Radioteletype
Radioteletype (RTTY) is a telecommunications system consisting originally of two or more electromechanical teleprinters in different locations connected by radio rather than a wired link.
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.
The US Department of the Navy successfully tested printing telegraphy between an airplane and ground radio station in August 1922.
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).
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.
These audio tones, then, modulate an SSB transmitter to produce the final audio-frequency shift keying (AFSK) radio frequency signal.
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").
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" The original (or "Baudot") radioteletype system is based almost invariably on the Baudot code or ITA-2 5 bit alphabet.
There are variations of the standard Baudot alphabet to cover languages written in Cyrillic, Arabic, Greek etc., using special techniques.
[15][16] Some combinations of speed and shift are standardized for specific services using the original radioteletype system: After World War II, amateur radio operators in the U.S. 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.
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).
In early 1949, the first American transcontinental two-way RTTY contact was accomplished on 11 meters using AFSK between Tom McMullen (W1QVF) operating at W1AW and Johnny Agalsoff, W6PSW.
Due to the efforts of Merrill Swan, W6AEE, of "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.
[23] 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.
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.
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 900 Hz or less.
The FCC Notice of Proposed Rule Making (NPRM) that resulted in the authorization of FSK in the amateur high frequency (HF) bands responded to petitions by the American Radio Relay League (ARRL), the National Amateur Radio Council, and a Mr. Robert Weinstein.
[27] Amateur radio operators outside of Canada and the U.S. began to acquire surplus teleprinter and receive permission to get on the air.
[29] Information on how to acquire surplus teleprinter equipment continued to spread and before long it was possible to work all continents on RTTY.
Most of the terminal unit equipment used for receiving RTTY signals was home built, using designs published in amateur radio publications.
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.
This 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.
Many amateur radio operators had equipment that was capable of being upgraded to 75 and 100 words per minute by changing teleprinter gears.
Some of the reasons for the failure of 100 WPM HF RTTY included poor operation of improperly maintained mechanical teleprinters, narrow bandwidth terminal units, continued use of 170 Hz shift at 100 WPM, and excessive error rates due to multipath distortion and the nature of ionospheric propagation.
[35] These symbol rates were later modified:[36] The requirement for amateur radio operators in the U.S. 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.
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.
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.
