Abstract: The apparatus is provided for inserting synchronizing data between blocks of serial data. A synchronizing data generating means produces a predetermined data sequence. Selector means are connected to said data generator and an input receives the serial data and is selected by said means or alternatively the means selects the synchronizing data. Control means are connected to an output of the selector means for producing an identification gap in the serial data before the start of the synchronizing data. The signal produced at an output of the control means comprises a data gap followed by the synchronizing data and the serial data.