DRMPLUS-0.95 is EXPERIMENTAL software for the decoding of DRM+ signals. The software is "still in development" and certainly not error free.
Since there are here no drm+ transmission I am depending on a few recordings. Right now, both the AAC and xHE-AAC audio can be decoded.
Anyway, the address sanitizer does not seem to give errors anymore, so it seems most errors are gone by now.
In the 0.95 version the technical data is redesigned and QtCharts are used to display the equalizer results and the symbol constellations.
Since these signals are meant to be transmitted in the FM band, the DRMPLUS decoder is therefore equipped with DRM+ and FM decoding
In the development of the software I have used (modified) parts of the drm decoder of my swradio-8 software.
Since in the region I live there are no DRM + transmissions, the software was tested with some input files, comments and suggestions are welcome.
The current (0.95) version of drmplus is - in principle - able to decode xHE-AAC encoded audio frames. *However, the ONLY source I currently have is a recording of a fragment of a transmission from Petersburg, The selection of audio channels seems to work well, selection of data is being worked on
Any suggestion - or better: a recording - is more than welcome
In version 0.95 one may configure the "faad" library for decoding AAC encoded fragments, or "fdk-aac" for decoding AAC or xHE-AAC encoded fragments.
The ".pro" file provides an option to configure the software to use the "faad" library or the "fdk-aac" library.
The ".pro" file contains in the "unix" section lines
#CONFIG += faad
CONFIG += fdk-aac
with which one (de)selects the library to be used. (Note that uncommenting both gives undefined results. Note further that if "faad" is selected, output of the xHE-AAC processor, that maps the audio super frames onto audio frames, is not passed on further.
The same applies to configuring the input device(s), currently there is support for
- pcm files, with 2 channels and a sampling rate od 192000,
- raw files (8 bits IQ), raw files (16 bits IQ) and raw files (32 bits IQ), all recorded with a sampling rate of 192000,
- the SDRplay devices that are supported by the 2.13 library
- RT2932 based "dab" sticks,
- Hackrf devices
- limeSDR
DRM (Digital Radio Mondiale) is a form of digital radio, it is known for a number of transmitters in the shortwave bands. Digital radio uses different techniques than AM and FM, the classic analog radio variants.
One variant of DRM, DRM+, differs from DRM in the bandwidth used. While DRM (Modes A .. D) mainly use a 10 Khz band (with variations from 4.5 to app 20 Khz), DRM + uses a 100 Khz bandwidth is is meant for e.g. the FM Band.
DRM - and DRM+ - can carry up to 4 services within this bandwidth, both audio - usually encoded as AAC or xHE-AAC - and data - pictures, slide shows and other data.
In the swradio-8 I already implemented a - to 10 Khz limited - DRM decoder, and some of that software was "recycled" to help in constructing a DRM+ decoder.
Since the DRM+ signals are in the FM Band, the DRM+ decoder is also equipped with an FM decoder: the main widget shows a button with which one selects the deceder.
DRM+ encodes its content in 4QAM and 16QAM. The decoding of these signals requires correction of the phase and - especially in case of QAM16 (and higher) correction of the amplitude.
The samples stream in the input is - using am FFT approach - mapped onto samples in the frequency domain, in this domain the real correction action takes place by an "equalizer".
The picture below shows an almost ideal 4QAM signal - derived from a recording from a synthetic signal. The constellation picture - taken from app a 100 samples - shows the 4 dots clearly. Each of the dots shows a position in an XY plane of samples where two bits can be extracted from a samples.
The equalizer output - i.e. the values computed to correct phase and amplitude of the incoming signal shows that almost straight lines.
The second picture shows the constellation and equalizer output for a recorded signal from St Peterburg. While some parts are encoded as 4QAM , the audio and data content is encoded in 16 QAM.
In 16QAM - as said - both phase and amplitude are important to extract the bits, each dot position will be mapped upon 3 bits.
The equalizer output shows that both phase and amplitude need some correction.
DRM+ is transmitted using OFDM. OFDM is an interesting technique, typical problems in decoding occur through
- (a) clock errors, that lead to having too many samples (or too few) for the OFDM symbol at hand, leading to a shift in the "first" sample of a symbol. With one of the example files, we encountered a shift of app 1 sample per 2 or 3 frames (a frame consists of 40 symbols). While an offset of a few samples is not killing, this example showed that after a few hundred frames, we had skipped 30 to 40 samples in the input. The software in the 0.6 version now reports on the (average) number of symbols (each symbol 480 samples on an inputrate of 192000) that pass per sample over- or underflow: the larger the number the better.
- (b) frequency drift. Frequency errors are typically handled in two steps, the "coarse" error, with a step size of the carrier distance in the decoded ofdm symbols, and a "fine" error, a resulting error in the range of this carrier distance. Drift in the frequency may lead to the "fine" error to grow such a way that it has effect on the coarse error.
Operation of the decoder is straight forward, on program start-up the main widget appears. One selects an input device and a decoder and a widget for controlling the decoder appears.
Assuming the DRM+ Decoder is selected: When a valid DRM signal is detected, the name(s) of the service(s) will be shown on the buttons in the middle of the widget. Some data will be shown as well, technical data on the left.
With the buttons on the bottom of the DRM+ Decoder widget one may make some really technical data visible, and some data on the selected service.
Data on the selected service will give information on the protection of the signal (just a technical term, skip if you are not familiar with it), i.e, the protection of the high and low protected parts as well as the offsets in the mux where to find the data. Furthermore, this widget will tell whether the signal was encoded as a 4QAM or a 16QAM signal (both supported).
With a combobox on the bottom of the decoder widget one may choose to see the constellation of the FAC signals, the SDC signals or the MSC signals. With the next button one may choose to see the equalizer output.
Supported devices are
- the SDRplay,
- RT2832 based so-called DAB sticks,
- the HackRF device,
- the limeSDR
Note that these devices will run with a samplerate of 11 * 192000 and some software filtering is applied - in the decimation process - to narrow the reception band to 192000 (Note that the SDRplay already has support for a bandwidth of 200 Khz)
The dump button will write out the input data as a PCM (.wav) file with a samplerate of 192000 Next to these devices, there is support for
- PCM based files, i.e. ".wav" files;
- "raw" i.e. "iq" files, files where the raw samples are 8 bit unsigned ints;
- "raw 16", i.e. files where the raw samples are 16 bit signed int values;
- "raw 32", i.e. files where the raw samples are 32 bit signed int values.
The standard way to build an executable is to use a qmake/make line of work.
The ".pro" file indicates the libraries that are needed
The software is "in development", its development is slightly hindered by not having live DRM+ transmissions in the neighbourhood. All testing is - up to now - done with some recordings of the signal.
It is most unlikely that here - in the Netherlands - a live DRM+ transmission can be heard, however, in some other countries, e.g. Russia.
Further input is needed to get the job done, any assistance is really appreciated here
Copyright (C) 2020
Jan van Katwijk (J.vanKatwijk@gmail.com)
Lazy Chair Computing
The drmplus software is made available under the GPL-2.0.
The drmplus software is distributed in the hope that it will
be useful, but WITHOUT ANY WARRANTY; without even the
implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU General Public License
for more details.