4. Apparatus as defined in claim 1, wherein said filtering means has a characteristic such that there is only one non-zero multiplying coefficient in the set of coefficients spatially separated by a number of words equal to the number of initial words per unit time divided by the reduced number of words per unit time.
6. Apparatus as defined in claim 4, wherein said filtering means includes means for producing digital outputs which are functions of the input digital words, and means for combining said digital outputs to produce said filtered words.