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Radioteletype

[rey-dee-oh-tel-i-tahyp]
Radioteletype (RTTY) is a telecommunications system consisting of two or more teleprinters using radio as the transmission medium.

The term radioteletype is used to describe:

  • either the entire family of systems connecting two or more teleprinters over radio, regardless of alphabet, link system or modulation,
  • or specifically the original radioteletype system, sometime described as "Baudot".

History

Landline teleprinter operations began in 1849 when a circuit was put in service between Philadelphia and New York City. Emile Baudot designed a system using a five unit code in 1874 that is still in use today. Teleprinter system design was gradually improved until, at the beginning of World War II, it represented the principal distribution method used by the news services.

Radioteletype evolved from these earlier landline teleprinter operations. Commercial RTTY systems were in active service between San Francisco and Honolulu as early as April 1932 and between San Francisco and New York City by 1934. The US Military used radioteletype in the 1930s and expanded this usage during World War II. The Navy called radioteletype RATT and the Army Signal Corps called radioteletype SCRT, an abbreviation of Single-Channel Radio Teletype. The Military used frequency shift keying technology and this technology proved very reliable even over long distances.

Starting in 1980s, teleprinters were replaced with computers running teleprinter emulation software.

Technical description of RTTY

A radioteletype station consists of three distinct parts: The Teletype or teleprinter, the modem and the radio.

The Teletype or teleprinter is an electromechanical or electronic device. 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.

Electromechanical teleprinters were quite heavy, complex and noisy and they have been replaced with electronic units. The teleprinter includes a keyboard, which is the main means of entering text and a printer or visual display unit (VDU). An alternative input device is a perforated tape reader and, more recently, computer storage media (floppy disks). Alternative output devices are tape perforators and computer storage media.

The line output of a teleprinter can be at either digital logic levels (a +5V signifies a logical "1" or mark and 0V signifies a logical "0" or space) or line levels (-80V signifies a "1" and +80V a "0"). When no traffic is passed, the line idles at the "mark" state.

When a key of the teleprinter keyboard is pressed, a 5-bit character is generated. The teleprinter converts it to serial format and transmits a sequence of a start bit (a logical 0 or space), then one after the other the 5 data bits, finishing with a stop bit (a logical 1 or mark, lasting 1, 1.5 or 2 bits). When a sequence of start bit, 5 data bits and stop bit arrives at the input of the teleprinter, it is converted to a 5-bit word and passed to the printer or VDU. In electromechanical teleprinters these functions required complicated electromechanical devices, but they are easily implemented with standard digital electronics using shift registers. Special IC's have been developed for this function, for example the 6402 and 6403. These are stand-alone UART devices, similar to computer serial port peripherals.

The 5 data bits allow for only 32 different codes, which cannot accommodate the 26 letters, 10 figures, space, a few punctuation marks and the required control codes, such as carriage return, new line, bell, etc. To overcome this limitation, the teleprinter has two states, the unshifted or letters state and the shifted or numbers or figures state. The change from one state to the other takes place when the special control codes LETTERS and FIGURES are sent from the keyboard or received from the line. In the letters state the teleprinter prints the letters and space while in the shifted state it prints the numerals and punctuation marks. Teleprinters for languages using other alphabets use also an additional third shift state, in which they print letters in the alternative alphabet.

The modem is sometimes called the terminal unit and is an electronic device which is connected between the teleprinter and the radio transmitter and receiver. The transmitting part of the modem converts the digital signal transmitted by the teleprinter or tape reader to one or the other of a pair of audio frequency tones. One of the tones corresponds to the mark condition and the other to the space condition. These audio tones, then, modulate an SSB transmitter to produce the final audio-frequency shift keying (AFSK) radio frequency signal. Some transmitters are capable of direct frequency-shift keying (FSK) as they can directly accept the digital signal and change their transmitting frequency according to the mark or space input state. In this case the transmitting part of the modem is bypassed.

On reception, the FSK signal is converted to the original tones by mixing the FSK signal with a local oscillator called the BFO or beat frequency oscillator. These tones are fed to the demodulator part of the modem, which processes them through a series of filters and detectors to recreate the original digital signal. The FSK signals are audible on a communications radio receiver equipped with a BFO (beat frequency oscillator), and have a distinctive "beedle-eeeedle-eedle-eee" sound, usually starting and ending on one of the two tones ("idle on mark").

From this analysis, it is clear that the transmission speed is a characteristic of the teleprinter while the shift (the difference between the tones representing mark and space) is a characteristic of the modem. Electronic teleprinters can readily operate in a variety of speeds, but mechanical teleprinters require the change of gears in order to operate at different speeds.

Today, both functions can be performed with modern computers equipped with digital signal processors or sound cards. The sound card performs the functions of the modem and the CPU performs the processing of the digital bits. This approach is very common in amateur radio, using specialized computer programs like MMTTY or MixW.

Before the computer mass storage era, most RTTY stations stored text on paper tape using paper tape punchers and readers. The operator would type the message on the TTY keyboard and punch the code onto the tape. The tape could then be transmitted at a steady, high rate, without typing errors. A tape could be reused, and in some cases - especially for use with ASCII on NC Machines - might be made of plastic or even very thin metal material in order to be reused many times.

The most common test signal is a series of "RYRYRY" characters, as these form an alternating tone pattern exercising all bits and are easily-recognized. Pangrams are also transmitted on RTTY circuits as test messages, the most common one being "The quick brown fox jumps over the lazy dog", and in French circuits, "Voyez le brick géant que j'examine près du wharf"

Technical specification

The original (or "Baudot") radioteletype system is based almost invariably on the Baudot 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 FSK (F1B). AFSK modulation (A2B, F2B) is used occasionally on VHF and 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.
  • In the past radio amateurs 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, although few of them remain active in this mode.
  • 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.

Early Amateur Radioteletype History

After World War II, amateur radio operators in the United States started to receive obsolete but usable Teletype Model 26 equipment from commercial operators with the understanding that this equipment would not be used for or returned to commercial service. US Amateur Radio operation began on 2 meters using audio frequency shift keying (AFSK). Operation on 80 meters, 40 meters and the other High Frequency (HF) amateur radio bands was initially accomplished using make and break keying since frequency shift keying (FSK) was not yet authorized. In early 1949, the first transcontinental two-way RTTY QSO was accomplished on 11 meters using AFSK between W1AW and W6PSW. FSK continued to remain off-limits on HF until February, 1953 when the FCC amended Part 12 of the Regulations. The amended Regulations permitted FSK in the non-voice parts of the 80, 40 and 20 meter bands and also specified the use of single channel 60 words-per-minute five unit code corresponding to ITA2. A shift of 850 hertz plus or minus 50 hertz was specified. Amateur Radio operators also had to identify their station callsign at the beginning and the end of each transmission and at ten minute intervals using International Morse Code. Use of this wide shift proved to be a problem for Amateur Radio operations. Commercial operators had already discovered that narrow shift worked best on the HF bands. After investigation and a petition to the FCC, Part 12 was amended, in March 1956, to allow Amateur Radio Operators to use any shift that was less than 900 hertz.

By the late 1950s, Amateur Radio operators outside of Canada and the United States began to acquire surplus teleprinter and receive permission to get on the air. The first recorded RTTY QSO in the UK occurred in September 1959 between G2UK and G3CQE. A few weeks later, G3CQE had the first G/VE RTTY QSO with VE7KX. This was quickly followed up by G3CQE QSOs with VK3KF and ZL3HJ. Information on how to acquire surplus teleprinter equipment continued to spread and before long it was possible to work all continents on RTTY.

During the early days of Amateur RTTY, the Worked All Continents – RTTY Award was conceived by the RTTY Society of Southern California and issued by RTTY Journal. The first Amateur Radio station to achieve this WAC – RTTY Award was VE7KX.. The first stations recognized as having achieved single band WAC RTTY were W1MX (3.5 MHz); DL0TD (7.0 MHz); K3SWZ (14.0 MHz); W0MT (21.0 MHz) and FG7XT (28.0 MHz). The ARRL began issuing WAC RTTY certificates in 1969.

By the early 1970s, Amateur Radio RTTY had spread around the world and it was finally possible to work more than 100 countries via RTTY. FG7XT was the first Amateur Radio station to claim to achieve this honor. However, Jean did not submit his QSL cards for independent review. ON4BX, in 1971, was the first Amateur Radio station to submit his cards to the DX Editor of RTTY Journal and to achieve this honor. The ARRL began issuing DXCC RTTY Awards on November 1, 1976. Prior to that date, an award for working more than 100 countries on RTTY was only available via RTTY Journal.

On January 7, 1972, the FCC amended Part 97 to allow faster RTTY speeds. Four standard RTTY speeds were authorized, namely, 60 (45 baud), 67 (50 baud), 75 (56.25 baud) and 100 (75 baud) words per minute. Many Amateur Radio operators had equipment that was capable of being upgraded to 75 and 100 words per minute by changing teleprinter gears. While there was an initial interest in 100 words per minute operation, many Amateur Radio operators moved back to 60 words per minute. Some of the reasons for the failure of 100 words per minute HF RTTY included poor operation of improperly maintained mechanical teleprinters, narrow bandwidth terminal units, continued use of 170 Hz shift at 100 words per minute and excessive error rates due to multipath distortion and the nature of ionospheric propagation.

The FCC approved the use of ASCII by Amateur Radio stations on March 17, 1980 with speeds up to 300 baud from 3.5 to 21.25 MHz and 1200 baud between 28 and 225 MHz. Speeds up to 19.2 kilobaud was authorized on Amateur frequencies above 420 MHz.

The requirement for Amateur Radio operators in the United States to identify their station callsign at the beginning and the end of each digital transmission and at ten minute intervals using International Morse Code was finally lifted by the FCC on June 15, 1983.

Slow by modern standards

RTTY is extremely slow by modern standards; a typical baud rate for RTTY operation is 45.45 baud (approximately 60 words per minute). This is one reason that RTTY has declined in commercial popularity, as faster, computerized transmission modes were developed, using less-expensive equipment.

The combination of low baud rate with robust FSK modulation makes RTTY highly resistant to most forms of radio interference, second only to Morse code. Part of this is due to the fact that FSK, like FM, always operates at maximum power. FSK is the single most demanding mode for transmitter equipment.

Spectrum efficiency



Primary users

Principally users that need robust shortwave communications

  • All Military Departments, all over the world, (using cryptography)
  • Diplomatic services, all over the world, (using cryptography)
  • Weather reports are transmitted by the US Coast Guard nearly continuously
  • RTTY systems are also fielded by amateur radio operators, and are popular for long-distance contacts

A very regular service transmitting RTTY meteorological information is the German Meteorological Service (Deutscher Wetterdienst or DWD) The DWD regularly transmit two programs on various frequencies on LF and HF in standard RTTY (ITA-2 alphabet). The list of callsigns, frequencies, baudrates and shifts (current January 2008) are as follows:

Callsign Frequency speed/shift
DDH47 147.3 kHz 50 baud/85 Hz
DDK2 4583 kHz 50 baud/450 Hz
DDH7 7646 kHz 50 baud/450 Hz
DDK9 10100.8 kHz 50 baud/450 Hz
DDH9 11039 kHz 50 baud/450 Hz
DDH8 14467.3 kHz 50 baud/450 Hz

The DWD signals can be easily received in Europe, N. Africa and parts of N. America.

Pronunciation

The pronunciation of RTTY is disputed

  • In very few applications, notably the U.S. military in WWII and the fifties, radio teletype is known by the acronym RATT (RAdio TeleType) rather than RTTY.
  • Some radio amateurs pronounce RTTY not by its initials but as "ritty".

Spectrum usage


Media

References

See also

Related technical references

Digital HF radio communications systems

  • Sailmail, a commercial HF mail system
  • SITOR, (SImplex Teleprinting Over Radio) a commercial RTTY variant with error control (the Radio Amateur version is called "AMTOR")
  • PACTOR, a packet SITOR variant, developed by Radio Amateurs in Germany
  • Hellschreiber, a FAX-RTTY hybrid, very old system from the 1930s
  • ACARS, used by commercial aviation – packet based
  • Navtex, used for maritime weather reports, with FEC error control code,
  • MT63, developed and used by Radio Amateurs and some government agencies
  • Olivia MFSK from the creator of MT63
  • PSK31 & PSK63 developed and used by Radio Amateurs
  • MFSK including COQUELET and PICCOLO, also referred to generically as Polytone
  • CLOVER2000 developed by HAL company, USA, for Radio Amateur application
  • Q15X25, a Radio Amateur created packet format(AX25), similar to the commercial X25 standard

Further reading


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