Digital Radio Mondiale

If you are a computer geek like me, the first thing you will think about when I tell you “DRM” is “Digital Rights Management”. You know, this process that consists in giving you both encrypted content and the keys to decrypt it, but in a really obfuscated way to attempt to discourage you from doing the decryption yourself… anyway, this is not the subject of this post. The DRM I want to talk about today is “Digital Radio Mondiale”. A strange acronym choice if you ask me, but there you go…

So what is “DRM” ? It is an attempt at keeping shortwave broadcast radio live and kicking, by broadcasting in digital format rather than using amplitude modulation. Gone are the days of listening to “Radio France International”, “BBC World Service” or “Voice of America” hunched over a small SW radio, trying to make out what was being said over the static… I will always remember a conversation in China back in 1997 when I clumsily tried to understand what sort of ideas my Chinese student friends had about Tibet, and the comment I got was “ah ah, my friend, you sound like you have been listening to Voice of America propaganda too much”. But I digress.

DRM uses AAC audio at a low bitrate – 16kbps mono, typically – and is able to stream data and basic text, in a manner similar to RDS. DRM in itself is a pretty well defined standard, and you can read all about it at http://www.drm.org/ but it is not widely used yet. As a bystander, it looks like getting agreement on standardisation of shortwave radio broadcasts is an extremely difficult and lengthy exercise: you have to get dozens of countries to agree and cooperate to broadcast information all over the globe, not hard to see why this can take time.

This means that the number of broadcasts today is actually fairly limited, and the number of commercially available shortwave receivers with DRM capabilities is even more limited, but the good news is that if you have a computer and a shortwave radio receiver with an IQ output, you will be able to listen to DRM fairly easily using an Open Source reference receiver implementation called “Dream“. You can head over tho the wiki referenced in that link to learn more about it and install it.

This is where things get a bit hairy: I mentioned above that DRM uses AAC: AAC is not license free, and for some strange reason, the DRM consortium chose it over other similar open codecs. This means that the downloadable versions of Dream do not contain what is needed to actually listen to DRM…

On a Mac, this means you will have to install libaac using homebrew – easy – but you will then need to compile Dream yourself since the binary on the Dream wiki is 32 bits only and all current Macs run a 64 bit OS, and the libaac you just compiled will be a 64bits lib. Yeah, I know, it’s a bit of a pain, but compilation works OK – I had to do a few nudges and updates to the code to make compile to the end, but I eventually managed.

Below is a screenshot of Dream running on my Mac: I have tuned my KX3 to a currently broadcasting station – Radio New Zealand – and listening to it on the computer. Dream is able to download the worldwide broadcast able by itself, and even tune your radio automatically if it is compatible with Hamlib!

Dream DRM decoder

The radio panel and various controls on the bottom is the web control interface of my radio, a work in progress and not something that is publicly available at this stage.

Dream is configured to use the I/Q output of the radio, channel is set to “I/Q Neg Zero”, and the sound device is the built-in input of the Mac: on my machine, Dream seems to have a bug and I need to select the entry directly above the one I actually want to use for both sound input and output, so if you have trouble, you might want to check this.

The upper right window is the spectrum output coming from the radio: you can see the DRM stream with its very characteristic visual signature, and lots of technical info on the decoder. The upper left window show the name of the station and radio text.

Sound quality is actually surprisingly good, as long as you have a decent signal! I would not go as far as saying it is worth going through all this trouble just  to listen to radio – there are definitely easier ways to do it in 2014 – , but the whole exercise is a good technical learning experience, and I will definitely be happy if I can listen to good audio quality western propaganda broadcasts next time I visit China!

http://www.aerodynes.fr/2014/02/28/drm-digital-radio-mondiale/