Digital Radio Mondiale


Digital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for analogue radio broadcasting including AM broadcasting—particularly shortwave—and FM broadcasting. DRM is more spectrally efficient than AM and FM, allowing more stations, at higher quality, into a given amount of bandwidth, using xHE-AAC audio coding format. Various other MPEG-4 codecs and Opus are also compatible, but the standard now specifies xHE-AAC.
Digital Radio Mondiale is also the name of the international non-profit consortium that has designed the platform and is now promoting its introduction. Radio France Internationale, TéléDiffusion de France, BBC World Service, Deutsche Welle, Voice of America, Telefunken and Thomcast took part at the formation of the DRM consortium.
The principle of DRM is that bandwidth is the limiting factor, and computer processing power is cheap; modern CPU-intensive audio compression techniques enable more efficient use of available bandwidth, at the expense of processing resources.

Features

DRM can broadcast on frequencies below 30 MHz, which allow for very-long-distance signal propagation. The modes for these lower frequencies were previously known as "DRM30". In the VHF bands, the term "DRM+" was used. DRM+ is able to use available broadcast spectra between 30 and 300 MHz; generally this means band I, band II and band III. DRM has been designed to be able to re-use portions of existing analogue transmitter facilities such as antennas, feeders, and, especially for DRM30, the transmitters themselves, avoiding major new investment. DRM is robust against the fading and interference which often plague conventional broadcasting in these frequency ranges.
The encoding and decoding can be performed with digital signal processing, so that a low-cost embedded system 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 — as well as RDS-type metadata or program-associated data as Digital Audio Broadcasting does. DRM services can be operated in many different network configurations, from a traditional AM one-service one-transmitter model to a multi-service multi-transmitter model, either as a single-frequency network or multi-frequency network. Hybrid operation, where the same transmitter delivers both analogue and DRM services simultaneously is also possible.
DRM incorporates technology known as Emergency Warning Features that can override other programming and activates radios which are in standby in order to receive emergency broadcasts.

Status

The technical standard is available free-of-charge from the ETSI, and the ITU has approved its use in most of the world. Approval for ITU region 2 is pending amendments to existing international agreements. The inaugural broadcast took place on June 16, 2003, in Geneva, Switzerland, at the ITU's World Radio Conference.
Current broadcasters include Akashvani, BBC World Service, funklust, Radio Exterior de España, Radio New Zealand International, Vatican Radio, Radio Romania International and Radio Kuwait.
Until now DRM receivers have typically used a personal computer. A few manufacturers have introduced DRM receivers which have thus far remained niche products due to limited choice of broadcasts. It is expected that the transition of national broadcasters to digital services on DRM, notably All India Radio, will stimulate the production of a new generation of affordable, and efficient receivers.
Chengdu NewStar Electronics is offering the DR111 from May 2012 on which meets the minimum requirements for DRM receivers specified by the DRM consortium and is sold worldwide.
The General Overseas Service of Akashvani broadcasts daily in DRM to Western Europe on 9.95 MHz at 17:45 to 22:30 UTC. All India Radio is in the process of replacing and refurbishing many of its domestic AM transmitters with DRM. The project which began in 2012 is scheduled to complete during 2015.
The British Broadcasting Corporation BBC has trialled the technology in the United Kingdom by broadcasting BBC Radio Devon in the Plymouth area in the MF band. The trial lasted for a year.
The BBC also trialed DRM+ in the FM band in 2010 from the Craigkelly transmitting station in Fife, Scotland, over an area which included the city of Edinburgh. In this trial, a 10 kW FM transmitter was replaced with a 1 kW DRM+ transmitter in two different modes, and coverage compared with FM. Digital Radio Mondiale was included in the 2007 Ofcom consultation on the future of radio in the United Kingdom for the AM medium wave band.
RTÉ has also run single and multiple programme overnight tests during a similar period on the 252 kHz LW transmitter in Trim, County Meath, Ireland which was upgraded to support DRM after Atlantic 252 closed.
The Fraunhofer Institute for integrated circuits IIS offers a package for software-defined radios which can be licensed to radio manufacturers.
The DRM standard is not permitted in the US on the mediumwave or the VHF bands since the FCC accepted HD Radio as standard for these bands. However, the shortwave band is not blocked, but is not used regularly.

International regulation

On 28 September 2006, the Australian spectrum regulator, the Australian Communications and Media Authority, announced that it had "placed an embargo on frequency bands potentially suitable for use by broadcasting services using Digital Radio Mondiale until spectrum planning can be completed" "those bands being "5,950–6,200; 7,100–7,300; 9,500–9,900; 11,650–12,050; 13,600–13,800; 15,100–15,600; 17,550–17,900; 21,450–21,850 and 25,670–26,100 kHz.
Since 2005, the United States Federal Communications Commission states in that: "For digitally modulated emissions, the Digital Radio Mondiale standard shall be employed." Part 73, section 758 is for HF broadcasting only.

Technological overview

Audio source coding

Useful bitrates for DRM30 range from 6.1 kbit/s to 34.8 kbit/s for a 10 kHz bandwidth. It is possible to achieve bit rates up to 72 kbit/s by using a standard 20 kHz wide channel. Useful bitrate depends also on other parameters, such as:
  • desired robustness to errors
  • power needed
  • robustness in regard to propagation conditions, etc.
When DRM was originally designed, it was clear that the most robust modes offered insufficient capacity for the then state-of-the-art audio coding format MPEG-4 HE-AAC. Therefore, the standard launched with a choice of three different audio coding systems depending on the bitrate:
  • MPEG-4 HE-AAC. AAC is a perceptual coder suited for voice and music and the High Efficiency is an optional extension for reconstruction of high frequencies and stereo image. 24 kHz or 12 kHz sampling frequencies can be used for core AAC which correspond respectively to 48 kHz and 24 kHz when using SBR oversampling.
  • MPEG-4 CELP which is a parametric coder suited for voice only but that is robust to errors and needs a small bit rate.
  • MPEG-4 HVXC which is also a parametric coder for speech programs that uses an even smaller bitrate than CELP.
However, with the development of MPEG-4 xHE-AAC, which is an implementation of MPEG Unified Speech and Audio Coding, the DRM standard was updated and the two speech-only coding formats, CELP and HVXC, were replaced. USAC is designed to combine the properties of a speech and a general audio coding according to bandwidth constraints and so is able to handle all kinds of programme material. Given that there were few CELP and HVXC broadcasts on-air, the decision to drop the speech-only coding formats has passed without issue.
Many broadcasters still use the HE-AAC coding format because it still offers an acceptable audio quality at bitrates above about 15 kbit/s. However, it is anticipated that in future, most broadcasters will adopt xHE-AAC.
DRM30, unlike HD Radio on the medium wave, allows multiprogramming.

Bandwidth

DRM broadcasting can be done using a choice of different bandwidths:
  • 4.5 kHz. Gives the ability for the broadcaster to do a simulcast and use the lower-sideband area of a 9 kHz raster channel for AM, with a 4.5 kHz DRM signal occupying the area traditionally taken by the upper-sideband. However the resulting bit rate and audio quality is not good.
  • 5 kHz. Gives the ability for the broadcaster to do a simulcast and use the lower-sideband area of a 10 kHz raster channel for AM, with a 5 kHz DRM signal occupying the area traditionally taken by the upper-sideband. However the resulting bit rate and audio quality is marginal. This technique could be used on the shortwave bands throughout the world.
  • 9 kHz. Occupies half the standard bandwidth of a region 1 long wave or medium wave broadcast channel.
  • 10 kHz. Occupies half the standard bandwidth of a region 2 broadcast channel, and could be used to simulcast with analogue audio channel restricted to NRSC5. Occupies a full worldwide short wave broadcast channel.
  • 18 kHz. Occupies full bandwidth of region 1 long wave or medium wave channels according to the existing frequency plan. This offers better audio quality.
  • 20 kHz. Occupies full bandwidth of region 2 or region 3 AM channel according to the existing frequency plan. This offers highest audio quality of the DRM30 standard.
  • 100 kHz for DRM+. This bandwidth can be used in band I, II, and III and DRM+ can transmit four different programs in this bandwidth or even one low definition digital video channel.

    Modulation

The modulation used for DRM is coded orthogonal frequency division multiplexing, where every carrier is modulated with quadrature amplitude modulation with a selectable error coding.
The choice of transmission parameters depends on signal robustness wanted and propagation conditions. Transmission signal is 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. OFDM modulation has some parameters that must be adjusted depending on propagation conditions. This is the carrier spacing which will determine the robustness against Doppler effect and OFDM guard interval which determine robustness against multipath propagation. The DRM consortium has determined four different profiles corresponding to typical propagation conditions:
  • A: Gaussian channel with very little multipath propagation and Doppler effect. This profile is suited for local or regional broadcasting.
  • B: multipath propagation channel. This mode is suited for medium range transmission. It is nowadays frequently used.
  • C: similar to mode B, but with better robustness to Doppler. This mode is suited for long distance transmission.
  • D: similar to mode B, but with a resistance to large delay spread and Doppler spread. This case exists with adverse propagation conditions on very long distance transmissions. The useful bit rate for this profile is decreased.
The trade-off between these profiles stands between robustness, resistance in regards to propagation conditions and useful bit rates for the service. This table presents some values depending on these profiles. The larger the carrier spacing, the more the system is resistant to Doppler effect. The larger the guard interval, the greater the resistance to long multipath propagation errors.
The resulting low-bit rate digital information is modulated using COFDM. It can run in simulcast mode by switching between DRM and AM, and it is also prepared for linking to other alternatives.
DRM has been tested successfully on shortwave, mediumwave and longwave.
There is also a lower bandwidth two-way communication version of DRM as a replacement for SSB communications on HF - note that it is not compatible with the official DRM specification. It may be possible in some future time for the 4.5 kHz bandwidth DRM version used by the Amateur Radio community to be merged with the existing DRM specification.
The Dream software will receive the commercial versions and also limited transmission mode using the FAAC AAC encoder.