Rotty - Technical Specification

Technical Specification

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.

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.

Some combinations of speed and shift are standardized for specific services using the original radioteletype system:

• Amateur radio transmissions are almost always 45.45 baud – 170 Hz, although 75 baud activity is being promoted by BARTG in the form of 4-hour contests.
• Radio amateurs have experimented with ITA-5 (7-bit ASCII) alphabet transmissions at 110 baud – 170 Hz.
• NATO military services use 75 or 100 baud – 850 Hz.
• A few naval stations still use RTTY without encryption for CARB (channel availability broadcasts).
• Commercial, diplomatic and weather services prefer 50 baud – 425 or 450 Hz.
• Russian (and in the past, Soviet Union) merchant marine communications use 50 baud – 170 Hz.
• RTTY transmissions on LF and VLF frequencies use a narrow shift of 85 Hz, due to the limited bandwidth of the antennas.