AMTOR is similiar to RTTY in many respects, but AMTOR can have FEC (Forward Error Correction) built in and also ARQ or (Automatic Query Request) to help ensure receiving stations receive error free text ARQ mode is used with two stations if error are received it automactically ask to send text again.but ARQ is not possible when receiving commericial AMTOR transmissions.
AMTOR use two frequencies which are filtered, each one corresponding to a binary digit (1 or 0). These two frequencies, separated by a shift, modulate the RF when transmitted and the modulation is called Audio Frequency-Shift Keying (AFSK), otherwise the modulation can be done directly on the RF carrier and it is called Frequency-Shift Keying (FSK).
-AMTOR FEC or NAVText is AMTOR with built in FEC to help ensure other station received what is being sent.
- AMTOR ARQ (called also SITOR-A for the navy (as a maritime mobile service system) or simply TOR)
AMTOR was developed in 1978 by Peter Martinez, G3PLX, with the first contact taking place in September 1978 with G3YYD on the 2m Amateur band. It was developed on homemade Motorola 6800-based microcomputers in assembler code. It was used extensively by amateur radio operators in the 1980s and 1990s but has now fallen out of use as improved PC-based data modes are now used and teleprinters became out of fashion.
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 »
AMTOR protocol utilizes a 7-bit code for each character, with each code-word having four mark and three space bits. If the received code does not match a four-to-three (4:3) ratio, the receiver assumes an error has occurred. In error detection mode, the code word will be dropped; in automatic repeat request mode, the receiver requests that the original data be resent.
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 translated the best possible way because no repetition can be requested. Therefore, receive quality is not generally very good.