AMTOR

AMTOR ARQ (called also SITOR-A for the navy (as a maritime mobile service system) or simply TOR)

Description :
Baud rate : 100. A character is composed of 4 “ space ” and 3 “ mark ” bits. A block of characters (called “ Block 1 ” or “ Block 2 ”) is composed of three characters (total duration: 3 x 70 = 210 ms) and a pause of 240 ms duration. During this pause, the receiving station emits a control signal of 70 ms duration to say “ please next block ” (symbol "ACK" for acknowledge) or to ask for a repetition of the last block, in case of erroneous character (symbol "NACK" for not acknowledge)
Speed : between 0 and 67 wpm (67 wpm corresponding to the 50 bauds RTTY) depending of the number of repetitions
Modulation : FSK two tones (“ mark ” and “ space ”, "mark" high) with a shift between tones of 170 Hz
Reception mode: USB
Character set : CCIR476-4 (32 letters, 32 figures and 6 control, idle or phasing signals) with no error reset character (“ fixed length ” characters).
In SITOR A, the character set may be extended to the Cyrillic characters by using the character for “ un-perforated tape ” (third shift)
In AMTOR A, the character set may be extended to the ASCII characters (32 to 128) by using the character for the “ un-perforated tape ”
Shape of pulse : rectangular
Bandwidth : about 800 Hz (due to rectangular shape),
Demodulation : non coherent
Synchronization : automatic using the signal
Correction code : yes via automatic repetition and easy detection of an erroneous character due to the particular content of a character (4 “ space ” and 3 “ mark ” bits)
Convolution code: no
Interleaving : no
Pmean/Ppeak : 1

Note: in AMTOR ARQ LISTEN, characters are tranlated the best possible way because no repetition can be requested. Therefore, receive quality is not generally very good.

AMTOR FEC (called also SITOR-B for the navy (as a maritime mobile service system) or simply FEC) collective B-mode

Description :
Baud rate : 100. A character is composed of 4 “ space ” and 3 “ mark ” bits. It is an uninterrupted stream of characters, contrary to AMTOR ARQ.
Speed : 71 wpm
Modulation : FSK two tones (“ mark ” and “ space ”, "mark" high) with a shift between tones of 170 Hz
Reception mode: USB
Character set : CCIR476-4 (32 letters, 32 figures and 6 control, idle or phasing signals) with no error correction character (“ fixed length ” characters). Small characters are transformed in capital characters and special characters as “ é ” are transformed in standard capital (in the example: “ E ”)
Shape of pulse : rectangular
Bandwidth : about 800 Hz (due to rectangular shape),
Demodulation : non coherent
Synchronization: automatic using the signal
Correction code : no
Convolution code: no
Interleaving : each character is repeated 5 positions later (that is 350 ms between 2 transmissions of the same character). This diversity in time allows the correction of an erroneous character. Its detection is easy due to the particular content of a character (4 “ space ” and 3 “ mark ”).
Pmean/Ppeak : 1
Lowest S/N : about -5 dB

Note : There are 2 FEC modes: the collective B-mode (described before) and the selective B-mode (little differences as an inversion of bits).

NAVTEX system (for information)

Description : it’s a world-wide system which allows the transmission of navigational and meteorological warnings, and urgent information through coastal stations. Transmissions take place on 518 kHz or 490 kHZ in AMTOR FEC mode with special specificities of the message.

The user will set his receiver on USB at 517 or 489 khz. The transmissions are done at fixed time, for example for NITON (GB): 7h00 and 19h00 UTC on 518 khz in English, 5h20 and 17h20 on 490 kHz in English, 7h10 and 19h10 on 490 kHz in French.

The character set of SITOR A and SITOR B/AMTOR FEC is the same of the one used for RTTY.
On SITOR A, the reception of the "un-perforated tape" is used by Russians ships to pass in traffic on Cyrillic. In that case, MULTIPSK passes to "letters".
The character set of AMTOR ARQ is an extended character set which allows the transmission of ASCII characters (32 to 128) thanks to the use of the "un-perforated tape".