An inexorable trend over the last decade or more has been the exodus of AM radio stations from the low frequency and HF broadcast bands. The bandwidth and thus audio quality at these frequencies puts them at a disadvantage against FM and internet streamed services, and the long-distance advantage of HF has been reduced by easy online access to overseas content. The world has largely moved on from these early-20th-century technologies, leaving them ever more a niche service.
Happily for medium- and long-wave enthusiasts there is a solution to their decline, in the form of DRM, or Digital Radio Mondiale, a digital scheme that delivers cleaner audio and a range of other services in the same space as a standard-sized AM channel. DRM receivers are somewhat rare and usually not cheap though, so news of an Android app DRM receiver from Starwaves is very interesting indeed.
DRM uses a licensed encoding scheme from the Fraunhofer Institute, and this product follows on from a line of hardware DRM receivers that Starwave have developed using their technology. It uses the Android device as a front-end for any of a number of SDR receivers, including the popular RTL-SDR series. It supports the VHF variant of DRM, though we’re guessing that since the best chance of finding a DRM channel for experimentation is on HF that an RTL-SDR with the HF modification will be required. We think it’s an interesting development because the growth of DRM is a chicken-and-egg situation where there must be enough receivers in the wild for broadcasters to consider it viable.
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!]
What does a Hackaday writer do when a couple of days after Christmas she’s having a beer or two with a long-term friend from her university days who’s made a career in the technical side of digital broadcasting? Pick his brains about the transmission scheme and write it all down of course, for behind the consumer’s shiny digital radio lies a wealth of interesting technology to try to squeeze the most from the available resources.
In the UK, our digital broadcast radio uses a system called DAB, for Digital Audio Broadcasting. There are a variety of standards used around the world for digital radio, and it’s fair to say that DAB as one of the older ones is not necessarily the best in today’s marketplace. This aside there is still a lot to be learned from its transmission scheme, and from how some of its shortcomings were addressed in later standards. Continue reading “Anatomy Of A Digital Broadcast Radio System”