Radioteletype
Encyclopedia
Radioteletype is a telecommunication
s system consisting originally of two or more electromechanical
teleprinter
s in different locations, later superseded by personal computer
s (PCs) running software to emulate teleprinters
, connected by radio
rather than a wired link.
The term radioteletype is used to describe:
In some applications, notably military and government, radioteletype is known by the acronym RATT (Radio Automatic Teletype).
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. The US Navy Department successfully tested printing telegraphy between an airplane and ground radio station in August 1922. Later that year, the Radio Corporation of America successfully tested printing telegraphy via their Chatham, MA radio station to the R.M.S. Majestic. An early implementation of the Radioteletype was the Watsongraph, named after Detroit inventor Glenn Watson in March 1931. 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 (Radio Automatic Teletype) 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.
From the 1980s, teleprinters were replaced by computers running teleprinter emulation software.
, 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 (such as 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 (+5 V signifies a logical "1" or mark and 0 V signifies a logical "0" or space) or line levels (-80 V signifies a "1" and +80 V 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 register
s. 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 also use 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 transceiver
. 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, traditionally 2295/2125 Hz (US) or 2125/1955 Hz (Europe). 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, 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. Pangram
s 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"
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:
s 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. "The Amateur Radioteletype and VHF Society" was founded in 1946 in Woodside, NY. This organization soon changed its name to "The VHF Teletype Society" and started US Amateur Radio operations on 2 meters using audio frequency shift keying (AFSK). The first two-way amateur radioteletype QSO
of record took place in May 1946 between Dave Winters, W2AUF, Brooklyn,NY and W2BFD, John Evans Williams, Woodside Long Island, NY. On the west coast, amateur RTTY also started on 2 meters. 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. While QSOs could be accomplished, it was quickly realized that FSK was technically superior to make and break keying. Due to the efforts of Merrill Swan, W6AEE, of the "The RTTY Society of Southern California" publisher of RTTY and Wayne Green, W2NSD, of CQ Magazine, Amateur Radio operators successfully petitioned the U.S. Federal Communications Commission
(FCC) to amend Part 12 of the Regulations, which was effective on February 20, 1953. 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.
The first RTTY Contest was held by the RTTY Society of Southern California from October 31 to November 1, 1953. Named the RTTY Sweepstakes Contest, twenty nine participants exchanged messages that contained a serial number, originating station call, check or RST report of two or three numbers, ARRL section of originator, local time (0000-2400 preferred) and date. Example: NR 23 W0BP CK MINN 1325 FEB 15. By the late 1950s, the contest exchange was expanded to include band used. Example: NR 23 W0BP CK MINN 1325 FEB 15 FORTY METERS. The contest was scored as follows: one point for each message sent and receipted entirely by RTTY and one point for each message received and acknowledged by RTTY. The final score was computed by multiplying the total number of message points by the number of ARRL sections worked. Two stations could exchange messages again on a different band for added points, but the section multiplier did not increase when the same section was reworked on a different band. Each DXCC entity was counted as an additional ARRL section for RTTY multiplier credit.
RTTY, later named RTTY Journal, also published the first listing of stations, mostly located in the continental US, that were interested in RTTY in 1956. Amateur Radio operators used this callbook information to contact other operators both inside and outside the United States. For example, the first recorded USA to New Zealand two-way RTTY QSO took place in 1956 between W0BP and ZL1WB.
By the late 1950s, new organizations focused on amateur radioteletype started to appear. The "British Amateur Radio Teletype Group", BARTG, now known as the "British Amateur Teledata Group" was formed in June 1959. The Florida RTTY Society was formed in September 1959. 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.
Amateur Radio operators used various equipment designs to get on the air using RTTY in the 1950s and 1960s. Amateurs used their existing receivers for RTTY operation but needed to add a terminal unit, sometimes called a demodulator, to convert the received audio signals to DC signals for the teleprinter.
Most of the terminal unit equipment used for receiving RTTY signals was homebuilt, using designs published in amateur radio publications. These original designs can be divided into two classes of terminal units: audio-type and intermediate frequency converters. The audio-type converters proved to be more popular with amateur radio operators. The Twin City, W2JAV and W2PAT designs are examples of typical terminal units that were used into the middle 1960s. The late 1960s and early 1970s saw the emergence of terminal units designed by W6FFC, such as the TT/L-2, ST-3, ST-5, and ST-6. These designs were first published in RTTY Journal starting in September 1967 and ending in 1970.
Amateur Radio operators needed to modify their transmitters to allow for HF RTTY operation. This was accomplished by adding a frequency shift keyer that used a diode to switch a capacitor in and out of the circuit, shifting the transmitter’s frequency in synchronism with the teleprinter signal changing from mark to space to mark. A very stable transmitter was required for RTTY. The typical frequency multiplication type transmitter that was popular in the 1950s and 1960s would be relatively stable on 80 meters but become progressively less stable on 40 meters, 20 meters and 15 meters. By the middle 1960s, transmitter designs were updated, mixing a crystal-controlled high frequency oscillator with a variable low frequency oscillator, resulting in better frequency stability across all Amateur Radio HF bands.
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.
In the 1950s through the 1970s, "RTTY art" was a popular on-air activity. It consisted of (sometimes very elaborate and artistic) pictures sent over rtty through the use of lengthy punched tape transmissions and then printed by the receiving station on paper.
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.
for Amateur operation of 45.45 baud (approximately 60 words per minute). It remains popular as a "keyboard to keyboard" mode in Amateur Radio. RTTY has declined in commercial popularity as faster, more reliable alternative data modes have become available, using satellite or other connections.
For its transmission speed, RTTY has low spectral efficiency
. The typical RTTY signal with 170 Hz shift at 45.45 baud requires around 250 Hz receiver bandwidth, over double that required by PSK31
. Because RTTY, using either AFSK or FSK
modulation, produces a waveform with constant power, a transmitter does not need to use a linear amplifier
, which is required for many digital transmission modes. A more efficient Class C amplifier may be used.
RTTY, using either AFSK or FSK modulation, is moderately resistant to vagaries of HF propagation and interference, however modern digital modes, such as MFSK
, use Forward Error Correction to provide much better data reliability.
A 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 May 2010) are as follows:
The DWD signals can be easily received in Europe, North Africa and parts of North America.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s system consisting originally of two or more electromechanical
Electromechanics
In engineering, electromechanics combines the sciences of electromagnetism, of electrical engineering and mechanics. Mechanical engineering in this context refers to the larger discipline which includes chemical engineering, and other related disciplines. Electrical engineering in this context...
teleprinter
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
s in different locations, later superseded by personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
s (PCs) running software to emulate teleprinters
Terminal emulator
A terminal emulator, terminal application, term, or tty for short, is a program that emulates a video terminal within some other display architecture....
, connected by radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
rather than a wired link.
The term radioteletype is used to describe:
- either the entire family of systems connecting two or more teleprinters or PCs using software to emulate teleprinters, over radio, regardless of alphabet, link system or modulation,
- or specifically the original radioteletype system, sometimes described as "BaudotBaudot codeThe Baudot code, invented by Émile Baudot, is a character set predating EBCDIC and ASCII. It was the predecessor to the International Telegraph Alphabet No 2 , the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of bits, sent over a...
".
In some applications, notably military and government, radioteletype is known by the acronym RATT (Radio Automatic Teletype).
History
Landline teleprinter operations began in 1849 when a circuit was put in service between Philadelphia and New York City. Émile BaudotÉmile Baudot
Jean-Maurice-Émile Baudot , French telegraph engineer and inventor of the first means of digital communication Baudot code, was one of the pioneers of telecommunications...
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. The US Navy Department successfully tested printing telegraphy between an airplane and ground radio station in August 1922. Later that year, the Radio Corporation of America successfully tested printing telegraphy via their Chatham, MA radio station to the R.M.S. Majestic. An early implementation of the Radioteletype was the Watsongraph, named after Detroit inventor Glenn Watson in March 1931. 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 (Radio Automatic Teletype) 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.
From the 1980s, teleprinters were replaced by computers running teleprinter emulation software.
Technical description of RTTY
A radioteletype station consists of three distinct parts: The Teletype or teleprinterTeleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
, the modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
and the radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
.
The Teletype or teleprinter is an electromechanical or electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
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
Punched tape
Punched tape or paper tape is an obsolete form of data storage, consisting of a long strip of paper in which holes are punched to store data...
reader and, more recently, computer storage media (such as 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 (+5 V signifies a logical "1" or mark and 0 V signifies a logical "0" or space) or line levels (-80 V signifies a "1" and +80 V 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
Serial communication
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
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 register
Shift register
In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, which has the output of any one but the last flip-flop connected to the "data" input of the next one in the chain, resulting in a circuit that shifts by one position the one-dimensional "bit array" stored in...
s. Special IC
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
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
Punctuation
Punctuation marks are symbols that indicate the structure and organization of written language, as well as intonation and pauses to be observed when reading aloud.In written English, punctuation is vital to disambiguate the meaning of sentences...
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 also use 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 transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
. 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
Audio frequency
An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...
tones, traditionally 2295/2125 Hz (US) or 2125/1955 Hz (Europe). One of the tones corresponds to the mark condition and the other to the space condition. These audio tones, then, modulate
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
an SSB
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
transmitter to produce the final audio-frequency shift keying (AFSK) radio frequency signal. Some transmitters are capable of direct frequency-shift keying
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
(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, 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
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
, 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
RY (test signal)
RYRYRYRY... is a character string that was widely used to test a five-level teleprinter or RTTY channel. The characters R and Y are "01010" and "10101" in 5-bit ITA2 code, also known as Baudot...
" characters, as these form an alternating tone pattern exercising all bits and are easily-recognized. Pangram
Pangram
A pangram , or holoalphabetic sentence, is a sentence using every letter of the alphabet at least once. Pangrams have been used to display typefaces, test equipment, and develop skills in handwriting, calligraphy, and keyboarding...
s are also transmitted on RTTY circuits as test messages, the most common one being "The quick brown fox jumps over the lazy dog
The quick brown fox jumps over the lazy dog
"The quick brown fox jumps over the lazy dog" is an English-language pangram, that is, a phrase that contains all of the letters of the alphabet. It has been used to test typewriters and computer keyboards, and in other applications involving all of the letters in the English alphabet...
", 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 codeBaudot code
The Baudot code, invented by Émile Baudot, is a character set predating EBCDIC and ASCII. It was the predecessor to the International Telegraph Alphabet No 2 , the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of bits, sent over a...
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
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
(F1B
Types of radio emissions
The International Telecommunication Union uses an internationally agreed system for classifying radio frequency signals. Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, and type of information transmitted on the carrier...
). 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 radioAmateur radioAmateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
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, although few of them remain active in this mode except in places like Africa or the South Pacific.
- Russian (and in the past, Soviet Union) merchant marine communications use 50 baud – 170 Hz.
- RTTY transmissions on LFLfLf or LF may stand for:* FlyNordic IATA airline designator* Nippon Broadcasting System, a radio station in Tokyo, Japan - JOLF* Lactoferrin, a protein* Laissez-faire* LeapFrog, an educational toy company...
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 operatorAmateur radio operator
An amateur radio operator is an individual who typically uses equipment at an amateur radio station to engage in two-way personal communications with other similar individuals on radio frequencies assigned to the amateur radio service. Amateur radio operators have been granted an amateur radio...
s 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. "The Amateur Radioteletype and VHF Society" was founded in 1946 in Woodside, NY. This organization soon changed its name to "The VHF Teletype Society" and started US Amateur Radio operations on 2 meters using audio frequency shift keying (AFSK). The first two-way amateur radioteletype QSO
Q code
The Q code is a standardized collection of three-letter message encodings, also known as a brevity code, all of which start with the letter "Q", initially developed for commercial radiotelegraph communication, and later adopted by other radio services, especially amateur radio...
of record took place in May 1946 between Dave Winters, W2AUF, Brooklyn,NY and W2BFD, John Evans Williams, Woodside Long Island, NY. On the west coast, amateur RTTY also started on 2 meters. 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. While QSOs could be accomplished, it was quickly realized that FSK was technically superior to make and break keying. Due to the efforts of Merrill Swan, W6AEE, of the "The RTTY Society of Southern California" publisher of RTTY and Wayne Green, W2NSD, of CQ Magazine, Amateur Radio operators successfully petitioned the U.S. Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) to amend Part 12 of the Regulations, which was effective on February 20, 1953. 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
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
. 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.
The first RTTY Contest was held by the RTTY Society of Southern California from October 31 to November 1, 1953. Named the RTTY Sweepstakes Contest, twenty nine participants exchanged messages that contained a serial number, originating station call, check or RST report of two or three numbers, ARRL section of originator, local time (0000-2400 preferred) and date. Example: NR 23 W0BP CK MINN 1325 FEB 15. By the late 1950s, the contest exchange was expanded to include band used. Example: NR 23 W0BP CK MINN 1325 FEB 15 FORTY METERS. The contest was scored as follows: one point for each message sent and receipted entirely by RTTY and one point for each message received and acknowledged by RTTY. The final score was computed by multiplying the total number of message points by the number of ARRL sections worked. Two stations could exchange messages again on a different band for added points, but the section multiplier did not increase when the same section was reworked on a different band. Each DXCC entity was counted as an additional ARRL section for RTTY multiplier credit.
RTTY, later named RTTY Journal, also published the first listing of stations, mostly located in the continental US, that were interested in RTTY in 1956. Amateur Radio operators used this callbook information to contact other operators both inside and outside the United States. For example, the first recorded USA to New Zealand two-way RTTY QSO took place in 1956 between W0BP and ZL1WB.
By the late 1950s, new organizations focused on amateur radioteletype started to appear. The "British Amateur Radio Teletype Group", BARTG, now known as the "British Amateur Teledata Group" was formed in June 1959. The Florida RTTY Society was formed in September 1959. 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.
Amateur Radio operators used various equipment designs to get on the air using RTTY in the 1950s and 1960s. Amateurs used their existing receivers for RTTY operation but needed to add a terminal unit, sometimes called a demodulator, to convert the received audio signals to DC signals for the teleprinter.
Most of the terminal unit equipment used for receiving RTTY signals was homebuilt, using designs published in amateur radio publications. These original designs can be divided into two classes of terminal units: audio-type and intermediate frequency converters. The audio-type converters proved to be more popular with amateur radio operators. The Twin City, W2JAV and W2PAT designs are examples of typical terminal units that were used into the middle 1960s. The late 1960s and early 1970s saw the emergence of terminal units designed by W6FFC, such as the TT/L-2, ST-3, ST-5, and ST-6. These designs were first published in RTTY Journal starting in September 1967 and ending in 1970.
Amateur Radio operators needed to modify their transmitters to allow for HF RTTY operation. This was accomplished by adding a frequency shift keyer that used a diode to switch a capacitor in and out of the circuit, shifting the transmitter’s frequency in synchronism with the teleprinter signal changing from mark to space to mark. A very stable transmitter was required for RTTY. The typical frequency multiplication type transmitter that was popular in the 1950s and 1960s would be relatively stable on 80 meters but become progressively less stable on 40 meters, 20 meters and 15 meters. By the middle 1960s, transmitter designs were updated, mixing a crystal-controlled high frequency oscillator with a variable low frequency oscillator, resulting in better frequency stability across all Amateur Radio HF bands.
During the early days of Amateur RTTY, the Worked All Continents
Worked All Continents
Worked All Continents, or WAC, is an amateur radio award given to those amateur radio operators who successfully complete two-way amateur radio communications with other amateur radio stations located in each of six continental areas of the world...
– 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.
In the 1950s through the 1970s, "RTTY art" was a popular on-air activity. It consisted of (sometimes very elaborate and artistic) pictures sent over rtty through the use of lengthy punched tape transmissions and then printed by the receiving station on paper.
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
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
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.
Comparison with Other Modes
RTTY has a typical baud rateBaud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
for Amateur operation of 45.45 baud (approximately 60 words per minute). It remains popular as a "keyboard to keyboard" mode in Amateur Radio. RTTY has declined in commercial popularity as faster, more reliable alternative data modes have become available, using satellite or other connections.
For its transmission speed, RTTY has low spectral efficiency
Spectral efficiency
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system...
. The typical RTTY signal with 170 Hz shift at 45.45 baud requires around 250 Hz receiver bandwidth, over double that required by PSK31
PSK31
PSK31 or "Phase Shift Keying, 31 Baud" is a digital radio modulation mode, used primarily in the amateur radio field to conduct real-time keyboard-to-keyboard informal text chat between amateur radio operators.- History :...
. Because RTTY, using either AFSK or FSK
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
modulation, produces a waveform with constant power, a transmitter does not need to use a linear amplifier
Linear amplifier
A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio...
, which is required for many digital transmission modes. A more efficient Class C amplifier may be used.
RTTY, using either AFSK or FSK modulation, is moderately resistant to vagaries of HF propagation and interference, however modern digital modes, such as MFSK
Multiple frequency-shift keying
Multiple frequency-shift keying is a variation of frequency-shift keying that uses more than two frequencies. MFSK is a form of M-ary orthogonal modulation, where each symbol consists of one element from an alphabet of orthogonal waveforms...
, use Forward Error Correction to provide much better data reliability.
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 operatorAmateur radio operatorAn amateur radio operator is an individual who typically uses equipment at an amateur radio station to engage in two-way personal communications with other similar individuals on radio frequencies assigned to the amateur radio service. Amateur radio operators have been granted an amateur radio...
s, and are popular for long-distance contacts
A 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
Lf
Lf or LF may stand for:* FlyNordic IATA airline designator* Nippon Broadcasting System, a radio station in Tokyo, Japan - JOLF* Lactoferrin, a protein* Laissez-faire* LeapFrog, an educational toy company...
and HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
in standard RTTY (ITA-2 alphabet). The list of callsigns, frequencies, baudrates and shifts (current May 2010) 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, North Africa and parts of North America.
Pronunciation
RTTY (in English) may be spoken as "radioteletype", by its letters: R-T-T-Y, or simply as "ritty".Media
Related technical references
- Asynchronous serial communication
- ModemModemA modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
- TeleprinterTeleprinterA teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
- Telex
- Types of radio emissionsTypes of radio emissionsThe International Telecommunication Union uses an internationally agreed system for classifying radio frequency signals. Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, and type of information transmitted on the carrier...
- UART
Digital HF radio communications systems
- ACARS, used by commercial aviation – packet based
- CLOVER2000CLOVER2000CLOVER is the name of a series or class of modem modulation techniques specifically designed for use over high frequency radio systems....
developed by HAL company, USA, for Radio Amateur application - HellschreiberHellschreiberThe Hellschreiber or Feldhellschreiber is a facsimile-based teleprinter invented by Rudolf Hell. Compared to contemporary teleprinters that were based on typewriter systems, the Hellschreiber was much simpler and more robust, with only two moving parts...
, a FAX-RTTY hybrid, very old system from the 1930s - MFSK including COQUELET and PICCOLO, also referred to generically as Polytone
- MT63MT63MT63 is a digital radio modulation mode for transmission in high-noise situations developed by Pawel Jalocha SP9VRC. MT63 is designed for keyboard-to-keyboard conversation modes, on HF HAM bands.- Features and Attributes :...
, developed and used by Radio Amateurs and some government agencies - NavtexNavtexNAVTEX is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships...
, used for maritime weather reports, with FEC error control code, - Olivia MFSKOlivia MFSKOlivia MFSK is an amateur radioteletype protocol designed to work in difficult conditions on shortwave bands. The signal can still be properly copied when it is buried 10 dB below the noise floor...
from the creator of MT63 - PACTORPACTORPACTOR is a radio modulation mode used by amateur radio operators, marine radio stations, and radio stations in isolated areas to send and receive digital information via radio. A robust network of PACTOR stations has been established to relay data between radio stations and the Internet,...
, a packet SITOR variant, developed by Radio Amateurs in Germany - PSK31PSK31PSK31 or "Phase Shift Keying, 31 Baud" is a digital radio modulation mode, used primarily in the amateur radio field to conduct real-time keyboard-to-keyboard informal text chat between amateur radio operators.- History :...
& PSK63PSK63PSK63 or Phase Shift Keying, 63 Baud, is a digital radio modulation mode used primarily in the amateur radio field to conduct real-time keyboard-to-keyboard informal text chat between amateur radio operators.- History :...
developed and used by Radio Amateurs - Q15X25Q15X25Q15X25 is a communications protocol for sending data over a radio link. It was designed by amateur radio operator Pawel Jalocha, SP9VRC, to be an open communications standard. Like all amateur radio communications modes, this protocol uses open transmissions which can be received and decoded by...
, a Radio Amateur created packet format(AX25), similar to the commercial X25 standard - SailmailSailmailSailMail is radio based e-mail system designed for yacht owners operating outside of areas of internet service. Much of its underlying technology is built upon the Winlink software originally developed by amateur radio enthusiasts....
, a commercial HF mail system - SITORSITORSITOR is a system for transmitting text messages. Although it uses the same frequency-shift keying modulation used by regular radioteletype , SITOR uses error detection, redundancy, and/or retransmission to improve reliability.There are two SITOR modes:* SITOR-A is used for point to point links...
, (SImplex Teleprinting Over Radio) a commercial RTTY variant with error control (the Radio Amateur version is called "AMTOR")
Further reading
- Getting Started on RTTY, Getting started on RTTY using MMTTY
- RTTY.COM, a repository of Amateur RTTY information
- British Amateur Radio Teledata Group (BARTG)
- RTTY Demodulator Development by Kok Chen, W7AY. A technology review for the early period until ca 1965.