26 April 2015

RTTY BAUDOT 50/450


The original (or "Baudot") radioteletype system is based almost invariably on the Baudot code or ITA-2 5 bit alphabet. The link is based on character asynchronous transmission with 1 start bit and 1, 1.5 or 2 stop bits. Transmitter modulation is normally FSK (F1B). Occasionally, an AFSK signal modulating an RF carrier (A2B, F2B) is used on VHF or UHF frequencies. Standard transmission speeds are 45.45, 50, 75, 100, 150 and 300 baud.
The word Teletype was a trademark of the Teletype Corporation, so the terms "TTY", "RTTY","RATT" and "teleprinter" are usually used to describe a generic device without reference to a particular manufacturer.

Common carrier shifts are 85 Hz (used on LF and VLF frequencies), 170 Hz, 425 Hz, 450 Hz and 850 Hz, although some stations use non-standard shifts. There are variations of the standard Baudot alphabet to cover languages written in Cyrillic, Arabic, Greek etc., using special techniques.


The correct measurement of the baudrate can't be done with the usual methods (ADP or quadrature AM detector) since we obtain a wrong value; as usual, the confirm comes from the raster-scan method. 

wrong manipulation speed from ADP
RTTY is an start-stop transmission (async), and the period is 1.5 stop bits: this is clearly visible  in the scan raster

manipulation speed from the raster

Given the FSK mode we have only two symbols, so baudrate (symbol per second) is equal to the speed in bps (bit per second). Some combinations of speed and shift are standardized for specific services using the original radioteletype system, see:

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