Please note that the documentation provided for download contains more information than the one written on this place here. For instance, the Operating Manual describes how you can decide whether the power of your machine is sufficient to demodulate DAB without interruptions, or the necessary adjustments in QIRX to enable DAB reception even on less powerful computers. We plan to update the documentation, as it is rather outdated.

QIRX like most SDRs processes so-called I/Q Data. In case you are not yet familiar with this important concept, we can highly recommend the tutorial "What is I/Q Data?" by National Instruments.

• TCP/IP Based: QIRX accepts I/Q-Data either from TCP/IP servers like the widely known rtl_tcp, or from pre-recorded files (both 8-bit and 16-bit) containing the raw I/Q-data.
  Both QIRX and its I/Q-Data Server may run on the same machine or on separate ones. The data servers can be started automatically without additional user actions, also when used remote via a LAN.
• Hardware Support: QIRX supports RTL-SDR dongles via the rtl_tcp software, and the RSP family of devices hardware from sdrplay. For the latter we wrote a TCP/IP server widely compatible with rtl_tcp, supporting 8- or 16-bit data up to 8MHz of bandwidth. We called it RSP2_tcp, and you may find the sources on Github.
• Dual Receiver: Within the selected bandwidth, e.g. 2.56MHz QIRX is able to operate two independent receivers simultaneously.
• Squelch: For each receiver, QIRX provides a digital squelch, enabling to monitor the selected stations – when not transmitting - without annoying background noise.
• Simplest Operating Principle: QIRX – using its AM, NFM or WFM demodulators – is purely FFT-based, with a NF lowpass filter only. This might change in a future version.
• Frontend Calibration: QIRX - thanks to its DAB-Mode - allows for calibration of the receiving frontend oscillator by one mouseclick. In that case, the tuned DAB transmitter serves as a highly accurate frequency reference. The working principle is covered in detail in the three-part tutorial "Calibrate your rtl-sdr in 15 seconds".
• Scanner: QIRX provides for Receiver 1 a simple scanner, being able to scan large frequency areas. This is still in an experimental state.
  In the "DAB-Only" version, a configurable scanner for all DAB-Frequencies is included.
• Spectrum: For each receiver, QIRX provides a spectrum viewer being able to show the HF, NF, or Waterfall (not for DAB) spectrum.
• Audio File Recorder: For all demodulators, the audio output can be saved to .wav files, independently for each of the both receivers. For DAB+ this allows for high-quality audio recordings. AAC-Audio (DAB+) can be saved into raw AAC files (playable with the popular foobar2000), thus saving a lot of disk space.
• Raw Data File Recorder and Replayer: The raw I/Q data can be saved to a file. A second file is recorded automatically, containing the GNSS position. It is possible to replay I/Q-data files, having been stored in 8-bit or 16-bit format (16-bit: same format as HDSDR). On the GUI, you can position ("seek") to any arbitrary timed position in the recorded data. As a consequence, long raw recordings become usable, usually consisting of very large (many GB) raw data files.
  With recordings having made in a mobile environment, the moving position of the recorder is displayed on the map.
• DAB and DAB+ Receiver: QIRX provides a comfortable DAB+ receiver ( Transmission Mode I ). It is -to the best of our knowledge- the first C# based SDR providing this facility. Some standard libraries like the Viterbi decoder are used as C/C++ packages, accessed via P/Invoke. QIRX decodes "Standard" DAB as well as DAB+ frames. Due to the particular properties of such a specialized receiver, the DAB has been "spinned-off" into a "DAB-only" project, also contained in the download. However, the DAB decoder remains in the standard distribution of QIRX, possibly with less features than the DAB-only version.
  
  DAB Spectrum Display
  • Constellation of the bits in a non-conventional linear display, showing the accuracy of the decoded bits with high resolution.
  • TII Carriers: This display shows the carriers of the Transmitter Identification in the Null-Symbol. QIRX has been the first SDR to display this information.
  • Channel Impulse Response (CIR), showing the relative delays of a signal having taken either different paths along its way to the receiving antenna, or stemming from different transmitters.
  Regrettably these features are not available in the Linux version, as the Mono environment does not support the MS-Chart of the Windows .NET runtime.
• Transmitter Identification: In DAB mode, QIRX detects and displays the Transmitter Ids (TII values). This should work also in complex mobile receiving environments. QIRX has been the first SDR providing this feature.
• TII Logging: In its DAB-Only version, QIRX offers a simple, text-based logger of the TII codes. The logging takes place in a fixed time raster of one second. The combination of the logging and the highly configurable DAB-scanner might be a helpful tool for DX-ing applications. The log files are ready for import into Excel (Tab as delimiter). The own position coming form a GNSS sensor is included in the logging.
• Map Integration: In its DAB-Only version, Transmitters with decoded TII values are displayed as icons on the map (Openstreetmap) at their geographical positions. . The data are contained in a database having been collected from OFCOM (UK) and DABLIST (other countries).
• GNSS Integration: If available, a GNSS sensor can be activated, showing the current own position on the map. Raw file recordings obtain a second file, logging the GNSS position once a second. Synchronous replay of recorded raw files together with their GNSS data is possible.
  TII logs also obtain the current GNSS data.
• Remote I/Q Data Server Automatic Startup: A separate executable, called "StubForRemoteStart.exe" can be used for an automatic start for a remote I/Q data server. Once started on the remote machine, it listens permanently to commands from the local QIRX for the creation and deletion of the I/Q data server process like rtl_tcp.
• Linux Port: As mentioned, QIRX runs under Linux, although with some restrictions. This was intended as a practical experiment of how simple or difficult it is to run non-trivial .NET software under Linux, also with respect to currently ongoing activities in the field (Mono, .NET Core, Xamarin). It is not clear yet if this line will be continued with QIRX, also in the light of the performance issues encountered.
  A short report has been added to our website.

As mentioned, the operating principle of QIRX is based on very simple Fast Fourier Transform (FFT)-based down conversion (Decimation) of the incoming I/Q data.

• Down-Conversion In QIRX, down-conversion takes place in the frequency domain. The user selects - as usual - a center frequency from the spectrum showing a signal of interest. Interesting signals have different bandwidths with respect to their modulation. For instance, a FM radio station has a much larger bandwidth than an AM station in the airband or a NFM station in the amateur radio band.

  For down conversion, a suitable part of the available spectrum of about 2MHz is cut out such that this spectrum piece satisfies the necessary bandwidth. To proceed with the following inverse Fourier Transform it needs to have a length of a power of two.

  An Inverse Fast Fourier Transform (IFFT) switches from the frequency domain back to the time domain, providing samples directly suitable to be fed into the demodulator. The demodulator provides the audio samples to be presented at the headphones output of the PC.

• Example1: Airband. Assuming a total available bandwidth of 2.048MHz, with a down-conversion factor of 128, a part of 2048kHz / 128 = 16kHz symmetrical to the desired center frequency is cut out of the spectrum. After IFFT time domain I/Q samples with a sample rate of 16kHz are obtained. After demodulation audio samples with a bandwidth of 8kHz are the final result.
Example2: FM Radio Stations. Assuming a total available bandwidth of 2.048MHz, with a down-conversion factor of 16, a part of 2048kHz / 16 = 128kHz symmetrical to the desired center frequency is cut out of the spectrum. After IFFT time domain I/Q samples with a sample rate of 128kHz are obtained. After demodulation audio samples with a bandwidth of 64kHz are the final result.
• Advantages, Disadvantages: The advantage of this method is its simplicity. For the necessary FFT and IFFT operations the fastest available library fftw is used, with a wrapper for C#. It is straightforward applicable for the use of two receivers operating simultaneously. Of course using two receivers is of little interest for WFM radio stations, but highly welcome when listening to other services like airband where stations always transmit only for a very short time. In QIRX, the primary intention for using the described method has been the demonstration of its simplicity.

  The disadvantage of the method of course is its lack of flexibility. In order to preserve its simplicity, one has to stick with very few available bandwidths. Bandwidth and Demodulators cannot independently be selected.

• Artifacts: The missing filtering in the frequency domain might cause artifacts after decimation (The term "decimation" usually implies Low-Pass filtering before down-conversion). Filtering in the frequency domain would have complicated the software considerably, as methods like Overlap-Add or Overlap-Save must be applied. However there are other causes for artifacts when dealing with I/Q data like I/Q imbalance. As a result the possible disadvantages caused by a missing FFT filter have been accepted.