[RTTY] Amtor

[Kok Chen]

On Apr 29, 2011, at 12:03 PM, Don AA5AU wrote:

> Amtor is a fun mode and one which I used on many occasions to get a new one on "rtty/digital".

Amtor (known as SITOR outside Amateur circles) is indeed a very interesting mode.

Although Amtor also uses the same character set as Baudot (including the LTRS and FIGS shifts), it does not use the 5-bit coding that we are familiar with in RTTY, but instead it uses a 7-bit Moore code (designated as CCIR-476) to encode the same 32 element character set (therefore, no lower case in Amtor, either).  Because of this redundancy, the code was constructed to separate the characters by Hamming distance of at least 2, and you can detect all single bit errors. 

Also, there are no start/stop bits in the 100 baud Amtor transmissions.  You can derive character sync from the Moore code itself (smart, eh?).

When errors are detected, the Amtor receiver uses ARQ to request a repeat.  The result is reliable, pristine print with sufficient automatic repeats.

Amtor also has a "broadcast" mode (no ARQ), called the SITOR-B mode (it was used by W1AW in the past when they transmitted Amtor Bulletins).  SITOR-B repeats a character (a rather arcane algorithm for the repeat position) after a short delay.  If a character error is detected at a character position but decodes properly in its repeated position, you can still uniquely determine what was transmitted.  If both locations produce errors, you can flag it in the printed output.  

Notice that because of the error detection capability for each character, there is no need to repeat a character three times.

I did not implement Amtor transmission into cocoaModem because by the time I became intrigued by the SITOR encoding, there was already no more Amtor activity on the ham bands, and only a sprinkling of SITOR-B transmissions.  So I implemented SITOR-B reception.  For a time, I used to copy the W1AW bulletins in addition to the Coast Guard bulletins.

Unfortunately, you can find even less Amtor activity now.  W1AW has replaced the Amtor bulletins by PSK31 and MFSK16 bulletins, so the only Amtor transmissions is pretty much reduced to NAVTEX coast guard transmissions now (e.g., the 518 kHz LF transmissions from Pt. Reyes in California).

Not many software modems implement Amtor.   You can look here for the cocoaModem interface for SITOR-B (same cross ellipse display as the RTTY interface, of course, HI):

http://homepage.mac.com/chen/w7ay/cocoaModem/UsersManual/sitorManual/index.html

73
Chen, W7AY