In the simplest form of FEC, each character is sent twice. The receiver checks both instances of each character for adherence to the protocol being used. If conformity occurs in both instances, the character is accepted. If conformity occurs in one instance and not in the other, the character that conforms to protocol is accepted. If conformity does not occur in either instance, the character is rejected and a blank space or an underscore (_) is displayed in its place.
Simple FEC is one of two modes used by radio amateurs in a self-correcting digital mode called AMTOR (an abbreviation for amateur teleprinting over radio). It is sometimes called Mode B. The other AMTOR mode, automatic repeat request (ARQ), involves handshaking and is also used with communications systems such as Global System for Mobile (GSM). In amateur radio, ARQ is sometimes called AMTOR Mode A.