, AMTOR , ASCII, RTTY,  and 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).
In RTTY 50 bauds with a shift of 450 Hz (and secondarily in 100 bauds), it is possible to decode the SYNOP/SHIP HF transmissions (WX information)

Description :
Baud rate : 45. A character is composed of a start bit (1 space ), 5 bits and a stop bit (1.5 mark )
Speed : 60 wpm
Modulation : FSK two tones ( mark and space , "mark" high) with a shift between tones of 23 Hz, 170 Hz (standard shift), 200 Hz or 850 Hz,
Note: in RTTY 45 bauds, shift=23 Hz, the modulation is specific and called "MSK" (Minimum Shift Keying). The demodulation is not done with two filters (as in FSK) but is done using phase measures.
The RTTY 45 bauds, shift 23 Hz MSK mode, from MMTTY, has been created by Makoto Mori, JE3HHT around 2003.
Receive mode : USB

Character set :ITA2 (32 letters and 32 figures) without error correction character ( fixed length characters). Small characters are transformed in capital characters and special characters as are transformed in standard capital (in the example: E ). The "Unperforated tape" character is obtained through the ASCII valor CHR(1). It might be used for a characters set change (russian characters to latin characters for example).
Shape of pulse : rectangular
Bandwidth : for the standard shift of 170 Hz, about 600 Hz (due to rectangular shape),
Demodulation : non coherent,
Synchronization : on start
Correction code : no
Convolution code : no
Interleaving : no
Pmean/Ppeak : 1
Lowest S/N : -5.5 dB
In 50 bauds, the speed is 67 wpm and the lowest S/N is -5 dB.
In 75 bauds, the speed is 100 wpm and the lowest S/N is -3.5 dB.
The 850 Hz shift mode is intended to old teleprinters.
For radio amateurs, the transmission speed is 45 bauds with a 170 Hz shift
+Very Popular
-Not all that senstive
-Can handle ionspheric effects pretty well.

Mode Menu
Mode Audio Sample
FFT Window Screenshot