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Overview

RTTY (Radioteletype) is a historic telecommunications system using the 5-bit Baudot character encoding. Operating at 45.45 baud (approximately 60 words per minute), RTTY has been used since the 1930s for maritime, aeronautical, and amateur radio communications.

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Technical Specifications

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Frequencies and Timing

Baud Rate: 45.45 baud
Data Bits: 5 (Baudot code)
Stop Bits: 1.5
Default Mark Frequency: 1585 Hz
Default Space Frequency: 1415 Hz
Frequency Shift: 170 Hz (standard)
From minimodem.c:819-826 and 922-930:
  • Data rate: 45.45 baud (exactly)
  • Character encoding: 5-bit Baudot
  • Stop bits: 1.5 (configurable)
  • Default shift: 170 Hz
  • Frequencies are configurable with --mark and --space options

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Framing Parameters

  • Start bit: 1 (space condition)
  • Data bits: 5 (LSB first)
  • Stop bits: 1.5 (mark condition)
  • Total character time: ~22 ms per character

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Baudot Character Encoding

RTTY uses the 5-bit Baudot (ITA-2) character set, which has two modes:
  • Letters mode: A-Z and some punctuation
  • Figures mode: Numbers and symbols
Special shift characters switch between modes:
  • LTRS (0x1F): Switch to letters mode
  • FIGS (0x1B): Switch to figures mode

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Common Applications

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Amateur Radio

RTTY is still actively used in amateur radio, especially for:
  • Contests and DX (long distance) communication
  • Emergency communications
  • Automatic relay stations

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Maritime Communications

Historically used for:
  • Ship-to-shore messages
  • Weather broadcasts
  • Navigational warnings

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Aeronautical Services

  • NOTAMs (Notices to Airmen)
  • Weather reports
  • Flight plan filing

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Usage Examples

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Receiving RTTY

minimodem --rx rtty

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Transmitting RTTY

echo "CQ CQ DE CALLSIGN" | minimodem --tx rtty

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Advanced Configuration

Custom Frequency Pair (Common Amateur Radio)
# 2125/1955 Hz (170 Hz shift) - popular on HF
minimodem --rx rtty -M 2125 -S 1955
200 Hz Shift (Amateur Radio Alternative)
# Some stations use 200 Hz shift
minimodem --rx rtty -M 1600 -S 1400
With Audio Level Adjustment
minimodem --rx rtty --volume 1.5 -q

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Standard Frequency Shifts

RTTY can use various frequency shifts depending on the service:
ShiftApplicationCommon Frequencies
170 HzStandard amateur, commercialMark 2125, Space 1955
200 HzAlternative amateurMark 1600, Space 1400
425 HzSome military systemsVaries
850 HzWide shift systemsVaries
The 170 Hz shift is the most common standard, especially in amateur radio. minimodem defaults to Mark=1585 Hz and Space=1415 Hz, but you should specify frequencies based on your application.

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Technical Details

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Timing and Synchronization

Each RTTY character consists of:
  1. Start bit: Space (high frequency) for 1 bit period (22 ms)
  2. 5 data bits: Mark or space for data (110 ms total)
  3. Stop bits: Mark (low frequency) for 1.5 bit periods (33 ms)
Total character time: ~165 ms (approximately 6 characters/second)

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Why 45.45 Baud?

The 45.45 baud rate comes from mechanical teleprinter constraints:
  • 60 WPM (words per minute) standard typing speed
  • 22.0 milliseconds per bit period
  • 1/0.022 = 45.45… baud

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Stop Bit Duration

The 1.5 stop bits provide:
  • Time for mechanical printers to reset
  • Synchronization margin
  • Improved reliability on poor links
Modern systems may use 1.0 or 2.0 stop bits. minimodem defaults to 1.5 for RTTY mode.

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Character Set Limitations

Baudot’s 5-bit encoding supports only:
  • 26 letters (uppercase only)
  • 10 digits
  • Limited punctuation
  • No lowercase
  • Special control characters (LTRS, FIGS, space, carriage return, line feed)

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Common Characters

Letters Mode: 
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Figures Mode: 
1 2 3 4 5 6 7 8 9 0 - ? : ( ) . , ' /

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Signal Quality Requirements

  • Minimum SNR: ~6 dB for reliable copy
  • Frequency stability: ±15 Hz maximum drift
  • Audio bandwidth: 1000-2500 Hz typical
  • Sample rate: 8 kHz minimum recommended

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Advantages of RTTY

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Robustness

  • Works well on noisy HF radio links
  • Narrow bandwidth (less than 500 Hz)
  • Simple error detection (invalid Baudot codes)

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Simplicity

  • Easy to implement
  • Low CPU requirements
  • Audible tones aid debugging
  • Human-readable at low speed

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Compatibility

  • Decades of installed base
  • Standard in amateur radio
  • Compatible with mechanical teleprinters

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Comparison with Other Protocols

ProtocolBaud RateData BitsStop BitsTypical Use
RTTY45.455 (Baudot)1.5Amateur radio, maritime
TDD45.455 (Baudot)2.0Telecommunications for deaf
Bell 1033008 (ASCII)1.0Modems, data links
Bell 20212008 (ASCII)1.0Caller ID, packet radio

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Troubleshooting

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Garbled Text

  • Check frequency shift (170 Hz vs 200 Hz)
  • Verify mark/space frequencies
  • Adjust audio levels
  • Check for interference

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No Output

  • Verify correct mark/space polarity
  • Check audio input levels
  • Ensure proper frequency shift setting

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Wrong Characters

  • May be in wrong shift mode (LTRS vs FIGS)
  • Check for inverted mark/space
  • Verify 45.45 baud rate
  • TDD - Uses Baudot at 45.45 baud with different frequencies
  • Bell 103 - Faster 8-bit protocol
  • Bell 202 - High-speed alternative

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See Also

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