SDR - Software Defined Radio

#Install dependencies
sudo apt-get install build-essential git libfftw3-dev cmake libusb-1.0-0-dev

#Fetch and build rtl-sdr, skip if already done (subdirectories will be created under the current directory).
git clone git://git.osmocom.org/rtl-sdr.git
cd rtl-sdr/
mkdir build
cd build
cmake ../ -DINSTALL_UDEV_RULES=ON
make
sudo make install
sudo ldconfig
cd ../..

#Disable the DVB-T driver, which would prevent the rtl_sdr tool from accessing the stick
#(if you want to use it for DVB-T reception later, you should undo this change):
sudo bash -c 'echo -e "\n# for RTL-SDR:\nblacklist dvb_usb_rtl28xxu\n" >> /etc/modprobe.d/blacklist.conf'
sudo rmmod dvb_usb_rtl28xxu # disable that kernel module for the current session

#Download OpenWebRX and libcsdr (subdirectories will be created under the current directory).
git clone github.com/simonyiszk/openwebrx.git
git clone github.com/simonyiszk/csdr.git

#Compile libcsdr (which is a dependency of OpenWebRX)
cd csdr
make
sudo make install

#Edit OpenWebRX config or leave defaults
nano ../openwebrx/config_webrx.py

#Run OpenWebRX
cd ../openwebrx
./openwebrx.py

RSC-5-C is the standard for digital terrestial radio in the United States. The physical layer and protocols are well documented on the NRSC’s website. The audio compression details are conspicuously absent considering that the goal of the standard is digital audio.
Overview

The development goal of NRSC-5-C was digital terrestial radio using the already allocated FM bandwidth. The result was an IBOC (in-band on-channel) system that supports both analog and digital signals within the existing FM allocations. This hybrid scheme is flexible and the station can choose to allocate more or less of the spectrum to analog or digital. Eventually, the entire spectrum allocation could be used for a pure digital signal.
Spectrogram
Figure 1: Spectrogram showing analog FM signal with digital sidebands

The system is split into several layers. The physical layer (layer 1) is responsible for finding the signal, decoding, and error correction. The multiplexer (layer 2) allows multiple layer 3 applications to share the bandwidth, giving priority to the real-time audio data. And the application layer (layer 3) is where the decoding of the audio data actually happens.

Our implementation of a NRSC-5 receiver is available in a GitHub repository. The source code is released under the GPL v3, and uses code from turbofec and reed-solomon, as well as the ao, faad2, and liquid-dsp libraries.

HD Radio is a high definition terrestrial digital broadcast signal that is only used in North America. It is easily recognized by the two rectangular blocks on either side of a broadcast FM station signal on a spectrum analyzer/waterfall display. Since HD Radio uses a proprietary protocol, finding a way to decode it has been difficult and so this signal has been inaccessible to SDR users for a long time. Back in February of this year we posted about Phil Burrs attempt, where he was able to create a partial implementation (up to layer 2) of the HD Radio standard, but didn’t get far enough to decode any audio in layer 3.

However, now cyber security researcher ‘Theori’ has created a full RTL-SDR based decoder for the HD Radio protocol. In his post Theori explains that the HD Radio system is split into three layers. Layer 1 finds the signals and does decoding and error correction. Layer 2 is a multiplexing layer, which allows various layer 3 applications to share the bandwidth. Layer 3 is the audio data layer. In his post he explains how these layers work in detail.

One of the main findings was the discovery of the audio compression codec. Theori found that the codec was essentially HE-AAC with some minor modifications. The modifications were minor enough that he was able to adapt the open source FAAD2 library for HD Radio audio decoding.

Theori’s code is open source and available on GitHub. The code includes the patch to modify FAAD2 for HD Radio and it is automatically applied during the build. A sample file for testing the decoder is also provided and we tested the decoder with the sample and it worked well. The decoding can also be performed in real time and examples of that are also on the git readme.

During my 2017 SWL Fest presentation, I mentioned that there is no way to decode either HD Radio or ATSC HD Television using software designed for the RTL2832U dongles. The explanation I provided is that both protocols are covered by patents and that the holders have not been forthcoming on providing necessary details to the open source community.

A cybersecurity researcher, Theori, has cracked the codec used by the NRSC-5-C standard for US based terrestrial digital radio. I am now listening to HD Radio via an RTL SDR dongle. It takes a decent signal, so I’m not getting too many stations using an inside whip antenna, but there are enough to experiment with. It also takes a better dongle with good frequency stability. An older dongle without the TCXO was not up to the task, even on an i7 based system.

The discovery is summarized on the RTL SDR Blog. You’ll need some familiarity with building packages under Linux to grab the source from github and to compile it on your system. So far, I’ve compiled under Debian x86_64, Fedora x86_64, and Raspbian! Next, I want to get it running under Cygwin so that I can use it on the Windows 10 computer in the radio room.

It is a blast to be able to decode the alternate program streams. Audio quality is better than Sirius XM.

Thanks Theori!

The Airspy HF+ is an upcoming product from the Airspy team that is intended to be a high performance HF/VHF receiver at a low price. Its frequency range will be DC to 31 MHz, and 60 to 260 MHz and the bandwidth will be about 660 kHz. So why choose the HF+ over the Airspy R2, Mini or SDRplay which all have larger frequency ranges and bandwidths? It seems the focus of the HF+ is to be an extremely high dynamic range receiver. This means that strong signals should almost never overload the receiver making it very good for DXing weak signals (listening to weak signals from very far away). On other receivers once you turn the gain up strong signals can block reception of the weaker ones.

Recently we saw the release of some of the first 3D renderings of the product. Now finally we have a photo of the actual PCB which is shown below. The RF sensitive innards are hidden away within a shielding can, but we know from the product page that inside are the switches, filters, tuner, ADC and 18-bit DDC.

AIRSPY HF+: COMPARED AGAINST COLIBRINANO, AIRSPY MINI, RSP2

Performance from the Airspy HF+ product page is stated as:

-141.0 dBm (0.02 µV / 50 ohms) MDS Typ. at 500Hz bandwidth in HF
-141.5 dBm MDS Typ. at 500Hz bandwidth in FM Broadcast Band (60 – 108 MHz)
-139.5 dBm MDS Typ. at 500Hz bandwidth in VHF Aviation Band (118 – 136 MHz)
-139 dBm MDS Typ. at 500Hz bandwidth in VHF Commercial Band (136 – 174 MHz)
-138 dBm MDS Typ. at 500Hz bandwidth in the upper VHF Band (> 174 MHz)
+26 dBm IIP3 on HF at maximum gain
+13 dBm IIP3 on VHF at maximum gain
110 dB blocking dynamic range in HF
95 dB blocking dynamic range in VHF

Update 7 Dec 2017: Under the Shield
As promised now that the Airspy HF+ is shipping and fully released we will show what is under the shielding can. Please take no note of the modded components and hacked in shorts as we had an early prototype unit which required some mods to achieve the full performance.



As some already guessed, the main chip is the STA709 which is a new digital tuner designed for automotive applications. The technology in the chip is fairly cutting edge, so combined with good PCB design and good DSP processing code is one of the secrets to the Airspy HF+.

Under the Airspy HF+ Shielding
Under the Airspy HF+ Shielding
Conclusions
The Airspy HF+ is an exceptional SDR and will truly please any DXers or people wishing to listen to weak stations. It is a relatively narrowband SDR (in comparison to say the Mini/R2 and RSP2) that can only tune up to 260 MHz, so don't expect to be able to use it as a wideband scanner for trunked radios for example. But on VHF it would perform very well on FM DX, airband voice scanning and for 137 MHz WX satellites. The reception on the HF+ is almost entirely unaffected by extremely strong pagers in the 157 MHz region.

Below 30 MHz the HF+ also shines. VLF to MW is the best we've seen on any sub $300 SDR. Overload is non-existent on broadcast AM, and no effects from the strong AM signals can be seen further up on the spectrum.

The closest competing unit to the HF+ in terms of price and use cases (designed for HF) is probably the ColibriNANO. But the ColibriNANO commands a decently higher price at $350 USD. Performance on HF seems similar, but we do have to give a slight edge to the HF+. The ColibriNANO also has the downside of poor LF/VLF reception (advertised response starts at 100 kHz), and heavily aliased VHF/UHF due to undersampling. A filter is needed for proper operation on VHF/UHF. That said the ColibriNANO itself is a very good SDR, but the HF+ certainly wins out in terms of value and general performance and we can't see any situation where the ColibriNANO would be a better choice at the moment.

The Airspy Mini/R2 and SDRplay RSP2 also generally perform well for the majority of signals, but will struggle when it comes to really strong signals. Comparing against these SDRs on weak signals near strong blockers really shows where the HF+ shines. But when compared against regular (non-weak) signals or in a tame RF environment without strong signals then it is pretty much impossible to determine which SDR is better.

So there is obviously some brand new cutting edge technology going on in this receiver with the polyphase harmonic rejection mixer and the sigma delta ADC which possibly even puts it on top of the very expensive direct sampling SDRs. On the HF+ weak and DX signals are noticeably more accessible. Performance for the price (expected $149USD) is phenomenal. This is a highly recommended SDR.

Disclaimer: We received the HF+ for free in exchange for an honest review, but are not affiliated with Airspy. We've been in contact with the Airspy team who have helped clarify some points about the architecture and technology used in the design.