N8MDP.com
Jonathan Tucker, BSEE, MBA
Auburn Township, Ohio USA

RTTY (Radio Teletype)
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An Amateur Radio radioteletype (RTTY) station consists of four distinct parts: A computer with a sound card, an modem interface such as the West Mountain Radio RigBlaster or the Tigertronics SignaLink, decoding software (See below), and an HF Radio. The sound card performs the functions of the modem and the PC performs the processing of the digital bits.

Historically, a Teletype or teleprinter was 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.

RTTY uses the Baudot code. The Baudot code uses data bits to represent letters, numbers and punctuation, much like your computer does. Unlike your computer, which uses eight bits for each character, the Baudot code uses only five, plus a start bit and stop bit. Using fewer bits speeds up transmission and reduces the chance of errors. Unfortunately, five 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, RTTY also includes two states, the unshifted or letters state and the shifted ornumbers 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 RTTY system prints the letters and space while in the shifted state it prints the numerals and punctuation marks.

During transmission, the modem converts the digital signal transmitted 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 directfrequency-shift keying (FSK) as they can directly accept the digital signal and change their transmitting frequency according to the mark or space input state.

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").

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.

For more information and a great tutorial on RTTY, I recommend reading IW5EDI's (Simone Mannini) great tutorial on RTTY.

Source: Wikipedia: http://en.wikipedia.org/wiki/Radioteletype

One of the more popular software tools for working RTTY is Ham Radio Deluxe and Digital Master 780. DM 780 makes it very easy to work RTTY. DM 780 gives you the ability to see multiple RTTY QSO's so you can quickly tune and call CQ or answer a CQ call.

The image on the right is a screen capture of DM780 capturing a RTTY QSO. The screen shows the typical RTTY transmit and receive windows, but more importanlty, the "Waterfall" plot. The watefall plot is a very useful tool because it represents a graphical window that allows you visualize the station that are active and transmitting. The stronger signals have brighter colors, while weaker signals will not be as bright. Using the waterfall graphic really is what makes RTTY1 operation easy to tune and enjoyable to work.

All that is really needed to run RTTY is an HF radio, a computer with a sound card, an interface that connects to the PC and the transceiver such as a RigBlaster or a homebrewed audio coupled solution and Ham Radio Deluxe.

HRD

A relatively new piece of software I found that runs RTTY (as well as PSK and MFSK) is Airlink Express from KR1ST. Airlink Express is a user friendly digital mode software package for the Amateur Radio Operator. The software is compatible with Microsoft Windows XP, Microsoft Windows Vista, and Microsoft Windows 7.

With the release of Windows Vista and Windows 7, Microsoft introduced a new sound architecture. Airlink Express is developed to support this new sound architecture, yet it maintains complete backward compatibility with Windows XP.

The DSP engine used in Airlink Express is MMVari by Makoto Mori, JE3HHT. This engine is very flexible and decodes as well, if not better, than any other soundcard mode engine currently available. It is used by many other software products like Logger32 and the N1MM contest logger.

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HRD

By far, one of the most popular RTTY programs around is MMTTY by JE3HHT, Makoto (Mako) Mori. Makoto is also the author of another very popular software called MMSSTV which is used to decode analog slow scan television pictures. MMTTY is very easy to use and it is FREE!

Tuning RTTY signals is not difficult. MMTTY makes it easy. MMTTY comes with a spectrum display (called FFT Display), an XY scope, and a waterfall display as shown below. 

The spectrum display (FFT) is useful for initial tuning to get close to a signal. The XY Scope is then used for fine tuning.  The two yellow vertical bars represent Mark and Space frequencies.  Teh final tuning goal is to line up the signal with these vertical bars.  Once this is accomplished, the signal indicator on the MMTTY screen should go above the squelch threshold and you will begin to see readable text begin to scroll across the screen.  It is that easy! .

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