Decoding NRSC-5 with SDR to Get In Your Car

NRSC-5 is a high-definition radio standard, used primarily in the United States. It allows for digital and analog transmissions to share the original FM bandwidth allocations. Theori are a cybersecurity research startup in the US, and have set out to build a receiver that can capture and decode these signals for research purposes, and documented it online.

Their research began on the NRSC website, where the NRSC-5 standard is documented, however the team notes that the audio compression details are conspicuously missing. They then step through the physical layer, multiplexing layer, and finally the application layer, taking apart the standard piece by piece. This all culminates in the group’s development of an open-source receiver for NRSC-5 that works with RTL-SDR – perhaps the most ubiquitous SDR platform in the world. 

The group’s primary interest in NRSC-5 is its presence in cars as a part of in-car entertainment systems. As NRSC-5 allows data to be transmitted in various formats, the group suspects there may be security implications for vehicles that do not securely process this data — getting inside your car through the entertainment system by sending bad ID3 tags, for instance. We look forward to seeing results of this ongoing research.

[Thanks to Gary McMaster for the tip!]

13 thoughts on “Decoding NRSC-5 with SDR to Get In Your Car

  1. HD is *not* high definition. The letters were chosen to “sound good”, but not to actually mean anything. Later it was suggested that they could stand for “hybrid digital”, but that was an after-the-fact thing.

    That said, it’s exciting to see the encoding finally cracked! Transmitting can’t be far behind!

    1. What I really want to see next is spam filtering for the display metadata. At least in New England area, some major broadcasters have discovered they can sell paid ad space in place of the title/artist/album/coverart (hawking things like discount tires and even hospitals), and rapidly update them to simulate scrolling for even longer ads. I haven’t seen the equivalent of GIF animation for ads in place of the coverart graphic…yet…

  2. I can’t fully articulate just how much I despise HDradio and its friend, Sirius XM. Just of such low quality!
    Most of the stations sound like copy of a copy. Like in the 00s when one would rip a CD and save it in MP3 format (probably 128kbps), and then burn it to another CD after accidentally leaving the “normalize all tracks” option enabled…
    Bleagh!

    Nice info, watch out for bad ID3 tags! Sanitize those things, automakers.

  3. HD radio is “HYBRID DIGITAL” not High Definition. they chose that name to instntionally confuse consumers. and it’s a massive failure. the fact that the latest JVC flagship double din radios do not come with it are telling enough that this HDFM is as dead as AM stereo.

  4. I actually work on a station that broadcasts HD radio. The simulcast is what you could call High Definition, the subordinate channels are lower quality. Since we broadcast speech, it’s harder to tell that the bitrate is lower.

    Going to play with this technique.

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