A detector for determining the validity of a received code, such as a Digital Supervisory Audio Tone (DSAT) code. A received DSAT code is serially shifted into a shift register (10) which outputs the received DSAT code in parallel format to a set of 24 exclusive-OR gates (12). Another shift register (11) receives a valid code, such as a valid DSAT code. The register (11) outputs the valid DSAT code, and right circular shifted (rotated) versions of the valid DSAT code, to the gates (12). The comparison signal from the gates (12) is provided to another register (13) and a counter (16) which together count the number of mismatches between the received DSAT code and the reference DSAT codes. The output of the counter (16) is provided to a comparator (17) which compares the number of mismatches against the allowable number of mismatches. The output of the comparator (17) is a validity signal (7) which indicates whether the received DSAT code matches any of the reference DSAT codes. Either the received code or the reference code may be used to provide the rotated codes.