RTTY

The RTTY mode is a traditional mode used by radio-amateurs like CW, AMTOR and ASCII. RTTY, ASCII 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).
   
    This mode is included into the software MULTIPSK by Patrick F6CTE, there is the description of this mode and its variants as well as the specifications coming from his software :

Click on the copy of the screen in order to listen to the corresponding audio signal.

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 « é »  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 and more rarely 50 bauds with a 425 Hz shift. Transmissions of Navy Mars Hams are done in 75 bauds, shift: 170, 183 or 200 Hz.

In commercial RTTY, the transmission speed may be, theoretically, 50, 75 or 100 bauds with a 85, 425 Hz, 450 or 850 Hz shift. In reality, only 50 and 75 bauds are available (in fact only 50 bauds).

Click on the copy of the screen in order to listen to the corresponding audio signal .

On several HF frequencies (for example DDK2 on 4583 Khz), SYNOP/SHIP WX information is transmitted in RTTY 50 bauds. This information can be decoded by this program, the WX station position being displayed on a map.

For 45 to 100 bauds speeds, mapping (ITA 2 standard) between the RTTY Baudot codes and the received or transmitted letters, figures and signs, as considered in this program, is as follows:

The small letters keystroke at the editor are translated in capital letters.

The 27 and 31 codes (shift to figures or to letters) are only managed by the program and are transparent to the user.

The 2 code (LF) is ignored because redundant.