Monday, September 14, 2015

AMTOR and SITOR

AMTOR (Amateur Teleprinting Over Radio) is a type of telecommunications system consisting of two or more teleprinters. It's is a specialized form of RTTY protocol known commercially as SITOR (Simplex Telex Over radio.) AMTOR utilizes FSK, with a frequency shift of 170 Hz, and a symbol rate of 100 Baud.. The ARRL called it an improved "errror-free" RTTY system. [LINK]. AMTOR is derived from CCIR recommendation 476-1 and was developed primarily for maritime use in the 1970s.
AMTOR was developed by single person, Peter Martinez G3PLX. His first successful AMTOR contact was with David Wicks G3YYD on the 2 meter band in 1978. It's unclear what contributions to the code base Wicks may have made, but he later wrote his own AX25 software to run on the same machine used in the AMTOR test, a home brew Motorola 6800. In 1980 the FCC granted an STA to four hams to use AMTOR: William Meyn K4PA, Charles Roettcher K3FLS, Marvin Leibowitz W3KET, and Walter Kaelin KB6BT.

Peter Martinez was a genius in his own right. (In print appearances prior to 1987 he went by J. Peter Martinez.) His hobbies are so complicated I honestly don't understand what he's doing; such as using pulse generators as ionospheric sounding transmitter to build ionograms of frequency reflection height. By comparison AMTOR was a small undertaking. He also developed PSK31 in 1998, another teletype communications protocol. And in 2010 he wrote part of a book on Cave radiolocation. His name appears in a number of IEEE texts. Martinez states in one of his many papers that he had been involved in RTTY in the 1960s but his pre-AMTOR resume is not well known. He has kept busy since then.

There are 32 characters in the RTTY teleprinter system. These are rendered by 32 combinations of 5 data bits. the problem is that the total number of combinations of 5 bits (in binary) is 32 [25]. So any bit error can transform a character into some other character. This error would be undetectable, a huge problem. AMTOR uses 7 data bits which renders 128 combinations. In a random data set only 25% of those would be recognizable as the 32 characters.  But AMTOR uses only combinations made of three zeros and four ones. So as a result errors are easily detected. There are 35 of these combinations leaving three extra to be used as controls. The result is so rugged that random noises generates a character less than 2% of the time.