I discovered mysterious hidden signals on a public radio channel (2013) [video]

Posted by joebig 10/27/2024

I discovered mysterious hidden signals on a public radio channel (2013) [video](media.ccc.de)

273 points | 67 comments

sriram_malhar 10/27/2024|

This was a such a refreshing talk. She was monitoring the FM channel using a spectrum analyser (as one does, of course, haha) and got curious about this strange signal showing up next to the FM radio Channel.

The talk is about her attempts to learn about the Radio RDS (Radio Data System) standard, using a sound card to decode signals, finding a bit of bus-related information in that stream is weakly encrypted and proceeding to chasing it down. Very entertaining.

I'm so glad such people exist. I wish I could be one of these fearless and supremely knowledgeable people!

stavros 10/27/2024|

> I wish I could be one of these fearless and supremely knowledgeable people!

There's no trick to it (or there's a massive trick to it), you just refuse to let a mystery go until you know what's happening, mostly because figuring things out is fun.

In the process, you gain a large amount of knowledge.

smetj 10/27/2024|||

It comes down to genuine, natural unforced interest (its there or not) and mastering the ability to systematically deal with and how to tackle the next unknown. After years this make you a true wizard.

deskr 10/27/2024||||

> mostly because figuring things out is fun

I also think that figuring things out is one of life's most rewarding experiences.

But I've found out a lot of people don't think like that. I've often been asked "but why are you doing this" on the topic of my "eccentric" projects. People often can't understand why I find it fascinating because they would only find it tormenting.

amelius 10/27/2024|||

What if the data is encrypted? Do you keep on trying in a brute-force way?

stavros 10/27/2024||

You do whatever you think is fun, what do you mean?

matthberg 10/27/2024||

I found on her website [1] that she released a RDS decoder tool [2] relatively recently (compared to the 2013 of the video at least). Looks like it works with a USB RTL-SDR [3] receiver or a pre-recorded file in a number of formats.

[1]: https://www.windytan.com

[2]: https://github.com/windytan/redsea

[3]: https://www.rtl-sdr.com/about-rtl-sdr

mbirth 10/28/2024|

I’ve used this to experiment with TMC messages and trying to unscramble TMCPro. Fun times!

Zigurd 10/27/2024||

Are there still apps and the underlying hardware in smartphones to receive and decode FM RDS? IIRC some phone chips had FM radio receivers, but there were limitations like needing a wired headphone plugged in so that the headphone cable could be used as an antenna. I know a USB SDR could do it, but it would be neat if this were still a latent capability in the phone.

Of course I did a search in the Play Store but it's crowded with streaming radio apps and SDR apps.

brokenmachine 10/28/2024|

Yes. My Samsung has an FM radio. A lot do.

https://www.gsmarena.com/results.php3?chkFMradio=selected returns 7010 models, lol.

edit: oops you mean specifically RDS. Of that I'm not sure. I just opened the "Radio" app on my phone (for the first time ever!) and you did have to have the headphones plugged in to receive, but I couldn't see anywhere in the really basic looking app for RDS info to show up.

0898 10/27/2024||

One thing I often wonder about with RDS: How do they stop the traffic alert (“TA”) feature being abused by stations to grab listeners?

looperhacks 10/27/2024||

I think nobody has to prevent abuse because listeners will either turn off traffic alerts or switch to a different channel.

Besides that, FM broadcasting isn't a lawless place and is regulated by the government. Abuse will most likely lead to some kind of penalty, but I can't be bothered to read through the laws to confirm it :P

fhars 10/27/2024|||

The same way the prevent stations from broadcasting 90 seconds or more of pure silence: you loose your license if you do.

GranPC 10/27/2024||

> broadcasting 90 seconds or more of pure silence

Have a link or info on this? Sounds interesting but can't find anything.

fhars 10/27/2024||

It is a rule in Germany, I know about it because we did some distributed cooperation stuff with community radio stations and some hacking events during the pandemic, and making sure that no matter what happens with the network between the speakers, the coordinating host, and server of the radio station, we never send out silence, was quite an important concern. I can't quote you the exact line of the law,though.

ARob109 10/27/2024|||

Not sure about TA, but definitely saw radio station in Indianapolis using RDS to broadcast advertisements. In between the artist/song info, ads for an injury lawyer appeared. Thought it was super lame use of RDS.

GJim 10/27/2024|||

They don't.

Back in the day, it wasn't uncommon for pirate radio stations to drum up listeners by using TA to advertise short snippets.

teeray 10/27/2024||

The almighty FCC fine is the deterrent

alfiedotwtf 10/27/2024||

"So I got to work"...

I love that hacker mindset :)

Aachen 10/27/2024||

Wow, was that 2013? I saw this talk irl, can't believe it has been that long

user3939382 10/27/2024||

I was picking up WCBS in NYC at 5.105 GHz, near the antenna at Empire State. Never could figure it out, their engineers didn’t know why either.

wildzzz 10/27/2024|

It was probably getting picked up by some component in a microwave transmitter and getting mixed into the signal.

mbirth 10/28/2024||

Not sure whether it’s part of her talk, but this demonstration of the RDS stream she uploaded on Soundcloud is pretty cool:

https://soundcloud.com/windytan-1/rds-mixdown

stavros 10/27/2024||

This was great, I love these rabbit holes people go down, and it's great when they then share the results. Very fun watch.

yarg 10/27/2024|

This makes me wonder, how do you reliably and securely encode steganographic content for distribution within a noisy medium?

ezcrypt 10/27/2024|

In this case I think it's mostly about using different sub-carriers (kind of a "channel in the channel"), so that the data information and the audio are separated in frequency and do not disturb each other. That's generally called Frequency Division Multiple Access (FDMA), IIRC.

Another more advanced technique is Code Division Multiple Access (CDMA), e.g. used by GPS and some mobile communication modulation schemes. It allows you to have multiple senders on a single radio carrier frequency, and the receiver "selects" which sender to listen to by knowing its "code".

There's also Time Division Multiple Access (TDMA), i.e. senders take turns sending content in allocated time slots.

More comments...