RTTY, short for Radio Teletype, is one of the oldest digital modes in amateur radio and wireless communications. It is based on the transmission of text characters over the air using a simple form of frequency shift keying (FSK). RTTY was originally designed for use with mechanical teleprinters, but has evolved over time into a fully digital mode.

 

Basic principle

• RTTY works by switching between two frequencies, each corresponding to a bit value: one frequency for '0' (mark) and another for '1' (space).
• The binary bits are grouped into combinations of five bits to encode letters, numbers and control characters. This is known as the Baudot code.

 

Baudot code

• The Baudot code is a 5-bit code that allows for 32 different characters. As this is not enough for all letters, numbers and characters, RTTY uses two 'layers' or modes:
  • LTRS (letters) for letters.
  • FIGS (figures) for numbers and special characters.
• A shift character switches between the two modes.

 

Modulation

• FSK (Frequency Shift Keying): In the original FSK modulation, the signal alternates between two fixed frequencies. Modern RTTY signals often use audio FSK (AFSK), in which the frequency shift occurs in the audio frequency before being transmitted via SSB (single sideband).

 

Transmission speed

• Transmission speed is measured in baud. The most common speed in amateur radio is 45.45 baud, which is about 60 words per minute.
• Typical frequency offsets (shifts) between mark and space are 170 Hz, but there are variants with other offsets, such as 85 Hz or 425 Hz.

 

Uses

• RTTY is primarily used in amateur radio for the exchange of short messages, contests and DX communications.
• Historically, it was also used for commercial, maritime and military communications before more modern digital modes were developed.

 

Pros and cons

• Advantages:
  • RTTY is easy to implement and has a long history in amateur radio.
  • It provides reliable transmission over long distances and is particularly useful where bandwidth is limited and signals are weak.

 

• Disadvantages:
  • Compared to newer digital modes, RTTY is less efficient and more susceptible to interference and multipath.
  • The fixed bandwidth and lower data rate limit the transmission rate.

 

Technical details

• Filtering: As RTTY operates in a narrow frequency band, precise filtering is essential to minimise interference and ensure clear decoding.
• Software: Today RTTY is mainly operated using software such as FLdigi, MMTTY or MixW on computers connected to the radio via a sound card.

 

RTTY in competition

• RTTY contests are very popular in amateur radio, where participants try to make as many connections as possible in a given time.

• These contests are a good way to improve RTTY skills and provide a challenge, especially in poor conditions.

 

In summary, RTTY is a classic and still widely used digital mode, characterised by simplicity and reliability. It has played an important role in the history of radio communications and remains an integral part of amateur radio.