What error-correction system is used in AMTOR FEC (Mode B)?

AMTOR (Amateur Teleprinting Over Radio) operates in two primary modes: ARQ (Mode A) and FEC (Mode B). ARQ stands for Automatic Repeat Request, where the receiving station specifically asks for missing or corrupted data to be sent again. FEC stands for Forward Error Correction, which is designed for broadcasting to multiple stations that cannot acknowledge receipt. To achieve error correction in FEC (Mode B) without a return link, the transmitting station simply sends every character twice, separated by a brief interval. The receiver compares the received characters to ensure they are valid and prints the correct ones. Distractors like checking an FCS or requesting repeats describe Packet radio or Mode A (ARQ), respectively. Mode B relies purely on this time-diversity redundancy (sending each character twice) to help receivers compensate for temporary signal fades or interference.