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.