Decoding radio: In the previous post I told about the 16 kbps data stream on FM broadcast frequencies, and my suspicion that its being used by the bus and tram stop display system here in Helsinki. Now its time to find out the truth.I had the opportunity to observe a display stuck in the middle of its bootup sequence, displaying a version string. This revealed that the system is called IBus and its made by the Swedish company Axentia. Sure enough, their website talks about DARC and how it requires no return channel, making it possible to use battery-powered displays in remote areas.Not much else is said about the system, though; there are no specs for the proprietary protocol. So I implemented the five-layer DARC protocol stack in Perl and was left with a stream of fully error-corrected packets on top of Layer 5, separated into hundreds of subchannels. Some of these contained human-readable strings with names of terminal stations. They seemed like an easy starting point for reverse engineering.