With superior audio quality and multimedia capabilities, DRM Radio pioneers global digital broadcasting, using COFDM for minimal interference. Initially facing adoption challenges, it has gradually gained popularity with sustained technological advancements.
Analogue radio technology was used in communication and broadcasting for decades. However, after the advent of digital electronics, a new technology emerged – the digital radio. It uses digital signals instead of analogue for broadcasts or communication. As analogue signals suffer from quality loss due to signal interference and obstructions, the digital radio, on the other hand, stands out in terms of audio superiority. It uses a digital radio transmitter, which encodes audio into digital radio signals and can be received using a digital radio receiver. A digital radio receiver encodes the signals back to audio signals using digital-to-analogue converter(DAC) conversion method. As a result, the audio is of a standard quality free of noise.
There are many types of digital radio technology used for radio communication, and one such is called digital radio mondiale (DRM).mondiale is the Italian and French word for ‘worldwide’. Just like the analogue AM or FM radio, DRM also broadcasts in AM or FM frequencies but in digital format. With superior transmission quality, it delivers premium audio. AM or FM bands are used, and mostly it uses the shortwave frequencies for AM.
However, DRM is spectrally and qualitatively efficient than AM and FM, allowing more stations into a given amount of bandwidth. Currently, the latest digital radio standard is DRM, a global, open (non-proprietary) system.
In September 1996, a meeting was held in Paris between the international broadcasters, among them Deutsche Welle, and manufacturers who wanted to contribute to the future of broadcasting in the bands below 30MHz (short, medium and long wave); also attended the meeting. Following that, another meeting was held in November, and many more attended the meeting. Finally, the name digital radio mondiale was chosen for the non-profit consortium and the radio technology. Radio broadcasters, including Radio France Internationale, TéléDiffusion de France, BBC World Service, Deutsche Welle, Voice of America, Telefunken (now Transradio) and Thomcast (now Ampegon), took part in the formation of the DRM consortium. Since then, many meetings were held around the world till 1999 which resulted in the formation of the International DRM Consortium in Guangzhou, China, and the DRM consortium became a sector member of the ITU (International Telecommunications Union).
In April 2001, the ITU ratified DRM as the digital standard for the broadcasting bands below 30MHz. In 2003, the world’s first DRM broadcaster Deutsche Welle, started the DRM shortwave broadcast in Geneva. In March 2005, the DRM consortium voted to extend the system to the very high-frequency (VHF) bands up to 108MHz, which are FM bands. Finally on 31 August 2009, DRM+ (Mode E) became an official broadcasting standard for FM bands. The technical specification was published by the European Telecommunications Standards Institute (ETSI). According to the specifications, DRM+ can be used for radio broadcast above 30MHz up to 174MHz. Being a digital transmission, it is possible to carry a low-definition video (mobile TV) through a 100kHz DRM+ channel at 0.7 Mb/s. Moreover, DRM+ provides the flexibility to upgrade the existing FM transmitters and enable DRM+ broadcast alongside the analogue FM. The DRM transmitters can consume 50%-90% less energy than the analogue transmitters, and that is why DRM is also green.
DRM uses coded orthogonal frequency division multiplex (COFDM) technology for radio transmission, which resolves the fading problem (common in analogue radio) with the help of cheap but efficient computer processing power.– DRM uses modern audio compression techniques that enable more efficient use of available bandwidth at the expense of processing resources. The AM broadcast used for DRM radio is called DRM30,the frequency range is 30kHz – 30MHz whereas the FM transmission is called DRM+ and has a frequency range of 30MHz – 300MHz. DRM broadcast is done using xHE-AAC audio coding format. It also uses various other compatible codecs like MPEG-4 and Opus codecs, but the standard now specifies xHE-AAC. As DRM radio provides a better radio transmission with superior audio quality, in many countries radio stations have switched from analogue to DRM, including BBC, All India Radio (AIR), Voice of Nigeria, Radio Romania International, China National Radio and many more are broadcasting in DRM around the world.
DRM can not only be used for radio broadcasting, but also for many useful multimedia applications such as DRM text messages, journaline advanced text with listener interactivity and geo-referenced information, slideshows, traffic updates via TPEG or TMC, and service logos via SPI.
The DRM Technology
DRM uses a different digital radio technology that takes advantage of the COFDM. This technology minimises the Doppler effect (frequencies offsets, spread: Doppler spread) and provides better radio broadcast quality.
COFDM is a telecommunications modulation scheme that divides a single digital signal across thousands of signal carriers simultaneously. The signals are sent as closely packed (non-overlapping) signals so they do not interfere with each other, which takes less bandwidth compared to frequency division multiplexing (FDM). It also uses a code that acts as a forward error correction (FEC) to eliminate data loss from radio frequency (RF) interference or distortion. It is an advanced version of the orthogonal frequency-division multiplexing (OFDM).
In OFDM multiple radio signals are transmitted simultaneously. Still, they are separated from each other so that the amplitude of one signal would only be at the peak level even when the amplitude of the other signal is low. This way, many signals can be packed close to each other, taking less bandwidth but can transmit independently. That is why the word ’orthodox’ is given, which means independent. However, in COFDM, the signals are not only transmitted using the OFDM method but also use FEC, and a code is transmitted for every signal. This code carries the information about the signal and the receiver.After receiving the code,the receiver accepts only the signals related to the code. This prevents the receiver from receiving any other similar frequency signals that might be RF noise or interference.
A DRM radio broadcast consists of DRM30 (30kHz – 30MHz) and DRM+ (30MHz – 300MHz). It covers most of the AM bands, including the analogue low-frequency band starting from 30kHz – 300kHz, and it includes the long wave (LW) broadcast that can travel more than 17,000 Km. It covers the medium-frequency (MF) band with a frequency range of 300kHz -3MHz, including the medium wave (MW) broadcast that can travel more than 2,000 Km. Above this comes the high-frequency (HF) band with a frequency range of 3MHz – 30MHz, and it includes the short wave (SW) broadcast that can travel thousands of kilometres from one continent to another by refracting from the ionosphere. After this band comes the VHF band with a frequency range of 30MHz – 300MHz, and the FM broadcast falls within this band, including the DRM+. Beyond this band is called the ultra-high-frequency (UHF) band with a frequency range of 300MHz – 3GHz, and many television broadcasts, cellphones, satellite communication, WiFi and Bluetooth work within this band. That is why the DRM radio broadcast does not use this band.
The DRM radio broadcast is done using advanced digital technology. On the transmitter side, first, the audio and data are processed using an encoder or multiplexer and using multiplex distribution interface (MDI) or distribution and communication protocol (DCP), data is modulated to COFDM signals and broadcasted using a DRM30 or DRM+ transmitter. On the receiver side, the receiver converts the radio signals back to MDI or DCP data using the demodulator. The data is decoded back to audio and data using the demultiplexer, which can be controlled using the receiver status and control interface (RSCI).
In DRM30, the range of bitrate is from 6.1 kbit/s (Mode D) to 34.8 kbit/s (Mode A) for a 10kHz bandwidth (±5kHz around the central frequency). It is possible to achieve bitrates up to 72 kbit/s (Mode A) by using a standard 20kHz (±10kHz) wide channel. A pure digital HD Radio can broadcast 20 kbit/s using channels 10kHz wide and up to 60 kbit/s using 20kHz channels. However, the bitrate depends on parameters like desired robustness to errors (error coding), power needed (modulation scheme), and robustness regarding propagation conditions (multipath propagation, Doppler effect).
When DRM was originally designed, it was clear that the most robust modes offered insufficient capacity for the state-of-the-art audio coding format MPEG-4 HE-AAC (high-efficiency advanced audio coding). Therefore, the standard launched with a choice of three different audio coding systems (source coding) depending on the bitrate:
- MPEG-4 HE-AAC: It is used for voice and music and the ‘high-efficiency’ is an optional extension for the reconstruction of high frequencies (SBR: spectral bandwidth replication) and stereo image (PS: parametric stereo). 24kHz or 12kHz sampling frequencies can be used for core AAC (no SBR), which corresponds to 48kHz and 24kHz when using SBR oversampling.
- MPEG-4 CELP: It is a parametric coder suited for voice only (vocoder) but robust to errors and needs a small bitrate.
- MPEG-4 HVXC: This codec is also a parametric coder for speech programs using an even smaller bitrate than CELP.
However, with the development of MPEG-4 xHE-AAC, an implementation of MPEG unified speech and audio coding (USAC), the DRM standard was updated, and the two speech-only coding formats, CELP and HVXC, were replaced.
Now MPEG-4 xHE-AAC has become stand codec for DRM radio and the xHE-AAC combines two MPEG technologies, high-efficiency AAC v2, and USAC. It is designed to support the delivery of mixed speech and general audio content, including music on mobile devices, radio broadcasts, and wired streaming services. xHE-AAC provides exceptional audio quality using low-bitrates, delivering a listening experience suitable for mobile devices and can scale up to offer audiophile-quality reproduction. The reduced bitrate helps broadcasters and mobile streaming audio providers distribute their content more efficiently. Consumers can enjoy high-quality audio reproduction at lower bitrates, which reduces their mobile data consumption and costs.
However, many broadcasters still use the xHE-AAC coding format because it still offers an acceptable audio quality, somewhat comparable to FM broadcast at bitrates above about 15 kbit/s, and it is anticipated that in future, most broadcasters will adopt xHE-AAC.
Moreover, as of v2.1, the popular Dream software can broadcast using the Opus coding format. This code has technical advantages over the MPEG codec like minimum latency issues while coding and decoding, and it is also royalty-free. It can be used as an open-source software. it is an alternative to the proprietary MPEG family – whose use is permitted at the discretion of the patent holders. Unfortunately, Opus has lower audio quality than xHE-AAC at low-bitrates, which are key to conserving bandwidth. In fact, at 8 Kbps, Opus sounds worse than analogue shortwave radio.
DRM broadcasting can be done using a choice of different bandwidths, including 4.5kHz, 5kHz, 9kHz, 10kHz, 18kHz, 20kHz, and 100kHz. Where the bandwidths 4.5kHz, 5kHz, 9kHz, 10kHz, 18kHz and 20kHz are used by DRM30, but 4.5kHz and 5kHz are of very poor quality. 100kHz is used by the DRM+,this bandwidth can be used in bands I, II, and III. DRM+ can transmit four different programs in this bandwidth or even one low-definition digital video channel.
DRM uses COFDM for modulation, and every carrier is modulated with quadrature amplitude modulation (QAM) and selectable error coding. The transmission parameters should be selected as per the required signal robustness and propagation conditions, which are affected by noise, interference, multipath wave propagation and Doppler effect. It is possible to choose among several error coding schemes and several modulation patterns: 64-QAM, 16-QAM and 4-QAM. COFDM modulation has some parameters that must be adjusted depending on propagation conditions. Choosing the suitable modulation will provide immunity to the signal against the Doppler effect. COFDM can affect the carrier spacing depending on the modulation mode; better spacing reduces the chances of Doppler effect. The DRM consortium has determined four different profiles corresponding to typical propagation conditions:
- A: It is a Gaussian channel with very little multipath propagation and Doppler effect. This profile is suited for local or regional broadcasting.
- B: It is a Multipath propagation channel. This mode is suited for medium-range transmission, and is frequently used these days.
- C: This mode is similar to mode B but has better robustness to Doppler (more carrier spacing). It is suited for long-distance transmission.
- D: This mode is similar to mode B but resists large delay spread and Doppler spread. Adverse propagation conditions exist on very long-distance transmissions, and the useful bitrate for this profile is decreased.
Choosing the right profile for the modulation mode ensures robustness against the Doppler effect and could maintain a consistent DRM broadcast.
Since DRM radio broadcast use COFDM technology, the broadcasted signals are less prone to RF interference and quality loss. This makes DRM radio one of the best technologies for radio broadcasting. The use of digital radio technology with OFDM and many latest audio codecs like MPEG-4 xHE-AAC, makes DRM the most advanced and superior radio in the world.
DRM Receivers
A DRM radio broadcast can only be received using a digital radio receiver. Such a receiver has a user interface consisting of buttons and a display screen. The receiver demodulates the OFDM signals of a DRM broadcast and decodes the audio and data that come with the modulated radio signals. The audio is converted to sound and played through the built-in speaker, while any data in the form of text or image is displayed on the receiver’s LCD display.
A DRM patent pool was formed in 2003 to facilitate a simple ‘one-stop’ licensing regime for manufacturers of DRM receivers. There is no financial or managerial link between the DRM Consortium and this pool of licensors. The licensing of DRM IPR is undertaken by VIA licensing, which acts on behalf of the licensor patent pool. The VIA website gives details of royalty fees for all classes of DRM equipment. There is no royalty charge for actual use of the system (broadcasting or reception).
Fig: 4. DRM receiver from Avion electronics
DRM receivers are available from many manufacturers – AV-DR-1401 (Avion Electronics), Gospel receivers from Gospel, Patronx’s SDR Titus II, and many DRM car receivers from Hyundai, Mahindra and Maruti Suzuki.
Benefits of DRM
DRM is a digital radio that uses the latest technology for radio broadcasting, with high-quality audio and data (text or image) that can be received using a DRM receiver. DRM’s benefits include universal access (connecting the whole country), flexibility, and greenness (energy efficiency).
Universal access (connecting the whole country)
DRM radio can broadcast to the whole country using AM bands with DRM30. As the LW,MW, or SW band radio signals can travel thousands of miles from one country to another, DRM30 broadcast can not only give access to news and information to the listeners of one country but also to the listeners of many other countries of different continents. Moreover, unlike any AM radio, a DRM30 can also deliver data, including text and images, to the DRM radio receivers in the form of news and information. This feature makes DRM a radio and an information provider, which can be helpful during any crisis like natural disaster and pandemic outbreaks. Such an information delivery system could alert the listeners about any crisis or danger.
Flexibility
DRM is also flexible compared to other digital radios, they do not have such flexibility. This flexibility makes DRM+ easy to implement. As DRM+ uses FM bands, it is possible to upgrade the FM transmitters to DRM+ without much effort and without affecting the current analogue FM broadcast. A DRM+ broadcast can also be carried out alongside the analogue FM broadcast, making DRM+ easy to implement and use in FM stations.
Green (energy-efficient)
DRM can be considered green, which means it consumes less power. DRM+ transmitters can consume 50%- 90% less power than other analogue FM transmitters. Such low power consumption of DRM radio makes it one of the best energy-efficient radio technologies in the world.
DRM was founded in the 90s, but due to the minimal use of digital audio and higher cost of digital electronics, worldwide use of DRM was not implemented everywhere then. But after the advent of newer technologies and cheaper digital audio electronic components like digital signal processors (DSP), DAC, and system-on-chip (SoC), the manufacturing of digital radio equipment has become possible with a cost-effective process. With the help of such digital electronics and state-of-the-art technology, DRM radio can now be used around the world to let you experience high-quality digital audio and information in the form of text and images.
Therefore, DRM radio is gradually on the rise, and with all such useful features, it is currently one of the best radio technologies that can provide the ultimate digital radio experience.
Debojit Acharjee The author is a software engineer and writer by profession.
