Neither the author or Just Amiga Monthly can be held responsible for any loss, injury or damage resulting from the use of information in this article. Faulty wiring can damage your computer. If in doubt consult an expert.
By far the most popular form of communication on the amateur radio bands is the spoken word. At any moment thousands of amateurs are talking to each, worldwide on the short wave bands or across town using microwave links. The second most popular mode, and some argue the most effective, is Morse code. Both speech ("phone") and Morse ("CW" for Continuous Wave) have in common the fact that they can and usually are sent and read by humans without machine aid.
Radio amateurs also use a variety of specialist modes. They have adapted or developed just about every known system of communication found commercially including Packet Radio, Facsimile, Slow-Scan Television, Fast Scan Television and of course RTTY. This article is about how to receive Radio Teletype, abbreviated to RTTY on your Amiga without breaking the bank.
The simplest way to receive RTTY is to go along to an amateur radio shop and buy a communications receiver and terminal unit (radio modem) complete with software. This solution will undoubtedly work well and will, depending on what you buy, leave your bank account around �1000 lighter! Sadly this is the solution adopted by many amateurs, one which turns the hobby from experimentation to consumerism. But anyone can put together a RTTY receive station (indeed even a transmit station). All you need is some simple electronics construction using easy to build kits. A whole station, including receiver and terminal unit can be built for as little as fifty quid. I'm just off on a Caribbean holiday with the small change....
Pronounced 'writ-eee' by enthusiasts, RTTY is used to carry amateur radio and commercial traffic across the world. Occasionally ASCII is used but more commonly each letter or number is encoded into Baudot Code. Also called the Murray Code after it's inventors this system was developed to allow hard copy of messages to be sent over telegraph links, particularly radio.
The teleprinters used originally were completely mechanical, bulky and very noisy. Thousands are still in daily use by press-agencies, governments, scientific expeditions and of course radio amateurs. The Baudot Code is therefore now not the fastest (nor most reliable) way of sending the printed page. Radio Amateurs use a speed of just 45.5 baud and commercial services are usually 50 or 75 baud. Just try making a mechanical teleprinter that runs at 9600 baud!
The slow speeds mean that any computer, (yes I mean any) computer can be used to decode Baudot. You can write the software in Basic, even Amiga Basic is quick enough! When a 1k ZX81 can do the job the Amiga would seem a little over-qualified! There are excellent PD RTTY receiving and transmitting program available for the Amiga so I wont go into many details of the actual structure of Baudot Code here. Sufficient to say its a 5 bit code which means that it hasn't got enough bits to even support upper and lower case. If you want to send a number you have to send first a figures shift code followed by one of the letter codes! Such high-tech! Figure shift followed by the code for J rings the bell on the teleprinter at the receive end, presumably to wake up the operator who has fallen asleep waiting for the next sentence.
Nevertheless RTTY has a certain charm and is actually quite addictive. It's fascinating to see messages appearing on your Amiga screen from thousands of miles away, particularly with simple home-made receiving equipment. If you already spend too long in front of your Amiga you had better stop reading right now.
RTTY is transmitted using two tones, usually with a 170Hz difference, for example 1445Hz and 1275Hz. This is a similar method to that used by a computer modem although the tone frequencies and shift are different. The slow baud rate means that the sound of a RTTY signal is quite distinctive when tuned in by a radio receiver- "jingle bells" is a good description.
This shows the RTTY receive setup I'm going to describe, apart from the Amiga you will need a receiver, a terminal unit and some software.
You may already have a suitable receiver which covers the short wave bands and there are books available listing the frequencies of commercial teleprinter services. Your receiver will need a BFO (beat frequency oscillator) to allow it to receive single sideband phone, CW and of course RTTY. Many popular short wave radios will do. If you haven't already got a receiver then don't despair. There are some simple construction kits available for building receivers and for the purposes of this article I'll concentrate on just a single amateur radio band, 14MHz, more commonly called "Twenty Metres".
Don't be put off by the simplicity or cheapness of these receiver kits, they can pull in amateur phone, CW and RTTY signals from right around the world. There are several manufacturers of suitable kits but the one I use, with excellent results is made by Kanga Products. Building simple short-wave receivers is a great way to get started in electronics construction and amateur radio. I wont go into the construction details here, Kanga supply very comprehensive instructions. You will need a small soldering iron and a pair of wire cutters and pliers.
The receiver also needs an antenna (aerial). An outside antenna is essential to pick up the best signals and to minimise the amount of interference from the computer. Anyone who has stood a medium wave transistor radio close to a computer will know that they generate radio interference. The Amiga is actually one of the best of a poor bunch and my A1500 produces few problems. In contrast I have to switch off my Amstrad FAX machine when I want to listen on the radio as it wipes out all trace of signals.
There are lots of textbooks on aerials and any number of different varieties. A simple but effective aerial for "Twenty" is the dipole and these are two typical setups.
The length of the dipole has been calculated for the centre of the twenty metre band and the signals are brought down to the receiver using television coax cable. Its important to waterproof the centre of the dipole with a sealant, I use a hot-melt glue gun. If water gets into the inside of the cable it will corrode and ruin it. The dipole can be supported at either end or in the centre with the ends brought down close to the ground (Inverted V). The ends can be bent around if space is tight but they should not be allowed to touch anything. Orange polypropylene string is an excellent material to use to tie the ends of the dipole up with. The higher the dipole is above ground the better will be the results.
With the receiver and dipole antenna you will hear amateur transmissions from around the world and its likely that the next part of the project will get delayed while you stop and listen in on what's going on on phone.
The audio output from the receiver is fed into a terminal unit. The terminal unit's job is the same as a modem, it converts the two tones into logic level signals that the computer can deal with.
The terminal I use is available from Maplin's in the UK and comes as a kit. The heart of the unit is a chip, the XR2211 which has been especially designed to be used in computer modems. The Maplin's modem produces TTL levels and needs an additional small circuit to allow it to drive the RS232 serial input on the Amiga. This small printed circuit board connects between the terminal unit board and the computer.
The terminal unit needs to be put into a metal case and its a good idea to make this a separate box to the receiver. Then if you get hooked on RTTY and want to buy or build a better receiver you can still use your terminal unit. The performance of the Maplin's terminal unit can be further improved by adding their audio filter kit between the receiver and TU although I've not bothered myself.
The terminal unit and RS232 driver can both be powered by using the +/- 12V available on the Amiga's serial socket as they take very little current. Beware! If you have made a mistake with wiring you could damage your computer. Neither I nor JAM will take any responsibility for blown up Amigas. Check and re-check your wiring and if in any doubt at all find an expert to help you. Most towns have amateur radio clubs and any of them should be pleased to encourage newcomers into the hobby.
These are the connections to the serial socket on the Amiga. Note that this RTTY program works by reading the data on pin 6 of the serail socket, the DSR (Data Set Ready) pin. This enables the software to raed the RTTY data without the use of an expensive baud rate converter. Other software will not work using this wiring.
The receiver should be powered separately, either from batteries or from a small suitable mains adaptor to prevent interference hash from the computer affecting reception.
If you have built things in the order I've suggested then you will have had some practice tuning the receiver. You will notice that as you tune across speech transmissions the pitch of the signal changes and there is only one correct point for the tuning. This is even more true when you are receiving RTTY. The frequency of the audio tones that comes out of the receiver needs to be set quite accurately. The Maplin's terminal unit has two LEDs which will help you tune the signal correctly although the final adjustment can be done by watching the screen on the computer. If sense rather than rubbish is printed on screen then the tuning is OK.
RTTY signals on "Twenty" are centred around a frequency of 14.090MHz. If you are not too sure of the accuracy of calibration of your receiver then this is at the point on the dial where morse signals stop and phone signals begin to be heard.
All radio amateurs have issued them a call-sign (mine is G4DCV) and the first letter or two is country designation. G for example is issued in the UK, F in France, W or K in America, VK in Australia etc etc.
There will of course be many signals received that are not in English although radio amateurs use a series of abbreviations called the Q code to allow them to exchange information regardless of language. Some of the common abbreviations you will see on screen with their translations include:
CQ CQ CQ- A general call to get a contact with another amateur. DE- From, QSO- Contact, QRM- Interference, QTH- Location., DX- long distance etc. In addition amateurs abbreviate many common words when using both Morse and RTTY. For example TKS for thanks, PSE for please, etc. It gets easier to understand after some practice! So TKS FER QSO 73 ES GD DX DE G4DCV actually means "Thank you for the contact best wishes and good long distance contacts from G4DCV).
The RTTY software I am use was written by a German radio amateur DL4ZBH. It is PD and can be downloaded from the Aminet:
rttyam.lhacomm/misc 50kDL4ZBH RTTY transmit & receive
The screen-shots show the program in action.
The screen is divided into two, an upper receive window and a lower transmit section. Functions like baud-rate and spooling of text to disk or printer are controlled by pull-down menus and the program multi-tasks properly. Its actually a very nicely written piece of software and one I would have been happy to pay for. The author has generously made it PD and includes a message to the effect that if you have paid anything at all for it then you are a twit! The Amiga Amateur Radio User Group has a membership of several hundred, publishes a regular newsletter and makes available a wide range of amateur radio related PD software. Membership is FREE (although donations are gratefully accepted of course!). If you are interested in amateur radio and the Amiga then membership is a must.
There are several organisations set up to help the radio amateur and these include:
The Amiga Amateur Radio User Group, (AARUG), 8 Orchard Close, South Wonston, Winchester, SO21 3EY. (PLEASE enclose an SAE).AARUG E-Mail
The Radio Society of Great Britain, (RSGB), Lambda House, Cranbourne Road, Potters Bar, Herts, EN6 3JE
The British Amateur Radio Teleprinter Group, (BARTG), Ffynnonlas, Salem, Llandeilo, Dyfed.
The 14MHz DC Receiver kit is availble from Kanga Products, 3 Limes Road, Folkestone, Kent, CT19 4AU. (0303) 276171
The Terminal Unit (order code LM95D ), RS232 Driver (order code LK17T), and optional Audio Filter (order code LM93D, ) kits are available from Maplin Electronics in the UK. Maplin also sell tools, cases etc. and their catalogues are availble from W H Smith.
In the UK, the magazines, Short Wave Magazine, Ham Radio Today and Practical Wireless contain articles of interest to RTTY and Packet Radio listeners as well as advertisments for equipment.