Abstract: The instant invention resides in a system, including an encoder and a decoder, for the transmission and retrieval of digital data over a transmission medium. The encoder includes an error correction encoding scheme which processes the data and produces an encoded bit stream including data and parity bits in either an interspersed or blocked relationship. A transmission bit stream is generated by modulo-2 adding the encoded bit stream with a predetermined sync sequence, which sync sequence is selected to provide optimum sync detection at the decoder site.The encoder modulo-2 subtracts said sync sequence from the received transmission stream, thereby producing a resultant which is monitored by an error correction detector. The detector produces an in- or out-of-synchronization indication as determined by the number of resultant bit stream detected errors.