Adding HD Radio Support to FM DX Webservers with an RTL-SDR

Thank you to Ivan (NO2CW) for submitting news about how he added HD Radio decoding capability to his FM DX Webserver receiver. The FM DX Webserver is a community of worldwide FM broadcast-band online receivers that mostly use SDRs based on the TEF6686 chip, with a few also using RTL-SDR receivers. HD Radio (aka nrsc5) is a proprietary digital audio standard used by FM broadcasters in North America. It's often observed by SDR users in a waterfall as the two rectangles flanking both sides of a broadcast FM signal. Ivan writes:

A major challenge has been that up until now none of the online receivers were HD Radio capable. I have now added this capability with the help of an RTL V3. The online receiver uses a combination of two receivers: a TEF6686 which is known to be highly sensitive and, when an FM HD Radio signal is present, an RTL V3 automatically kicks in to provide HD radio capability opening up HD1, HD etc audio streams together with bundled album art, traffic and weather. The program was initially conceived by discord user seehed and then further developed by me.

The core code for this plugin was developed by GitHub user seehed and can be found here: https://github.com/Seehed/NRSC5_HDRadio. I had to make a few tweaks to get it fully working.

How the Setup Works: The hardware setup requires both a TEF6686 receiver and an RTL dongle operating simultaneously.

I tested an RTL-SDR V4 first, but the necessary DLLs worked much better with an RTL-SDR V3 in my current build.

By default, the server uses the TEF6686 for browsing the FM band for analog signals. When a user tunes to a frequency where an HD Radio signal is present, the logic switches to "HD Mode" and the RTL V3 takes over. The NRSC5 library is used to process the digital signal. In this mode, users will notice the change in audio quality and the available subcarrier streams (HD1, HD2, HD3, etc.) become visible and selectable.

How Users Can Set It Up: To replicate this, a user would need:

  • A working FM DX Webserver with a TEF6686.
  • An RTL-SDR V3 plugged into the same host.
  • The NRSC5 library installed.
  • The plugin from the GitHub link above installed and configured in the webserver directory.

Ivan also notes that currently his Miami NO2CW receiver is the only one on the network supporting HD Radio.

HD Radio on FM DX Webserver. Receiving HD Radio inside the fm dx webserver application.

HD Radio Added to FM DX Webservers via an RTL-SDR
HD Radio Added to FM DX Webservers via an RTL-SDR
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Demod: An iOS/iPadOS Client for rtl_tcp, SpyServer and KiwiSDR

Thank you to Alphonse Romero for submitting news about the release of his new iOS and iPadOS SDR receiver app called Demod. Demod is a network client that connects to remote SDR servers, so it can be tried out immediately with no dongle needed at all. 

The app works as a client for rtl_tcp, SpyServer (Airspy and RTL-SDR), and KiwiSDR, all in a single app. It features a Metal-accelerated waterfall and spectrum display, AM/NFM/WFM/USB/LSB/CW demodulation, and adjustable bandwidth, gain, and squelch. Other features include frequency memories with CSV import and export, AirPlay and Lock Screen controls, and native layouts for both iPhone and iPad. It can also browse and connect to public SpyServers directly from within the app, which is how it can be used with no hardware of your own. The developer notes that it collects no data, with no ads or tracking.

Alphose writes that Demod is a solo project and is available on the US App Store for US$9.99. More information and screenshots can be found on the project website, and the app itself is available on the App Store.

The developer has also provided us with 15 free codes that we will be giving away in one week. To enter the giveaway, simply comment on this post (make sure to include your email address in the comment so that we can send you the code), the X post, or the Facebook post. (UPDATE: ALL CODES HAVE BEEN ALLOCATED NOW, THANKS!)

Demod iOS SDR App
Demod iOS SDR App
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NyxScope: A Windows Multi-Protocol SDR Decoder Program with Multiple Digital Native Decoders

Recently, we've learned about NyxScope, a multi-protocol SDR receiver program for Windows that comes with multiple native decoders built right into the software. Their own description describes this all-in-one program best:

You get spectrum and waterfall, multiple concurrent VFOs, trunked-radio following, digital voice, aviation and marine tracking, paging, ISM sensors, HD Radio, and transcription, in one binary.

NyxScope includes decoders for P25 Phase 1 and Phase 2 voice, EDACS and NXDN control channels, ADS-B, AIS, ACARS, POCSAG, FLEX, LoRa CSS PHY + LoRaWAN MAC, Morse, RDS, CTCSS/DCS, Iridium decoding (with voice), and also includes a signal classification tool, Bluetooth LE scanner, and Whisper voice-to-text transcriber. It also outsources decoding of other protocols to mature decoding software such as multimon-ng, rtl_433, dsd-neo, nrsc5, direwolf, dumpvdl2, dump978, rs41mod where required, noting that the outsourced decoders are bundled with the software, meaning no extra installation work is required.

The software supports RTL-SDR, HackRF, Airpsy, bladeRF, SDRplay, Fobos and PlutoSDR. It also supports using multiple SDRs used in parallel.

The software does not appear to be open source, but it is provided for free with a limitation of 3 concurrent VFOs and a limitation on recording, transcribing, and pager messages. A perpetual per-machine license for $89.95 can be purchased to lift these restrictions and add access to their FCC frequency lookup database.

AI-Disclaimer: While the developers have not noted any use of AI tools, we suspect that AI was used in the creation of this software.

NyxScope Screenshot Scanning the 800 MHz Band
NyxScope Screenshot Scanning the 800 MHz Band
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Testing WSPR HF Propagation with a Raspberry Pi 3B+ and TAPR Shield Running WsprryPi

Thank you to Simone Spadino (SWL ID I8926BA), who wrote in to share his low-cost WSPR setup. WSPR (Weak Signal Propagation Reporter) is an amateur radio mode designed to test propagation paths using very low-power transmissions, with receiving stations reporting spots in real time to the WSPRnet map.

Simone's setup consists of a Raspberry Pi 3B+, which can be made to modulate a square wave RF signal on one of its GPIO pins via the WsprryPi software. Because a square wave generates many harmonics, he used the 20m WSPR shield from TAPR, which provides a low-pass filter to remove them. A dipole made from simple electrical monopole wire was connected to the shield via 50 ohm coax, and the signal was transmitted at 14.0971 MHz.

Transmitting from his rooftop in the center of Bari in southern Italy, Simone had his beacon received in England, Finland, Cyprus, and the Canary Islands, all more than 3100 km away. He repeated the experiment a week later with similar results. This is a reminder of how far low-cost hardware can transmit with a resilient protocol like WSPR.

Simone's WsprryPi Setup
Simone's WsprryPi Setup
WSPR Range with the Raspberry Pi 3
WSPR Range with the Raspberry Pi 3
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InmarScope: An Inmarsat AERO and STD-C Decoder with Multichannel Decoding and Automatic Call Following

Over on the SignalsEverywhere YouTube channel, Sarah Rose has released InmarScope, a multichannel L-band Inmarsat decoder that connects directly to an RTL-SDR, Airspy, or HackRF. The software can receive and decode both aeronautical (AERO) and maritime (STC-C / EGC) traffic at the same time. Decoders are dropped directly onto the aligned FFT and waterfall by holding CTRL and left-clicking, and the software lets you stack Aero MSK (600/1200 bps), high-rate OQPSK (10500 bps), AMBE voice (8400 bps), and Inmarsat-C BPSK decoders side by side.

One of the more interesting features is automatic voice-call following. By monitoring the 10500 baud forward-link channels, InmarScope can receive C channel voice assignments and automatically retune the SDR to the assigned frequency, lock the carrier, decode and record the AMBE call, and then hop back to where it was. There is also a two-SDR mode that dedicates a second radio to voice with a live split-view spectrum so one radio stays on the P control channel while the other tunes to voice calls. For assignments that never get broadcast, there is also a Call Hunter feature that uses a squelch threshold to automatically drop the decoder when a call appears. When a call is playing, the built in flight map also decodes the aircraft hex ICAO address and looks it up on airplanes.live, showing the plane's position and route in real time.

Recent updates have added a community-editable band plan, message search and filtering, an IQ recorder that also captures the seconds before you hit record, and a web dashboard for browsing decoded data from a phone.

The software is completely open source on GitHub, and the C++ code can be compiled from source, or a precompiled Windows build is available on sarasforge.dev for $15, with Sarah's Patreon patrons getting it free.

We note that Inmarsat signals such as AERO and STD-C/EGC can be received with our RTL-SDR Blog L-band Patch Antenna, which is available in our store.

Multi-Channel Voice Following Inmarsat Decoder for Windows!

Decoding Inmarsat in 2026

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QuadRF Now Crowdfunding on Crowd Supply for $499

Back in June and January, we posted about QuadRF and ScaleRF (previously known as open.space) and their mission to unlock affordable open-source Earth-Moon-Earth (EME) bounce communications for the amateur radio public.

Their core product, QuadRF, is a 4-element coherent antenna tile with built-in SDR and processing on a Raspberry Pi 5. Apart from being able to be chained together to make a massive coherent beam-steering antenna capable of working with EME, a single tile can also be used for C-band (4.9–6.0 GHz) direction finding and spatial signal visualization. QuadRF can also transmit, enabling point-to-point communication and the creation of mesh networks.

Recently, their product was launched on the CrowdSupply crowdfunding platform, and it has already far exceeded their $100,000 funding goal. The QuadRF kit is priced at $499 and includes the required Raspberry Pi 5, a tripod, a 32GB microSD card, and a power supply. The QuadRF Mobile expansion pack costs $149 and lets you take the QuadRF on the go. They are also selling a QuadRF six-pack for $594, although as this is designed for research, it does not include the enclosure, antennas, Pi 5, or phased-array structure. 

QuadRF - A 4x4 MIMO SDR tile for spatial RF vision & beamforming that scales as a phased array

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OpenStint: An Open-Source RC Car Lap Timing System Using an RTL-SDR or HackRF

Thank you to Attila for submitting news about OpenStint, an open-source lap timing system for radio-controlled (RC) car racing that uses an RTL-SDR or HackRF One as its receiver. In RC racing, each car carries a small active near-field transponder that transmits a unique identifier on 5 MHz using BPSK modulation. A wire loop embedded in the track acts as the receiving antenna, picking up each transponder's signal only within a short distance, which allows the exact crossing point to be detected. OpenStint digitizes this signal with the SDR and performs the decoding and pass detection entirely in software, with sufficient accuracy for RC racing.

Professional timing systems work on the same principle, but rely on dedicated proprietary hardware and software, with even entry-level systems typically costing thousands of dollars. OpenStint is compatible with the transponders used by the vast majority of RC racing clubs (MyLaps RC3 and RC4), while also supporting its own fully open-source transponder design. A complete decoder can be built from inexpensive off-the-shelf components, typically consisting of an RTL-SDR dongle, a simple loop interface, and a laptop or even a Raspberry Pi 3. The software has also been tested with existing timing software including LapBeeps, RCGTiming, and ZRound.

Besides the decoder itself, the project documents an open-source transponder protocol, includes an ATtiny-based transponder reference implementation, and describes the signal processing used for reliable pass detection. Documentation and source code can be found on the OpenStint website and over on the project's GitHub page, with the open transponder design available here.

Atilla also sells the transponder boards on the OpenStint website's sales page for a reasonable €30/panel + shipping (8 pcs per panel), and notes that larger quantities can easily be manufactured by JLPCB.

AI-Disclaimer: We note that Claude is listed as a contributor to the code.

OpenStint: RC Car Lap Timing with RTL-SDR or HackRF
OpenStint: RC Car Lap Timing with RTL-SDR or HackRF
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Airspy 2026 Summer Sale: 20% Off All Airspy Products

Airspy has just announced its annual summer sale, offering 20% off all Airspy products from June 27 to July 4, 2026. 

  • Airspy R2: US$169.00 US$135.20
  • Airspy Mini: US$99.00 US$79.20
  • Airspy HF+ Discovery: US$169.00 US$135.20
  • Airspy SpyVerter R2: US$49.00 US$39.20
  • YouLoop Antenna: US$39.95 US$31.96

The sale is active at all participating resellers, which includes our own store, where we have the YouLoop on sale for US$31.96, including free shipping to most countries worldwide (excluding tariffs!). 

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