How to decode shortwave data streams

Turn strange bleeps on the airwaves into text and pictures

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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