In early September we posted about Oona Räisänen’s deinvert which is a tool that can be used to unscramble voice audio that has had voice inversion scrambling applied to it. Voice inversion works by scrambling the voice frequencies so that a simple eavesdropper will have trouble listening in. A special descrambling radio is required to listen in. This provides very little real security, but may be enough to stop people with cheap scanners from listening in. Oona’s deinvert tool allows us to take a scrambled audio sample recorded with an RTL-SDR or any other radio and decramble the inversion.
In her latest blog post Oona explains how her deinvert software works and how it can also be used to decode the more difficult split-band inversion technique. She also writes that at the default quality level, the deinvert software is fast enough to run in real time on a Raspberry Pi 1.
However, being without a directional coupler Tomi looked for other options and realized that cheap TV antenna network taps are also directional couplers. Taps are commonly used with Cable and Satellite TV installations to split a signal from an antenna over multiple TVs. They are designed as directional couplers to ensure that unwanted signals do not feed back into the antenna system and so that there is a pass through port to continue the strong signal down a long cable.
Note that there is a difference between a tap and a splitter. Taps are used when multiple devices need a signal over a long run of cabling. A splitter divides the signal strength by the number of out ports and can feedback unwanted signals into the system.
Taps vs. Splitter Example (Source: http://forums.solidsignal.com/showthread.php/5843-Solid-Signal-s-WHITE-PAPER-The-NEW-DIRECTV-Residential-Experience)
In his tests Tomi found that TV taps worked acceptably well to determine the resonance frequency of an antenna that he was testing. Taps can be found for as cheap as $2 on sites like eBay, although for some listings it is unclear over what frequency range they work well at as sellers assume that they will be used for TV frequencies.
Over on YouTube Christopher Bridges has uploaded a video showing him using a PlutoSDR and a GNU Radio program to transmit a DVB-S signal, which is then received with an RTL-SDR. DVB-S is a digital video broadcasting standard designed for satellite transmissions and digital amateur television video (DATV) also uses DVB-S in the 1.2 GHz amateur band. In this example the PlutoSDR transmits at 1.28 GHz.
Chris uses the rtl_sdr command line software to receive the raw IQ data at 1 MSPS, and then uses the leandvb software to decode the raw IQ file directly into a video file.
If you’re interested in TXing DVB-S/DATV but don’t have a transmit capable SDR, then we note that even a Raspberry Pi just by itself can be used to transmit it with rpidatv.
Linux gnuradio QPSK DVBS PlutoSDR + rtl MacBook leansdr
Thanks to Marcin Jakubowski for submitting news about his new software tool called iqToSharp which is a simple tool that allows you to convert rtl_sdr IQ files into the SDR# IQ format. The rtl_sdr command line tool records raw IQ files but by default they are not compatible with the format used by SDR# so a conversion is required.
This is useful as for example you could set a command line script to record an entire band for a few hours on a portable Linux device like a Raspberry Pi, and then use the converter to listen to the file on SDRSharp at a later time. Recording the raw IQ file allows you to record all signals within the entire bandwidth at full quality.
Note that IQ files can become very large so for archiving compressing them with FLAC can be useful. You might also be interested in the SDR# FilePlayer plugin which allows you to easily skip back and forth in time through a recorded IQ file.
Thanks to Doug Ward (@dsward) for letting us know about his new RTL-SDR compatible MacOS based app called LocalRadio. LocalRadio is an open source web browser based app that connects to a MacOS server running an RTL-SDR. The software allows you to listen in on any frequency supported by the RTL-SDR in AM or FM modes, and audio is capable of being streamed to multiple devices via a built the LAME MP3 encoder, EZStream and Icecast server. It does not provide an FFT or waterfall display however.
The software introduction reads:
LocalRadio is an experimental, GPL-2 licensed open-source application for listening to “software defined radio” on your Mac and mobile devices. With an inexpensive RTL-SDR device plugged into the Mac’s USB port, LocalRadio provides a casual listening experience for your favorite local FM broadcasts, free music, news, sports, weather, public safety and aviation scanner monitoring, and other radio sources.
LocalRadio’s easy-to-use web interface allows the radio to be shared from a Mac to iPhones, iPads, Android devices, and other PCs on your home network. No additional software or hardware is required for sharing with mobile devices, simply use the built-in mobile web browser to connect to LocalRadio and tune to your favorite stations. You can also listen to LocalRadio audio on your Apple TV and other AirPlay-compatible devices.
LocalRadio does not provide features like FFT waterfalls, panadapters, or signal recording that are found on other SDR software. For those features, GQRX for Mac is highly recommended. GQRX is a good way to discover radio frequencies that can be used with LocalRadio.
LocalRadio is intended for use as in-home entertainment, using a local area network with a private IP address. It has not been tested with a public IP address, particularly for security testing, therefore it is not recommended for that purpose. For simply listening to LocalRadio on the Mac with the RTL-SDR device plugged in, no network is required at all.
Thank you to Adrian for submitting his video about using the Android App called QRadioLink and an RTL-SDR to decode digital amateur radio voice transmissions. Adrian writes that in the video the RTL-SDR connects to the Android phone with a USB OTG cable and uses a sample rate of 1 MSPS. He also writes the following about QRadioLink:
QRadioLink is a building platform which allows experimenting with VHF-UHF SDR transceivers using different modulation schemes for digital data transmissions. So far digital voice and text transmission is supported, using either a narrow band modem and Codec2 or a high bandwidth modem and Opus. Supported hardware includes the RTL-SDR, Ettus USRP, HackRF, BladeRF and in general all devices supported by libgnuradio-osmosdr.
QRadioLink running on Android (Debian chroot) with RTL-SDR
Over on YouTube Mike from the SDRplay team has created a tutorial video that shows how to use the SDRuno EXTIO edition. SDRuno is the official software of the SDRplay line of products and can be freely downloaded from the SDRplay website. The EXTIO edition allows other non-SDRplay SDR units to freely be used with SDRuno. The only restrictions are that the maximum bandwidth is artificially restricted to 2.5 MHz and some DSP filters are missing.
In the video Mike shows how to set up the SDRuno workspace to work with an RTL-SDR dongle and demos reception of some signals. Note that the EXTIO dll file for the RTL-SDR mentioned in the video is the same one required for HDSDR, and can be downloaded from the dll table on the HDSDR website.
If you’re interested in more, Mike has a full SDRuno tutorial series available on the SDRplay YouTube channel which mostly focuses on usage with the SDRplay units, but could be applicable to the EXTIO version as well.
SDRuno EXT/IO Edition for a range of SDRs and dongles
Thanks to Juan Moreno for letting us know that his online MOOC (massive open online course) on RTL-SDR is starting on September 25. The course is presented by Juan and three of his colleagues from the Technical University of Madrid. It will focus on SDR 101 knowledge such as digital signal processing with the aide of an RTL-SDR to help with practical learning. In their video they also mention that MATLAB and Simulink will be required and used for most of the course, so it will probably be a fairly technical beginners course at a University level of learning. Their description of the course reads:
SDR is a reality around us. It is present in a lot of systems everywhere and is a versatile technology which can be used for many things (not only academics and industrial). The purpose of this course is to introduce students into general-purpose SDR tools. The SDR hardware platform chosen for this course is the RTL-SDR. It is worldwide available, it’s cheap ($15) and there is a lot of help in the Internet. But, as far as we know, there is no other MOOC focused on an introduction to SDR as this MOOC. Here we will not only learn about SDR but also a lot of related areas like antennas, digital signal processing, radio frequency and communication electronics.
The website and registration forms seem to all be in Spanish or Portuguese, but the course will be presented in entirely in English. Google Translate can easily be used to help with the signup process. The course is completely free and students that complete 75% of assignments will receive a free participation certificate. A more official accomplishment certificate can be obtained for a 50 Euros.
