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 ARQ (called also SITOR-A for the navy (as a maritime mobile service system) or simply TOR)
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 translated the best possible way because no repetition can be requested. Therefore, receive quality is not generally very good.
MORE ON AMTOR FEC SITOR-B for the Navy CLICK HERE
+Handles ionspheric effects fairly well
-Not used much
-not used much
ARQ MODE or SITOR-A
FFT Screen Shot
Mode Audio Sample