Plugins for SDRuno

I develop the software on a Linux box. Windows versions of the programs are usually done using a cross-compilation facility (mingw) on this box. I am not very familiar with Windows and personally I am not happy with Windows.

Nevertheless, since I am an enthousiastic user of the SDRplay device, and the plugin interface for plugins in SDRuno is reasonably easy, I converted some of the decoders that were written for the sw software to plugins for the SDRuno environment.

Plugins are there for weatherfax, FT8 psk, rtty, navtex, cw, drm and acars plugins.

All plugins except the acars plugins share the same model for input handling: The SDRuno provides an interface for (complex) samples that are decimated to 192000 Ss. Most plugins deal with signals with a small bandwidth, e.g. psk31 signals have (in theory) a bandwidth of less than 50 Hz, and DRM takes 10 KHz, so even for 192000 additional filtering and decimation is required.

The group of plugins for cw, rtty and psk are relatively simple. The real problems with handling the kind of signals for these plugins is in accurate tuning. An offset of 20 to 30 Hz is sufficient to make decoding of e.g. psk31 signals, especially qpsk32 signals, impossible. For an amateur RTTY signal the space between mark and space is 170 Hz, so a tuning offset of even less than 100 Hz ruins decoding here.

cw psk rtty

Of course, each of the plugins has a number of switches for settings, such as filterdepth, normal or reverse encoding etc etc.

Another plugin is the one for decoding navtex> signals. Navtex signals are usually transmitted on 518 KHz by coastal services. The encoding of the signal is simple, a 100 baud FSK signal with a width between Mark and Space of 170 Hz. A few layers of protection and error correction ensure that only real Navtex messages are shown.


When invoking the plugin, the SDRuno is automatically set to the frequency of 518 KHz. The plugin has some selectors to bypass some validity checks on the messages.

A more complex plugin is the weatherfax plugin. On shortwaves there is still a number of stations transmitting weather information. With my limited antenna equipment I receive stations transmitting such charts on 3588, 4610, 7880 and 8020 KHz regularly, all from this region. In other regions there are obviously other frequencies. The US National Weather Service publishes a document Worldwide marine radio facsimile Broadcast Schedule which can be found on the internet.


A weatherfax transmission starts with a tone on a given frequency (usually 450 Hz), followed by a simple pattern with which the receiver can synchronize. Then, line by line the image is transmitted in either AM or FM mode. The precise parameters are detemined by the mode, the picture here was transmitted in WeFax576 mode. The transmission btw shows that there is a small clock error.

A transmission with 2 lines a second and 1200 lines takes 10 minutes, so when testing these was a need for a button with which the sync procedure could be overruled. A button on the widget - when touched - tells the software that it should behave as if a sync was detected.

Note that the picture on the computer screen is reduced in size, however, the program has the original picture "in core" and when dumping the picture, it will have the original size.

The FT8 plugin decodes - as the name suggests - FT8 messages and shows the decoded messages on the screen. If you fill in your home location (station identification or, as I do, a(n official) NLXXX number) the software - when switched on - sends data about the caller on the received messages to the PSK recorder. (the button callsign shows - when touched - a small field where the callsign can be written).


To ease tuning, the plugin is equipped with presets, where frequencies like 14075, 21075 KHz etc are listed. The plugin allows user manipulation of the preset list, so deleting and frequencies is supported. Contents of the list is obviously saved between invocations of the plugin.

A plugin for one of the more interesting broadcasting schemes is the plugin for decoding DRM (Digital Radio Mondiale). As mentioned elsewhere, DRM is a scheme for transmitting digital radio on short and medium waves.


The plugin shows the decoding of the Voice of Nigeria, which as far as I know is not in the air anymore. The widget shows that the transmission is in Mode C, with spectrum occurrence 3, telling that the transmission was 10 Khz wide, and telling what the layout of the digital raw data frames is. DRM transmits - next to the payload, a number of signals with which the software is - in principle - able to decode the payload.

The widget also shows that the format in which audio was encoded was AAC, and the bit pattern used was QAM16, with a resulting 12000 Hz signal. The transmission contained text as well, as can be seen at the bottom line.

More technical details state that the receiver was -272 + 27.5 Hz off the frequency transmitted, the time was 18:37 (UTC).

The picture finally shows that all required steps for synchronization were found OK. The FAC, Fast Access Channel, the SDR, Service Description Channel and the AAC could all be decoded without errors or with sufficient error correction. If any of these synchronizations is faulty, then it is unlikely that there is any audio output.

One comment is in order: the plugin does not have any control. All settings have to be detected by the software, by interpreting the incoming signal. Of course, the setting of the frequency was done in SDRuno itself. Frequency correction - as said here about 250 Hz - is done in software.

The most recent plugin (actually, there are two versions implemented) is for decoding Acars signals, aircraft communication signals in the 130 MHz band, These is a lively (automatically generated) conversation between planes and airfields, and each and every event - such as open a door, close a latch - is signaled by the plane and transmitted. These messages are encoded, obviously augmented with plane identification and time - and encoded messages are then transmitted.

A few years back I rewrote the excellent software of Thierry Leconte in C++, ir order to create support for devices like the SDRplay and - recently - derived these plugins. So, copyrights of some parts of the plugin code are Thierry's.

acars acars

The left picture shows the "simple" version, the decoder decodes the signals - if any - on a single selected frequency. Acars frequencies are standardized per continent, the plugin lets you choose the continent and then shows a list of frequencies commonly used in that continent.

The picture on the right is slightly more advanced, it handles more channels simultaneously. The picture shows that signals on 131725KHz, 131525KHz and 131825KHz are being dealt with. The bandwidth selected for the SDRplay is 2 MHz, and a group of transmitters, within the width of 2 MHz can be selected. In a few cases the maximum frequency distance between transmitters is more than the 2 MHz, and then more entries for the continent are made.