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
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