Digtal File Transfer Over Radio
Ham DRM (Digital Radio Modial)
To begin a discussion of DRM (not to be confused with DMR – a totally different thing) we must understand the Digital Radio Mondial principals of operation. Digital Radio Mondial (or Digital Radio World from the original translation) was orginally designed for International Shortwave Broacast stations that wanted high quality audio from their broadcasts well known for channel deficiencies like noise, fading, doppler effect shifting, and other distortions of the audio signal.
The standard was formed by a consortium in co-operation with the International Telecommunication Union (ITU). The system offers the radio stations and new service providers access to the multimedia age for all kind of radio stations, from high power broadcasters with large target areas and long distances reception to low power line-of-sight broadcasters.
The bandwidth of a [International Broadcast] DRM bandpass signal is less than 20 kHz in the DRM30 mode and less than 100kHz in the DRM+ mode, and the number of carriers used in the OFDM-modulation [OFDM being the abbreviation of orthoginal frequency division multiplexing] is relatively small. These features motivate a real-time software implementation of a DRM-receiver for conventional personal computer (PC) or smartphones/tablets using affordable input devices like the FunCubeDongle Pro+ or a sound card connected to a standard radio receiver with an IF ouput.
The original DRM project
This software project implements a working software DRM30 receiver with, at least, the basic features (DRM+ support is planned). Since this project was created at a university and the fundamental idea of such an institution is to teach and stimulate the creativity, this source-code is free under the GNU-General Public License (GPL). Dream is a development project which uses the open source model to improve DRM technology. The main aim of this project is to implement and test new research results on an existing system, whereby the synchronization and channel estimation is of special interest.
The programming-language is C++. The code runs under Mac OSX, Microsoft Windows
and Linux. Start of the project was June 2001.
Dream is a software implementation of a DRM receiver. It is capable of making perfect DRM transmissions of 10 and 20 KHz bandwidth. Reception requires an adaptor to be connected to the receiver’s IF stage. It was Created at Darmstadt University of Technology in Germany and Released under the GNU General Public License.
The intended audience of the Dream software are people who are interested in
how to decode a DRM stream and want to learn from the algorithms used in this
software and people who want to help us improving the performance of the
receiver and the source code.
HamDream is software modified from Dream by Cesco HB9TLK. HamDream uses only 2.5 KHz bandwidth. HamDream is the basis for all the 2.5 khz DRM programs. The project is outdated and will not be supported any more.
A word about ODFM
In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, used in applications such as digital television and audio broadcasting, DSL internet access, wireless networks, power line networks, and 4G mobile communications.
In coded orthogonal frequency-division multiplexing (COFDM), forward error correction (convolutional coding) and time/frequency interleaving are applied to the signal being transmitted. This is done to overcome errors in mobile communication channels affected by multipath propagation and Doppler effects. COFDM was introduced by Alard in 1986 for Digital Audio Broadcasting for Eureka Project 147. In practice, OFDM has become used in combination with such coding and interleaving, so that the terms COFDM and OFDM co-apply to common applications.
OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. OFDM was introduced by Chang of Bell Labs in 1966. Numerous closely spaced orthogonal sub-carrier signals with overlapping spectra are emitted to carry data. Demodulation is based on Fast Fourier Transform algorithms. OFDM was improved by Weinstein and Ebert in 1971 with the introduction of a guard interval, providing better orthogonality in transmission channels affected by multipath propagation. Each sub-carrier (signal) is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate. This maintains total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
The main advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and use echoes and time-spreading (in analog television visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs) where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be re-combined constructively, sparing interference of a traditional single-carrier system. [--extracted from an explanation on Wikipedia]
The OFDM modulation mode is part of the reason digital modes in ham radio, like MT-63 have become so popular. MT-63 uses up to 64 orthogonal sub-carrier signals with overlapping spectra in a select-able bandwidth arrangement from 500 to 2000 cycles wide. Each bandwidth has a throughput limitation, the widest being the highest throughput. See our discussion on this mode elsewhere on this site.
FreeDV is a Digital Voice mode for HF radio. You can run FreeDV using a free GUI application for Windows, Linux and OSX that allows any SSB radio to be used for low bit rate digital voice. There are several reports of the new FreeDV 700D mode outperforming SSB at low SNRs. At high SNRs FreeDV 1600 sounds like FM, with no annoying analog HF radio noise.
Speech is compressed down to 700-1600 bit/s then modulated onto a 1.25 kHz wide signal comprised of 16 QPSK carriers which is sent to the Mic input of a SSB radio. The signal is received by an SSB radio, then demodulated and decoded by FreeDV.
FreeDV was built by an international team of radio amateurs working together on coding, design, user interface and testing. FreeDV is open source software, released under the GNU Lesser Public License version 2.1. The modems and Codec 2 speech codec used in FreeDV are also open source.
DRM can deliver FM-comparable sound quality, but on frequencies below 30 MHz (long wave, medium wave and short wave), which allow for very-long-distance signal propagation. VHF is also under consideration, under the name “DRM+”. DRM has been designed especially to use portions of older AM transmitter facilities such as antennas, avoiding major new investment. DRM is robust against the fading and interference which often plagues conventional broadcasting on these frequency ranges.
The encoding and decoding can be performed with digital signal processing, so that a cheap embedded computer with a conventional transmitter and receiver can perform the rather complex encoding and decoding.
As a digital medium, DRM can transmit other data besides the audio channels (datacasting) – as well as RDS-type metadata or program-associated data as Digital Audio Broadcasting (DAB) does. Unlike most other DAB systems, DRM uses in-band on-channel technology and can operate in a hybrid mode called Single Channel Simulcast, simulcasting both analogue signal and digital signal.
DRM broadcasting can be done on different bandwidths:
- 4.5 kHz or 5 kHz which are half channels. The idea is to offer a possibility for the broadcaster to do simulcast and use a full 10 kHz channel for AM, plus a 5 kHz half-channel sideband for DRM. However the resulting bit rate and audio quality is less (approximately 8-16 kbit/s).
- 9 kHz or 10 kHz which are the standard bandwidth of an AM broadcasting channel so existing frequency plan can be reused (approximately 17-35 kbit/s).
- 18 kHz or 20 kHz which correspond to a coupling of two adjacent channels. It offers the possibility to offer a better audio quality or to multiplex audio channels in the same transmitter (approximately 31-72 kbit/s).
DRM Comparison (Amateur Radio)
THE DRM mode has become very popular. Why has the DRM mode become so
popular? It takes no (or an insignificant) amount of time to decode/encode.
This is not like RDFT. Everyone likes the decoding on the fly so that you
can see instantly how the picture came through. The file data is sent faster,
3 times faster than RDFT and 2 times faster than DIGPAL. Now, small standard
.jpeg and .gif files can be sent in their original size. DRM allows larger
files to be sent in less time which means better quality pictures in about
the same amount of time. When using DRM your ID (callsign) is sent continually.
This would allow others to identify the transmitting station and turn an
antenna for better reception. It allows viewing digital images with missing
data (blocks) or progressively viewing as the data is received. This is somewhat
like analogue SSTV. There are no critical periods. You could miss the start
or end of a DRM transmission and still have enough data to be useful. Without
the problem of “Bad block Zero”, it is possible to expect even
large files to make it through in spite of poor band conditions. A replay
is exactly like the original. It would have the same file name, and the same
Note: There is also no picture slant adjustment required with digital image modes which is often necessary with analogue SSTV.
If segments of a file are missing, a station may repair the file manually.
During a DRM replay, those stations who have missing segments can also receive
If conditions are very bad. it is possible to get incremental repair (i.e. the repair data does not have to be received 100%). A partial repair may be repeated until the file is complete. The “repair data segments” can be sent multiple times increasing the chance of getting all the segments even under poor conditions (sound familiar). Under conditions of QSB/fading multiple instances may be sent. This makes it more likely for a successfully received transmission.
During transmission it is advisable to set the wave level for minimal ALC. If too much ALC is present, there will be a tendency to overdrive the transceiver signal resulting in the receiving stations MSC indicator bar remaining ‘RED’. Over a period of time RDFT or Redundant Digital File Transfer has been the primary means to send digital pictures over ham radio. Barry Sanderson KB9VAK developed the RDFT mode. Since then, the mode has been adopted by DSSTV and SSTV-PAL Multi Mode from Erik VK4AES in Australia by the DIGTRX program from Roland PY4ZBZ in Brazil and by the DigiACE software from Martin Emmerson G3OQD in the U.K. DRM mode has gained so much popularity that the RDFT mode has become nearly obsolete.
WinDRM is the current software by Cesco and it uses either 2.3 KHz or 2.5 KHz bandwidth. It also has a digital voice mode. HamDRM is a Windows DLL program by Cesco based on his WinDRM program. It serves as an engine to be used with other graphical user interfaces that wish to support the DRM mode. WINDRM allows as many as eight different files to be sent in a single transmission.
DRM is very forgiving. It can tolerate QSB and QRM.
Since it uses real time decoding, it is possible to monitor the success of the received file as it comes in. The total number of segments, the number of segments received, and the last segment number decoded are displayed as received. The display of the signal to noise ratio (SNR) allows the user to make adjustments to the receiver during transmission and see if it improves the SNR and optimise reception.
A robust mode is available for use when there is heavy QRM or QRN.
A high speed mode is available for use on VHF/UHF or when conditions are very good on the HF bands. A SNR of better than 18 is required for this 64 QAM mode.
Reed-Solomon error correction is an error – correcting code that works by over sampling a polynomial constructed from the data. The polynomial is evaluated at several points, and these values are sent or recorded. Sampling the polynomial more often than is necessary makes the polynomial over-determined. As long as it receives “many” of the points correctly, the receiver can recover the original polynomial even in the presence of a “few” bad points.
EasyPal software uses the DRM (Digital Radio Mondiale) encoding to allow the
sending of image files over voice channels and is the work of Erik, VK4AES SK.
The EasyPal GUI (graphical user interface) was originally written with Delphi (a Pascal IDE from the makers of Turbo-C) and compiled on Windows. It is only available on the Windows platform.It works best when run on a 2 GHz or faster CPU with Windows. Although less stable, it should run on most fast computers with Windows latest releases. It is designed to be easy to setup and use.
DRM as used in EasyPal, allows very fast data transmissions with error correction, enabling very accurate decoding, and a means to request missing or incorrect blocks, this is all done within EasyPal, and very simple to do.
Images up to 1280x1024 are sent in a little over 1 minute, Easypal does compress images to shorten transmission times, but you have the ability to adjust the compression if unwanted material is present in the image.
Erik Sundstrup, VK4AES is now silent key as of 14th march 2015. Having suffered a catastrophic lightning strike just before his death, the Delphi source code is no longer available and further development of the project has been halted on the orignal HamPal and EZPal project. We will miss Erik.
In an effort to incorporate the best of both worlds and create the availability
of digital SSTV type modes on the Linux operating system, Johan Maes, ON4QZ of
Everberg, Belgium started developing the DRM functionality in QSSTV after
studying the excellent work Ties Bos (PA0MBO) did on txrxamadrm.
The software is NOT based on the hamdrm.dll but on DIORAMA ( http://nt.eit.uni-kl.de/static/diorama/index.html ) and adapted by PA0MBO. For more information , see the manual of txrxamadrm.
It now runs on all GNU Linux based machines including the Raspberry Pi 2, 3 and 3B running Raspian Jessie and Ubuntu MATE.
Operation is very similar to EZPal even though the graphical user interfaces (GUIs) are very different. The similarities are the transmission mode of digital files (like images) over radio using DRM, being practically the same.
It is on the basis of these two similar software applications that we will begin our experiment with digital file transfer over radio. You are encouraged to download and install the appropriate software and SWL on 14.230 and 14.233 for digital image transmissions.