Operating Environments

Please note that the complete installation and startup procedure is documented in the Operating Manual, available in the Download section.

QIRX is able to work in the following environments:

• Stand-Alone: Although QIRX works with a TCP/IP frontend, it can be operated - together with rtl-tcp or any other I/Q-data server - on the local machine. In that case, the tcp/ip address is the one of the localhost, i.e. 127.0.0.1. The QIRX software is able to start and stop the local rtl-tcp.exe automatically, without any further user action. The USB driver of the RTL-dongle must have been installed.
• Single Remote: In this configuration, the rtl-tcp server runs on a remote machine, for instance at an antenna location with better reception possibilities. Also in this case the QIRX software is able to start and stop the remote rtl-tcp.exe automatically, without any further user action.

  This environment might also be useful in case the installation of the USB driver (e.g. the Zadig driver) is not desired on the machine running QIRX. No particular speed requirements exist for a PC with rtl-tcp as I/Q data server.

• Dual Local and Remote: QIRX does not need any Registry entry. As a result, independent QIRX instances can run on the local machine, receiving their I/Q-data from different rtl-tcp servers. In such a situation it might be possible to watch up to four stations simultaneously (one station in each receiver, each QIRX instance providing two receivers) across twice the bandwidth of a single QIRX instance. The usual cheap RTL-SDR dongles are able to cover up to about 2.5MHz. Of course, the two receivers operate independently, their data are not phase synchronized (i.e. no coherent operation).
• DAB+: QIRX works in DAB+ Mode I. It provides either fully automatic or manually controlled frequency synchronization. Additionally - to save processing power - it is possible to tune the fine frequency deviation in the dongle instead of the software. In this case, the phase shift caused by the non-perfect frequency synchronization (due to the limited tuning possibilities of the rx dongle) is compensated by additional control loops in the software.

  The sampling rate error is always corrected.

Digital Audio Broadcast (DAB+) Demodulator

In DAB, radio stations grouped together are called an "Ensemble". Each Ensemble covers a bandwidth of 2.048MHz, thus making DAB an ideal candidate for reception with a RTL-SDR dongle. No down-conversion is necessary, the whole spectrum is used. The various radio stations within an ensemble are called "Services". To receive a DAB radio station, a user has to select the Ensemble and the Service. Ensembles are transmitted on published frequencies. DAB+ (in contrast to DAB) uses an enhanced Reed-Solomon error correction within a so-called superframe.

For a more detailed description, please consult the Technical Report available in the download section.

The picture shows a nearly ideal spectrum of a DAB+ ensemble, 20-fold averaged (the degree of average is GUI-controllable).

• Comparison with the Standard: The shown spectrum has been scaled and overlaid in Photoshop for a comparison with a theoretical spectrum published in the Standard. The coincidence is excellent. However, very often the DAB spectra look not so ideal due to disturbances like multipath reception.
• Coarse timing: DAB signals are organized in frames, composed of symbols. The frame start is indicated by a transmission of a number of zero-intensitiy I/Q samples, the "Null Symbol". This fact is used to roughly find the start of a frame. As a peculiarity, in the Null Symbol the "Transmitter Identification" TII is sent with a low power. This allows for identifying the different transmitters contributing to a receiving situation. It might be noted that in the DAB SFN (Single Frequency Network) the TII is the only information transmitted in an ensemble differing between the different transmitters involved. QIRX - in its special distribution - is the first and currently only DAB SDR providing this information.
• Coarse Frequency: DAB imposes stringent synchronization requirements in time and frequency on the receiver. As a result, the center frequency must be identified down to at least 1kHz. The frequency of the receiver hardware might deviate considerably from its nominal frequency. Usual cheap RTL-SDR dongles show frequency deviations of 50ppm and more.

  In previous versions of QIRX (until 0.9.1) the center frequency has been found by inspection of the spectrum shape and searching for the central "dip" (suppressed central carrier). While this usually worked well, the method has been omitted in order to speed up synchronization also in more adverse receiving conditions like strong distorted central parts of the spectrum due to multipath.
  Starting with version 0.9.2, the coarse frequency synchronization is performed by scanning the receiving frequency around a reasonable (e.g. 50kHz) range of the center frequency, and searching for a correlation peak with the Phase Reference Symbol. With steps of 1kHz and some interpolation the accuracy achieved is about 250Hz.

• Fine timing: To find the start of a frame within the accuracy of a sample, the Phase Reference Symbol is used. It is always sent with identical, constant values of its samples, overlaid by noise due to the HF transmission. The undisturbed values are stored in the software, already in the frequency domain. The two blocks are compared by using a fft-based cross correlation ("Phase Correlation"). In this way the exact start of a frame can be identified.
• Fine Frequency: While the “Coarse Frequency Synchronization” is responsible for finding the frequency of the transmitter down to 1kHz, the “Fine Frequency Synchronization” should find it down to 1Hz. This corresponds to an accuracy of 5*10e-9. This high accuracy becomes possible because additional information is sent within the symbols of a frame, the "Cyclic Prefix" (CP). Each symbol is prepended by a CP, being a copy of the last (504 in mode I) samples of the symbol. Frequency shifts in the Hz region are obtained by measuring the phase shift between the CP and the end of the frame.
• Permanent Correction: The fine timing adjustment is checked after every frame and corrected if necessary. The fine frequency deviation also is frequently checked, at a much lower rate. Its permanent correction imposes a high processing load on the PC. Therefore an alternate solution is offered in QIRX as an option, letting the hardware make this correction. Details can be found in the Technical Report.

  Coarse timing and frequency checks are only performed after a complete synchronization loss.

DAB Bit Constellation

In DAB the information like audio is transmitted digitally. The receiver obtains the bits by a calculation of an angle of a subcarrier with its predecessor. Ideally, this angle has one of the four values 45°, 135°, -135°, -45°. These four possible values correspond to two possible bits with the values 0, 1, 2, 3. The arrangement of the bits with respect to their angles is called “constellation”.

Usually the constellation is displayed showing the bits as dot heaps in a polar diagram, like in the following picture, from a report by Andreas Müller, ETH Zurich. The dot heaps show the single bits, the heaps being separated by 90 Degrees.
QIRX uses a different, linear display for an improved visual control of the synchronization accuracy.

The picture shows an example: The spectrum (upper part) shows regions of strong multipath reception reducing the signal strength around 178 MHz. The constellation (lower part) shows – for each subcarrier – how well the bits are arranged on their correct position. Each dot corresponds to a single bit value. The reduced signal strengths around 178MHz clearly shows a much larger scattering of the bits off from their correct values. In a polar display it would not be possible to assign regions of large scattering to the frequency regions (subcarriers) with reduced signal strength.

This kind of constellation display can be used to obtain additional information.

• Sampling Rate Error: A sampling rate error shows up in the constellation by displaying the bits - depending on their subcarrier - with a misalignment from their ideal positions. In a polar plot, this would show up as a slight distortion, not easily being recognized. The sampling rate error is in the same order of magnitude as the frequency error of the dongle, assuming both are derived from the same clock.

  In QIRX, the sampling rate error is permanently corrected.

• Constant Angle Error: Slightly changing the frequency (e.g. in manual mode) results in a shift of the whole constellation by a constant angle. QIRX permanently monitors the constellation and corrects this type of error. In a polar plot, this error would show up in a rotation of the whole constellation.
• Constellation Worst Case: The worst case for a bit decoder is the one where all of the bits are arranged between two quadrants. In such a situation clearly no unambiguous decoding is possible. QIRX' control loop will shift the constellation into an unambiguous state, seemingly showing a properly aligned constellation. However, this loop cannot have any knowledge whether it shifted the bits into the correct quadrant. The solution to this problem consists in rotating the whole constellation by values of 90 degrees until a reception becomes possible. As soon as this happens, the rotation is stopped. The Fast Information Channel (FIC) is used as an indicator whether a proper decoding took place. The FIC bits are CRC-protected, making a reliable decision about the correct quadrant possible.

Transmitter Identification

As already mentioned, the Transmitter Identification Information (TII) is sent with low power in the Null symbol. The details about how the TII is coded can be found in the Standard. The following is to illustrate how the QIRX software presents the TII information to the user in different receiving situations.

• Simple Receiving Situation: The picture shows on its left side the "DAB+" dialog, enhanced by the "Transmitter Identification" part. As the "Show TII Spectrum" checkbox is selected, the TII spectrum is displayed in the window on the right usually showing the constellation.

  The four red-framed boxes in the spectrum all contain the same coded information about the transmitter. This information consists of a "Main Id" and a "Sub Id". The Ids are displayed in the table of the dialog.

  Only a single transmitter contributes to the received signal. The number in the "Strength" column is an approximate value how much an average signal strength exceeds the selected "Threshold" value.
• Medium Complexity Receiving Situation: The next picture shows a somewhat more complex receiving situation. Three different transmitters contribute to the reception of the "BR Bayern" ensemble. They are listed according to their relative strength in the table of the "DAB+" dialog. All three transmitters show the same "Main Id". The reason might be that "Bayerischer Rundfunk" assigns to transmitters in a certain region the same Main Id and different Sub Ids. However, in Germany no common organizational scheme exists for these Ids.
• High Complexity Receiving Situation: The last picture of this series shows a highly complex receiving situation, having been recorded during a flight. Although the frequency spectrum looks fair enough, not less than seven different transmitters are contributing to the received signal. The indicated center frequency does not correspond to the shown ensemble, as the picture has been taken from a raw-data playback where frequencies are irrelevant.
  Such a situation makes a synchronization more unreliable, because signals violating the guard interval contribute to Inter Symbol Interference with its accompanying negative impact on synchronization.
• Related Problems: The sketched "complex" receiving situation leads to the rather difficult problem where to position optimally the "FFT-Window". The term "FFT-Window" used in this context means the positioning of the FFT start in the time domain sample stream, in order to make best use of the guard interval when several transmitters contribute to a reception. In mobile environments receiving situations change rapidly. As a consequence, when such decisions are to be made on a symbol-by-symbol basis, a pure Software-based solution is limited by the possible performance.
  Nevertheless, the simple possibility to identify different transmitters in recorded raw-data opens the door to tackle these and other problems in a systematic way.