SDR: RTTYculously Easy


Scanning through my newly available low frequency bands, I happened upon a friendly bunch of ham operators from Europe, conversing in something called RTTY, often pronounced “ritty”, which stands for Radio TeleTYpe. It’s a basic method of sending text messages over the air. Luckily, I happened to have an application on hand which can decode this and many other data encoding types, it’s called MultiPSK (but since writing this article, I’ve picked up fldigi, which offers a similar set of decoders – this software stuff is all a matter of taste). Before you can decode anything, you’ll need to send the audio to an audio input, using either another soundcard or a piece of software like Virtual Audio Cable, it’s a simple process but somewhat outside of the scope of this article, all you really need to do is get the audio coming out of SDR# back in to another app. XP users may sidestep this requirement by setting the MultiPSK or fldigi recording source to Stereo Mix or similar, should your sound card support it.

Now, as much as I appreciate the time and effort the creator of this software has put into this piece of software, graciously offered for free so idiots like me can pretend to be ham operators, I am going to have to ask you all to shield your eyes and/or obtain some beer goggles. Ready? Great, here goes. You can click it to embiggen it if you’re brave enough.

Still there? OK, good. I’ll run through a little of what you’re seeing here, because I realise at first glance it’s probably going to look like someone dropped a bag of mushrooms and got happy with the spray can tool in MS Paint. Let’s start at the bottom. That’s the SDR# window, it’s where you’ll be controlling the radio itself, you can see a few important bits and pieces happening here. In the spectrum analyser window, that’s the blue gradient graph with the jagged edge, you’ll see that I’m tuned to about 3.592MHz, that’s what the red line signifies. You’ll also see a greyed out vertical bar to the left of it, that means I’m in what’s called Upper Side-Band mode, you can tell because the bar is to the right of, or higher in frequency on the graph than, the red line. It’s a bit complex to explain here, so here’s a wiki link.

Slightly below that, you can see what’s called the waterfall display, and that displays the signal over time. Think of the two graphs as being a 3D object folded flat, the waterfall is what you’d see if you could somehow look at the spectrum analyser graph from above, standing up and looking down at the top edge of your monitor. In that waterfall display, you can see some fairly bright yellow lines streaking down the screen directly below the greyed out area, they indicate that a couple of seconds ago, as time passes downwards, there was a signal there which would have also appeared on the spectrum analyser as a spike, or a peak, sticking up above the rest. This is the RTTY signal that I was decoding.

Let’s look a bit closer at what we’re dealing with here. RTTY works by transmitting a series of dots and spaces, called Baudot code, it’s a bit like morse code and ASCII got drunk and funky together. That’s the reason we see two yellow lines in this signal, because like morse code, or even binary data, we have two states to represent, and each gets its own frequency 170Hz apart from the other. One line is the dots, the other is the spaces. If we follow those dots and spaces, according to the Baudot standard, we’ll end up with text. That’s exactly what MultiPSK is doing in the top half of the image. If you look at the colourful bar a little below 1/3 of the way down the window you can see the two bright orange lines, those correspond with the yellow lines in SDR#, and there are a pair of cyan markers just to the right of each of those orange lines, they’re supposed to sit on the orange lines to tell MultiPSK which pair of Baudot streams it should be looking at, there might be others audible at the same time, or some other signal/noise which could confuse matters.

Once you sit those lines in the right place, you’ll end up with something like the text I’ve highlighted in cyan below all that.

This rabbit hole goes deep. Very deep. Pick an aspect of radio and be prepared to chase it until your brain melts out of your ears. Now we’ve got some messages from a ham radio operator, what do they mean? It just looks like random characters! Well, you’re right, it does, so sometimes if you don’t know what you’re looking for it can be quite difficult to tell what’s random radio noise being interpreted as garbage and what’s actually being said, but thankfully this one isn’t too tricky. Let’s break it down:


Working backwards, CQ is quite a common thing to see between hams, it basically means “Hey, I’m here, anybody there? I’m open for replies”. IW1AYD is the guy’s handle, his radio callsign, that’s usually in a standard format so it’s pretty easy to pick out amongst other stuff. Callsigns themselves are pretty informative too, looking closer at this chap’s callsign, we know he’s Italian, because it begins with an I. We also know, thanks to this page, that the W after the I means that he’s an Italian with a radio licence which until 2006 was only valid on the VHF/UHF bands, but now he’s able to use HF too. Which is good, because before I read that I expected to have to rewrite this breakdown. From the same page, we can see that because he has a 1, he’s on the Italian mainland up in the north-west of the country, and the AYD is his own personal ID. So he’s a north-western Italian who earnt himself a special level licence some time between 1972 and 2005. Cool.


So what about WAE? What’s that for? As it happens, it means he’s calling anybody in Europe, quite simple really. But what does all this mean, as a complete message? Well, he’s participating in an activity called DXing, it’s a popular pastime for many hams, and the object of the game is to collect contacts from as many people as you can, the further away the better. The name DXing is telegraph shorthand for “long distance”, which makes sense, given what it now describes. It can be just a hobby, to collect QSL cards, essentially ham radio postcards which are exchanged as proof that contact was made with a given operator or location, or as a contest during which participants compete for the most points gained from hitting far-flung radio contacts. As it happens, there’s one going as I write this message, a weekend-long WAE DX contest (also in PDF) which includes RTTY on 3.5MHz. Bingo. That’s probably what we’re seeing! If you look at the image below, you can see some previous CQs I’d recorded, some of which the software had helpfully translated the country codes for. Little did I realise at the time, it’s quite possible they could also have been competing in the contest.

Good luck everyone!

I hope you’ve been informed, entertained or at least staved off boredom for a brief period.

‘Til next time, 73

Please don’t kill me for stealing operator slang… : D


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