satisfy appropriate conditions),they are called pseudo-random.