How to decode shortwave data streams

22nd Mar 2009 | 09:00

How to decode shortwave data streams

Turn strange bleeps on the airwaves into text and pictures

Introducing MultiPSK

Have you ever tuned into shortwave radio bands? If so, you might have come across some strange-sounding periodic signals, from rhythmic bleeps and gurgles to scratching sounds.

If you guessed that they were some sort of data transmission, you're right. But who transmits them, and why? In this guide, we'll show you how to use your PC to decode these mysterious signals and enter the world of data on the airwaves.

You'll soon be able to decode radio teletype signals, interpret Morse code, unscramble radiofax transmissions of weather charts and watch colour slow-scan television (SSTV) images form.

Recognising the modes

We're going to use MultiPSK to decode data signals from a radio. This package supports various types of data transmission, but if you don't select the correct ones it won't be able to interpret them, so you need to be able to recognise some of the key modes by ear

To help out here, we've prepared samples of several data modes including: Morse 25 WPM, RTTY 50 baud, Fax 120 LPM 288 IOC and SSTV Robot 2. By listening to these samples, you'll find out what Morse code, radio teletype, radiofax and SSTV transmissions sound like when you tune them in on a radio.

You can download the samples here:

Morse 25 WPM
RTTY 50 baud
Fax 120 LPM 288 IOC
SSTV Robot 2

Although there are lots more data modes, these four are the ones that we'll be concentrating on in this guide. If you want to get a feel for how some of the others sound, MultiPSK can also encode data (for example, typed text) into audio files. Do this and have a listen to the resulting files if you're curious about other modes.

Introducing MultiPSK

First, you need to learn how to use MultiPSK. You can use your sample data files to get to grips with it. This is easier than decoding live radio signals, as you won't have to cope with the interference and varying signal strengths that you'll encounter on the airwaves. To use MultiPSK on live data, you would need to connect the audio output of your radio to the line or microphone input of the soundcard in your PC.

To provide a close approximation of this, use a patch cable to loop the soundcard's line output to its input. After doing this, if you play back a sample file using a media package, that signal will be recognised by MultiPSK. Start up MultiPSK and you'll find that it opens with the Configuration screen displayed.

We can accept the default settings for now, so click on the 'RX/TX Screen' button towards the bottom left. This will take you to the screen we'll be using for decoding data.

In the block of controls in the upper-right corner, click on the 'CW' button (CW stands for Continuous Wave and is a term that radio amateurs use somewhat inaccurately to refer to Morse code). Now try playing back the Morse code sample file.

You'll notice that the 'waterfall' display in the centre of the screen, which was previously showing very little, suddenly bursts into life. This display shows a graph of audio frequency against time, and is used for selecting a signal to decode.

Before you can do this, however, you'll need to adjust the Windows audio level ('Start | All Programs | Accessories | Entertainment | Volume control') until the Level display in the centre top of the MultiPSK screen shows a value just short of 100 per cent and the adjacent Over (overload) display doesn't show green.

Once the audio level is correct, play back the Morse code file again. When you see a red line appear in the waterfall display, click on it to select that signal for decoding. The text represented by the Morse code will appear in the large text area at the bottom of the screen. Having mastered Morse code, you can now do the same with the other three sample files.

There are a few differences, though. With Morse code, MultiPSK can figure out how fast the data's being sent and adjust accordingly. With many of the other modes, you have to specify certain parameters. With RTTY, for example, you have to select the correct transmission speed – the sample provided is 50 baud. And with HF Fax, you have to select values for LPM (lines per minute) and IOC (index of cooperation).

For the above sample file, use 120 and 288 respectively. The other difference concerns what you see in the waterfall display. Morse code uses a simple on/off modulated signal, which means that it appears as a single frequency in the waterfall display. RTTY, on the other hand, uses two frequencies to represent binary ones and zeros, while Fax uses a pair of frequencies to represent black and white components.

The upshot of all this is that many signals appear on the display as two lines, even though only one is being transmitted at once. It makes a difference which of the lines you click on, so if nothing sensible seems to happen, try clicking on the other one.

Let's decode some live data!

Radio teletype (RTTY)

In terms of decoding live data, RTTY is probably the easiest mode to use. First, you need to set up your shortwave receiver, ideally with a decent external antenna, and connect its line output (or headphone output) to the line input or microphone input of your soundcard.

RTTY is transmitted slowly to cope with the varying atmospheric conditions encountered on the shortwave bands: it's not uncommon for RTTY streams to be transmitted at 50 baud, which is 50 bits per second (and you thought that old dial-up modems were slow at 56kbps per second!). However, the encoding is a predecessor to ASCII, and it uses only five bits per character. This gives it a clear speed advantage over 7-bit or 8-bit ASCII, but it requires the use of Shift symbols to switch between letters and figures.

Even then, the character set contains only numerals, upper-case letters and a limited set of punctuation symbols.

Now tune in to an RTTY transmission. One of the most consistent signals you'll find is the German Weather Service in Hamburg on 7.646MHz. Try decoding it in MultiPSK (50 baud). This and other similar radio stations around the world transmit weather forecasts for nautical use. You'll also find that RTTY signals are plentiful on the amateur bands, but there is a difference. The weather stations transmit information for reception by anyone who happens to be listening.

Radio amateurs, on the other hand, exchange information with the stations they've established contact with, so it's basically a dialogue. When listening on the amateur bands you may be able to hear both sides of the exchange. Often, however, you'll only hear one station (probably the one closest to you), and there will be a period of silence while the other station responds.

Morse code

Morse code differs from the other types of data transmission in several respects. Although it can be generated automatically, as a relic of an age before electronics it can also be sent by hand and received by ear. This is how it's often used on the amateur bands. The upshot of all this is that the timing of the dots and dashes and the spaces between them won't be perfect, and the automated decoding of hand-sent Morse is quite a challenge.

MultiPSK does a pretty good job, but it sometimes makes mistakes an experienced human operator wouldn't. It also tends to try to interpret radio noise as Morse, so you'll often see garbage in the received text window when no Morse code is being received.

Another difference is that Morse code signals occupy a very narrow bandwidth, so you might see several of them in the waterfall display. If so, you'll be able to flip between them.

Finally, because Morse is slow even compared to RTTY (30 words per minute is considered reasonably fast), abbreviations reminiscent of text messaging and various coding systems are used. In time, you'll learn to interpret Morsespeak, but you might struggle at the start. Morse is no longer used commercially, so you'll only find it in the amateur bands.

HF Fax and SSTV

HF Fax and SSTV modes deal with the transmission of pictures rather than the transmission of text.

Without going into all the mind-numbing details, let's just say that Fax is used for black-andwhite images – normally line diagrams such as weather charts – while SSTV is used for colour photographic images. Both modes are slow, with images taking several minutes to be transmitted.

As with RTTY, this is to allow the data to be transmitted within the limited bandwidth and poor conditions of fading and interference often present in the shortwave bands. Two stations that gave us good signals were GYA (the Joint Operational Meteorological & Oceanographic Centre, which broadcasts from Northwood, England) on 4.610MHz, and DDH in Offenbach, Germany on 3.855MHz (also on 7.880MHz).

These stations transmit weather charts that are used for shipping. Neither station transmits continually, so you might have to wait for a transmission to start.

One quirk of Fax transmissions is that the image is sometimes skewed; it may be shifted horizontally so that the left-hand edge of the image is not at the left-hand side of the window in MultiPSK. The six Slant buttons ('///', and so on.) and two Shift buttons ('<' and '>') that appear in Fax mode can be used to correct these distortions.

SSTV is used almost exclusively by radio amateurs. There are a few spot frequencies in each of the amateur bands that tend to be used for this mode. Lists of frequencies are available, but in our experience one of the best is 3.733MHz (also try 3.730MHz).

A group of French radio users convene here each morning from about 7.30 to 9.30 to exchange images. We were able to receive several good quality pictures from the group.


First published in PC Plus, issue 278

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