Just an update to my painfully slow learning process about Radio Data System (RDS), the FM-broadcast system for side-channel data.  As a recap, RDS can be used to send a variety of different data types, including FM channel niceties like the currently playing song (or the next song), station call-letters, weather alerts.  The most interesting to me is Traffic Message Channel, TMC, because RDS-TMC broadcasts are free updates about current traffic conditions.  The actual traffic info is gleaned from road sensors, traffic lights, even emergency responders.

Supporting RDS-TMC in any free software OS or app requires libraries to read and decode the data, and hardware that supports it.  The latter is the first topic of new info; it seems as though the Nokia N900 phone’s built-in FM receiver supports RDS.  That’s excellent news, as Maemo is the most open, just-like-desktop-Linux phone platform and has a developer community that’s one-hundred-and-crazy-percent motivated.

I’ve also stumbled upon a developer who is working on FM RDS support at codeplex; he is writing Windows code though.  The base library is http://www.codeplex.com/FM  and he is blogging about it as well.  Unfortunately, the code is only available under the Microsoft Public License (MsPL), which means it cannot be reused inside GPL’ed libs or applications.  But at least there’s knowledge.

Obviously, with HTC (ie Android) and Maemo devices supporting RDS generally, there are great possibilities for basic RDS usage, like having the FM tuner show RDS song info… supposedly Martin Grimme’s fmradio Maemo app  does read RDS data, but I haven’t found it to work for any of my local stations (this is not a bug report, though — it could easily just be my lousy stations).  Reading the Python code makes it look really straightforward, but it’s not immediately usable by other apps.  Which I care about because the more interesting possibility to me is RDS-TMC on these platforms.

Sources of data

That brings me to the next subject for update: a polished RDS-TMC stack with supported hardware still needs a live, broadcasting source of TMC information.  I’m having a great deal of trouble tracking down accurate info on TMC FM broadcasts in North America.

The Wikipedia entry has been updated several times since my last post, but is thin on references — and how lame is it that Wikipedia is my best source of information?  Allegedly “INRIX,” Navteq Traffic, Clear Channel and Tele Atlas all broadcast FM TMC … somewhere.  Where exactly is not specified.  Even the Navteq Traffic wiki page lists a few big cities, but without an external reference.

Worse, the suggestion is that commercial TMC broadcasts are often encrypted in one way or another.  And you guessed it, there is little in the way of public information about that….

License compatibility of data

An even bigger  problem, however, is the legality of using TMC data.  The trouble hinges on the fact that TMC messages use location tables to compactly encode where events are happening, and the location tables are not available for free.  Then again, it also hinges on the fact that private companies like Navteq Traffic collect the road sensor data and sell that as a service to the people who actually broadcast the FM signal.

In the long run, there is at least a possible workaround for the TMC location table nastiness: TomTom has started an open source location referencing project called OpenLR.  I wrote about it in September.

As I mentioned last time, there are also alternative methods to grab rough traffic information, such as Floating Car Data aggregated by GPS-equipped cars and phones. I still don’t think there’s a free software project collecting that info.

The End!  I know I’m only barely scratching the surface of this subject; it’s way outside my usual beat so I’m doing a lot of lernin along the way.  Comments welcome.

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