digital radio

digital radio

digital radio, audio broadcasting in which an analog audio signal is converted into a digital signal before being transmitted; also known as digital audio broadcasting (DAB) and high-definition radio. Digital radio reception is virtually free of static and fading, pops, and hisses; overall, adjacent stations do not interfere within one another, audio clarity and volume are improved, and weather, noise, and other interference cease to be a factor. Digital radio can be land based (or terrestrial) or transmitted via satellite. In either case, a special receiver is required to decode the multiplexed signal; the receiver may contain a small display that provides information about the audio content (such as the name of the artist or title of the music).

The land-based technology was first deployed in Great Britain in 1995, and has since become established throughout Europe. The first satellite-based digital radio system was WorldSpace, which orbited the first of its three geostationary earth orbit (GEO) satellites, AfriStar, in 1998. Each satellite transmits three overlapping signal beams carrying more than 40 channels of programming; most of world (except mainly North America and Australia) is covered. The first satellite-based system to provide a mobile subscription digital radio service covering the United States was XM Satellite Radio, which orbited two GEO satellites in 2001. XM's ground station transmits digital signals to its satellites, which retransmit them directly to radio receivers on the ground. The receivers unscramble the signal, which contain up to 100 channels of digital audio. In metropolitan areas where tall buildings, overpasses, and other obstacles can interfere with the signals when, for example, the receiver is in a moving vehicle, a network of ground-based repeaters retransmit the signals. The receiver also buffers the signal briefly so that if it loses the satellite signal it can use one from a repeater to maintain a continuous broadcast. Sirius Satellite Radio, which launched national service to the United States in 2002, employs three satellites in inclined elliptical orbits instead of GEO satellites.

Digital Radio Mondiale (DRM) is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly shortwave. DRM can fit more channels than AM, at higher quality, into a given amount of bandwidth, using various MPEG-4 codecs.

Digital Radio Mondiale is also the name of the international non-profit consortium designing and implementing the platform. Radio France Internationale, TéléDiffusion de France, BBC World Service, Deutsche Welle, Voice of America, Telefunken (now Transradio) and Thomcast (now Thomson SA) took part at the formation of the DRM consortium.

The principle of DRM is that bandwidth is the limited element, and computer processing power is cheap. So modern CPU-intensive audio compression techniques enable more efficient use of available bandwidth.

Features

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 analog signal and digital signal.

Status

The LW/MW/SW standard has been approved by the IEC, and the ITU has approved its use in most of the world. Approval for ITU region 2 (North and South America and the Pacific) is pending amendments to existing international agreements. The inaugural broadcast took place on June 16, 2003, in Geneva, Switzerland, at the ITU's annual World Radio Conference.

Current broadcasters include Vatican Radio, BBC World Service, Deutschlandradio, biteXpress, HCJB, Radio Canada International, Deutsche Welle, Radio Netherlands, Radio Telefís Éireann (RTÉ), Radio Exterior de España, Rai and Radio New Zealand International.

Until now DRM receivers have typically used a personal computer. A few manufacturers are presently producing stand alone DRM receivers (Sangean, Morphy Richards, Starwaves). Kenwood and Fraunhofer presented a prototype standalone receiver chip in September 2006. It will be produced by STMicroelectronics. Himalaya demonstrated their two models in 2006.

Morphy Richards is mass producing DRM receivers, which are being promoted by the broadcaster Deutsche Welle . The receivers cost under £169.99 in the UK (as of October 2008). At the time of writing, Morphy Richards are only distributing these sets around Germany, Austria and the UK, but Europe-wide distribution is expected shortly, with grey market importers using German sourced radios to distribute across Europe.

Digital Radio Mondiale is being considered by Ofcom for introduction in Britain in 2012 on the present AM medium wave band.

The British Broadcasting Corporation BBC Is trialing the technology by broadcasting BBC Radio Devon in the Plymouth area. The trial will last for a year from the end of April 2007.

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..." being "5950–6200, 7100–7300, 9500–9900, 11650–12050, 13600–13800, 15100–15600, 17550–17900, 21450–21850 and 25670–26100 kHz

Technique

Audio source coding

Useful bit rates with DRM range from 8 kbit/s to 35 kbit/s for a standard 10 kHz bandwidth channel. It is possible to achieve bit rates up to 72 kbit/s by using 20 kHz wide channels. (For comparison, pure digital HD Radio can broadcast 20 kbit/s using 10 kHz width and up to 60 kbit/s using 20kHz bandwidth) Useful bit rate depends also on other parameters like wanted robustness to errors (error coding), power needed (modulation scheme), robustness in regard to propagation conditions (multipath, doppler). So DRM offers the possibility to use different audio coding system (source coding) depending on the bitrate:

  • MPEG-4 HE-AAC (High Efficiency - Advanced Audio Coding). AAC is a perceptual coder suited for voice and music and the High Efficiency is an optional extension for reconstruction of high frequencies (SBR: spectral bandwidth replication) and stereo image (PS: Parametric Stereo). 24 kHz or 12 kHz sampling frequencies can be used for core AAC (no SBR) 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 (vocoder) 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.

All codecs can optionally be combined with spectral band replication.

Broadcasters have some freedom of choice depending on the material they send. The most commonly used mode is HE-AAC (also called AAC+) that offers an acceptable audio quality somewhat comparable to FM broadcast.

Bandwidth

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).

Modulation

The modulation used for DRM is COFDM (Coded Orthogonal Frequency Division Multiplexing), where every carrier is modulated with QAM (Quadrature Amplitude Modulation) with a selectable error coding.

The choice of transmission parameters depends on signal robustness wanted, 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 (which cause frequencies offsets, spread: Doppler spread) and OFDM guard interval which determine robustness against multipath propagation (which cause delay offsets, spread: delay spread). The DRM consortium has determined 4 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 (more carrier spacing). 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 more the carrier spacing is the more the system is resistant to Doppler effect (Doppler spread). The more the guard interval is the more the system is resistant to long multipath propagation (delay spread).

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 (e.g. DAB or FM services).

DRM has been tested successfully on shortwave, mediumwave (with 9 as well as 10 kHz channel spacing) and longwave.

Mode OFDM Carrier spacing (Hz) Number of carriers Symbol length (ms) Guard interval length (ms) Nb symbols per frame
9 kHz 10 kHz 18 kHz 20 kHz
A 41.66 204 228 412 460 26.66 2.66 15
B 46.88 182 206 366 410 26.66 5.33 15
C 68.18 * 138 * 280 20.00 5.33 20
D 107.14 * 88 * 178 16.66 7.33 24

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.

The Dream software will receive the commercial versions and also limited transmission mode using the FAAC AAC encoder.

Error coding

Error coding can be chosen to be more or less robust.

This table shows an example of useful bitrates depending on protection classes, OFDM propagation profiles (A or B), carrier modulation (16QAM or 64QAM) and channel bandwidth (9 or 10 kHz):

Protection class A (9 kHz) B (9 kHz) B (10 kHz) C (10 kHz) D (10 kHz)
64-QAM 16-QAM 16-QAM 64-QAM 16-QAM 64-QAM 16-QAM 64-QAM
0 19.6 kbit/s 7.6 8.7 17.4 6.8 13.7 4.5 9.1
1 23.5 10.2 11.6 20.9 9.1 16.4 6.0 10.9
2 27.8 - - 24.7 - 19.4 - 12.9
3 30.8 - - 27.4 - 21.5 - 14.3

DRM Plus

While DRM currently covers the broadcasting bands below 30 MHz, the DRM consortium voted in March 2005 to begin the process of extending the system to the VHF bands up to 120 MHz. DRM Plus (DRM+) will be the name of this technology. Design, development and testing are expected to be completed by 2007-2009.

Wider bandwidth channels will be used, which will allow radio stations to use higher bit rates, thus providing higher audio quality. One likely channel bandwidth is 50 kHz, which will allow DRM Plus to carry radio stations at near CD-quality. A 100 kHz DRM+ channel has sufficient capacity to carry one mobile TV channel: it would be feasible to distribute mobile TV over DRM+ rather than DMB or DVB-H.

References

See also

External links

DRM in general

DRM broadcast transmitter manufacturers

DRM software

DRM radio techniques

DRM's COFDM

Index

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