HamRadio Tech

DIGITAL MODES: the new generation of HF HAM Radio !

AMTOR

AMTOR - BY TONY LONSDALE VK2DHU

AMTOR is a specialised form of RTTY. 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.

In the early 1980's, Peter Martinez, G3PLX, made several minor changes to the SITOR protocol and called it AMTOR.

AMTOR improves on RTTY by incorporating a simple Error Detection technique. The system remains relatively uncomplicated but AMTOR performs well even in poor HF conditions. While there can still be many errors in AMTOR data, the Error Detection helps a lot and the result is quite tolerable for normal text mode conversations because of the high redundancy in plain language text. Certainly much better than RTTY. But for more critical data such as program code, or even some technical information messages, NO errors can be tolerated.

There are two modes used in Amtor: ARQ and FEC.

ARQ
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This mode is a little different in that it is a Synchronous protocol, which means both stations are synchronised to each other's signals.

In ARQ mode (Automatic Repeat Query), sometimes called Mode A, data is sent in groups of 3 characters. Although each character is only 5 bits (same as for RTTY), two additional control bits make it up to 7 bits per "character" and they are set so there are always 4 marks and three spaces in every transmitted character. If the receiving station gets some other combination it knows an error has occurred. The 40 percent overhead is considered worthwhile to get some error detection. This technique can identify a lot of errors that might occur but is not as thorough as the methods used in PACTOR and Packet which we look at later.

The receiver responds to each 3 character group by sending either an ACK (ACKnowledge) code (if OK) or a NAK (Negative AcKnowledge). Each time the transmitting station gets a NAK, that 3 character group is sent again.

If you listen around on the HF bands in the recognised Data Segments of the bands, you might hear a chirp-chirp sound that identifies an ARQ transmission. Even when there is no data actually being transmitted, the transmitting station continues to send idle "chirps" to maintainthe link.

Your AMTOR equipment probably supports a Listen Mode too and that allows you to monitor another ARQ session even though you are not participating in the session with the usual acknowledgements. Of course that means you don't get the opportunity to say "NAK" if you don't copy something properly!

FEC
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In FEC mode (Forward Error Correcting), sometimes called Mode B, the sending station sends each character twice so this mode provides a means of transmitting to several stations at once. The receiving station does not acknowledge the data received. If a receiving station matches both instances of a character, that character will be printed, otherwise some error symbol is printed. This mode does not provide for the receiver to ask for the missing data to be retransmitted. An FEC transmission sounds more like a Baudot RTTY signal.

The two stations need to keep in phase with each other so each FEC transmission is started with several sets of "phasing pairs" and these are sent at regular intervals even while there is no data being transmitted.

FEC Mode is still better than ordinary RTTY but its error detection is not as reliable as that in the ARQ Mode.

AMTOR systems are still limited to the technology of the 60s with limitations such as the character set and the maximum transmission rate (100 baud) geared to the mechanical teleprinter. The Error Detection technique provides improved accuracy over the "vanilla" RTTY mode, but is still not entirely reliable. It is perhaps better termed Error Reduction than Error Detection and has limited application for critical data.

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