Frame information is recovered from a stream of binary data formed at a transmitter from a block of mxn information bits comprising m groups of n bits plus m parity bits, one for each said group of n bits, these parity bits having then been scrambled by performing an exclusive-OR operation in a gate on each parity bit with a respective bit of an m-bit parity scrambling code, said block of bits together with the scrambled parity bits forming a frame for transmission over a transmission link. At a receiver new parity bits are produced in a parity bit generator at the incoming bit rate from the incoming stream of binary data by forming a new parity bit from each successive moving group of n+1 incoming bits. The new parity bits are cyclically distributed to n+1 m-bit shift registers and a framing signal is produced when one of the shift registers holds m bits substantially corresponding to the m bits of the parity scrambling code.
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