7. Apparatus as defined in claim 6, wherein said monitoring means utilizes the digital output producing means and the combining means of said filtering means, and further including means for comparing the combined digital outputs with a reference signal, thereby to determine whether the input digital words have a characteristic similar to filtered words.
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.