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AMTOR is a specialised form of RTTY protocol. The term is an acronym for Amateur Teleprinting Over Radio and is derived from the commercial SITOR system (Simplex Telex Over radio) developed primarily for maritime use in the 1970s. AMTOR was developed around the SITOR protocol in the early 1980s and is used by amateur radio operators today
AMTOR improves on RTTY by incorporating error detection or error correction techniques. The protocol remains relatively uncomplicated and AMTOR performs well even in poor and noisy HF conditions. AMTOR operates in one of two modes: an error detection mode and an automatic repeat request (ARQ) mode.
The AMTOR protocol utilizes a 7-bit code for each character, with each code-word having four mark and three space bits. If the received code does not match a four-to-three (4:3) ratio, the receiver assumes an error has occurred. In error detection mode, the code word will be dropped; in error correction mode, the receiver requests that the original data be resent. AMTOR also supports FEC in which simple bit-errors can be corrected.
AMTOR utilizes FSK, with a frequency shift of 170 Hz, and a symbol rate of 100 Baud.
AMTOR is rarely used today, as other protocols such as PSK31 are becoming favored by amateur operators for real-time text communications.

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