Abstract: An auto-correlation apparatus for producing N auto-correlation values R(k), (k=0, . . . N-1) of an auto-correlation function of a sequence of digital samples x(n), (n=-.infin. . . . 0, . . . p) of an input signal, each sample being encoded as words of "b" binary bits, each value R(k) being defined by the formula: ##EQU1## wherein M is the number of digital samples. The apparatus comprises a random access memory for storing at respective addresses therein words (X,R) each of which is a concatenation of a word representing the value of a signal sample (X) and a word representing an auto-correlation value (R). A multiplier cooperating with an adder performs multiplying and summing operations on the M most recent signal sample words in accordance with the formula for R(k). A memory addressing and sequencing member supplies an addressing cycle to the memory for each received signal sample words, and controls the operations of the multiplier and adder.