Abstract: A first counter operating at the PCM sampling frequency counts out the normal minimum period between successive synchronization words in the data stream. In a following "window" the detection of another synchronization word produces a check signal and in all other cases a provisional check signal. In such other case the data for the next period are marked as dubious. A second counter counts out twice the minimum normal interval between synchronization words and in a following "window", the appearance of a synchronization word produces a verification signal. The occurrence of a verification signal one normal interval after a provisional check signal removes the marking of intervening data as "dubious". If no verification signal is then produced, the intervening data are marked and treated as erroneous.

Abstract: The digital signals representing successive picture points of a digital television video signal is caused to pass through, in succession, two delay circuits each providing a delay of one television line interval. This makes three places available where shift registers may be connected for stepping the digital signals at picture point intervals, so that a two-dimensional array of neighboring picture points located on adjacent portions of three television lines can be made simultaneously available for correcting the signal of the central picture point if it is found to be defective. A defective picture point is caused to be represented by a null value of digital signals by an error-correcting circuit.