Rotty - Technical Description of RTTY

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 heavy, complex and noisy, and 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 registers. Special integrated circuits 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. 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"