By now I've rambled quite a lot about RDS, the data subcarrier on the third harmonic of the 19 kHz FM stereo pilot tone. And we know the second harmonic carries the stereo information. But is there something beyond that?

Using a wideband FM receiver, like an RTL-SDR, I can plot the whole demodulated spectrum of any station, say, from baseband to 90 kHz. Most often there is nothing but silence above the third pilot harmonic (denoted 3f here), which is the RDS subcarrier. But two local stations have this kind of a peculiar spectrogram:

There seems to be lots of more data on a pretty wide band centered on the fourth harmonic (4f)!

As so often with mysterious persistent signals, I got a lead by googling for the center frequency (76 kHz). So, meet the imaginatively named Data Radio Channel, or DARC for short. This 16,000 bps data stream 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.

To find out more, I'll need to detect the signal first. Detecting L-MSK coherently is non-trivial ("tricky") for a hobbyist like me. So I'm going to cheat a little and treat the signal as continuous-phase but non-coherent binary FSK instead. This should give us good data, even though the bit error probability will be suboptimally high. I'll use a band-pass filter first; then a splitter filter to split the signal band into mark and space; detect both envelopes and calculate difference; lowpass filter with a cutoff at the bitrate; recover bit timing and synchronize using a PLL; and now we can just take the bits out by hard decision at bit middle points.

Oh yeah, we have a bitstream!

Decoder software for DARC does not exist, according to search engines. To read the actual data frames, I'll have to acquire block synchronization, regenerate the pseudorandom scrambler used and reverse its action, check the data against CRCs for errors, and implement the stack of layers that make up the protocol. The DARC specification itself is luckily public, albeit not very helpfully written; in contrast to the RDS standard, it contains almost no example circuits, data, or calculations. So, a little recap of Galois field mathematics and linear feedback shift registers for me.

But what does it all mean? And who's listening to it?

Now, it's no secret (Signaali 3/2011, p. 16) that the HSL bus stop timetable displays in Helsinki get their information about the next arriving GPS positioned buses through this very FM station, YLE 1. That's the only thing they're telling though. I haven't found anything bus-related in the RDS data, so it's quite likely that they're listening to DARC.

DARC is known to be used in various other applications as well, such as DGPS correction signals. So decoding it could prove interesting.

Sniffing the packet types being sent, it seems that quite a big part of the time is being used to send a transmit cycle schedule (COT and SCOT). And indeed, the aforementioned magazine article hints that the battery-powered displays use a precise radio schedule to save power, receiving only during a short period every minute each. (This grep only lists metadata type packets, not the actual data.)

$ perl darcdec-filters.pl | grep Type: | sort | uniq -c 88 Type: 0 Channel Organization Table (COT) 8 Type: 1 Alternative Frequency Table (AFT) 8 Type: 2 Service Alternative Frequency Table (SAFT) 1 Type: 4 Service Name Table (SNT) 8 Type: 5 Time and Date Table (TDT) 112 Type: 6 Synchronous Channel Organization Table (SCOT) $ █

At the moment I'm only getting sensible data out at the very beginning of each packet (or "block"). I do get a solid, error-free block sync but, for example, the date is consistently showing the year 1922 and all CRCs fail. Other fields are similarly weird but consistent. This could mean that I've still got the descrambler PN polynomial wrong, only succeeding when it's using bits from the initialization seed. And this can only mean many more sleepless coding nights ahead.

(Continued in the next post)

(Pseudotags for Finns: HSL:n pysäkkinäytöt eli aikataulunäytöt.)

Update 1/2019: There's now a basic DARC decoder on GitHub, it's called darc2json.