Decoding radio-controlled bus stop displays

Oona Räisänen (a/k/a Windytan) is a self-taught signals and electronics hacker from Helsinki, Finland, who is fascinated by mysteries, codes and ciphers, and vintage tech. She’s previously written regarding the use of digital transmissions carried on FM broadcast subcarriers as a means of supplying data to digital information signs used at bus stops.

Her research revealed that these remote controlled transit signs are on a system called IBus made by the Swedish company Axentia. The signals use a proprietary protocol known as Data Radio Channel or DARC. Her efforts have thusfar revealed that the data is sent “using a 16,000 bps data stream that uses level-controlled minimum-shift keying (L-MSK), which can be thought of as a kind of offset-quadrature phase-shift keying (O-QPSK) where consecutive bits are sent alternating between the in-phase and quadrature channels.” It appears that some of the decoded data is human readable, containing a list of terminal stations and similar data.

Oona used a RTL2838 DVB stick for receiving the signals. This is fascinating stuff. We hope to read further details on this reversing process in her future posts.

For the latest details visit Windytan’s absorptions blog.

(Here’s a link to United States Patent 6,307,890 for High density FM subcarrier modulation with standardized network layer which may be of interest.)

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