International Digital Audio Broadcasting Standards: Voice Coding and Amateur Radio Applications Here’s a digital-voice standard for broadcast and Amateur Radio—read all about it! By Cédric Demeure and Pierre-André Laurent his paper presents a new digi- existing demonstrator to be tested on for much better quality by offering net tal system for use in terrestrial an across-the-Atlantic link, together bit rates compatible with up-to-date Taudio broadcasting in the fre- with the associated modem and voice audio compression techniques like quency bands below 30 MHz and its coding techniques. AAC-MPEG 4 (near FM quality). potential use for Amateur Radio. The A new coherent COFDM (coded Such a broadcasting scheme may system is based upon a COFDM mo- orthogonal frequency-division multi- even be implemented using existing dem; it was elaborated as a derivation plexed) modem for use in high- power transmitters in a transmission of the DRM modem. DRM (Digital quality audio broadcasting in the mode named Simulcast. This mode con- Radio Mondiale) is a worldwide consor- frequency bands below 30 MHz is pro- sists in transmitting a program in a tium proposing through ITU a new posed by DRM as a candidate to replace given frequency channel by using an standard for digital radio broadcasting the current AM modulation that is AM compatible analogue waveform and for frequencies below 30 MHz. It is now notoriously perturbed by multipath, the new digital signal simultaneously. the only digital standard adopted by Doppler shift and fading. The main This broadcasting system was ini- ITU for HF digital radio broadcasting. characteristics of this system are to tially elaborated within the NADIB A specially adapted version that fits in- be compatible with existing chan- consortium (European Eureka 1559 side a 3-kHz channel was derived for nelization within these bands and with project: NArrow Band DIgital Broad- Amateur Radio. This paper presents the current transmitter technology (5-kHz casting) and was adopted and en- spacing). Therefore, it should be a bet- hanced within the DRM consortium as Thales Communications SA ter alternative than satellite broadcast- a candidate for international normal- 66 Rue du Fossé Blanc ing at higher frequencies because it is ization at ITU. Real transmission on 92231 Gennevilliers Cedex cheaper, offers better propagation and a standard 100-kW, shortwave AM France indoor coverage. It also opens the door transmitter was demonstrated at the Jan/Feb 2003 49 1997 IBC conference in Amsterdam. designed to fit in with the existing AM • Minimum disturbance of AM us- Since then, numerous field trials have broadcast band plan, based on signals ers on the same band: This leads to the been performed. The latest series with 9 or 10-kHz channelization. It design of a signal which, as seen by an (May/June 2002) involved transmis- has modes requiring as little as 4.5- AM receiver, must be as noise-like as sions from Canada, the Caribbean or 5-kHz bandwidth, plus modes that possible. The frequency spectrum of the and Europe to Australia and New can take advantage of wider band- transmitted signal must also be as com- Zealand. widths, such as 18 or 20 kHz. pact as possible to minimize jamming Thales, with its two affiliates Thales • Better audio quality: The aim is to in the adjacent channels. Communications SA and Thales Broad- obtain near-FM quality within a much • Low complexity: This is essential cast and Multimedia SA, was within the narrower frequency bandwidth. The for having receivers of low complexity initial group of companies to work on improvement upon analogue AM is im- and low power consumption, espe- the standard.1 Going for digital broad- mediately noticeable. DRM can be used cially in countries where batteries are casting allows the introduction of new for a range of audio content, including rare and expensive. services such as Program Associated multi-lingual speech and music. • Graceful degradation: If desired, Data (PAD), RDS type alternative fre- • Simple-to-use receivers, especially graceful degradation may be obtained quency switch (AFS) management, when it comes to HF programming by the use of hierarchical coding. This single frequency network (SFN), simul- (frequent frequency changes due to option, although compatible with the taneous data channels for various ser- propagation conditions) system design, is not described in this vices such as picture transmission, and • Low-cost equipment to quickly paper. so forth. This paper describes work per- reach the mass market DRM benefits for the listeners are formed by Thales Communications • Text messaging similar to RDS at the following: France to adapt this standard and the FM for simple PAD (Program Associ- • FM-like sound quality with the associated modem to 3-kHz Amateur ated Data) transmission AM reach Radio needs. • Data applications: an open path • Improved reception quality for new applications of this medium • Flexible use of radio, whenever DRM Standards with a large geographical coverage and wherever you want it DRM stands for Digital Radio • Future enhancements: a clear path • No change to existing listening Mondiale, where the spelling of the last for new ideas with an open standard habits: same frequencies, same listen- word is on purpose to show its inter- The DRM standard has been de- ing conditions (fixed, portable and mo- national nature. DRM is a worldwide signed taking into account a number of bile radio), same listening environment consortium established in Geneva, technical constraints, among which are: (indoors, in cities, in dense forests) Switzerland, to promote a unique LF, • Short access time for the receiver: • Low-cost receiver, low energy con- MF and HF audio-broadcasting stan- The listener shall not wait for more sumption dard. See www.drm.org for more than a few seconds before getting ac- • Easy tuning with selection by fre- details. This site contains two reference cess to the desired program, and shall quency, station name or programming ITU papers on the subject.2, 3 obtain radio broadcasting information • More diverse program content, DRM’s members include: broad- even faster. using the full capabilities of new digi- casters and broadcasting associations; • Maximum quality (objective and tal features network operators; research insti- subjective): In the allowed transmis- • Wide receiver range with more tutes; component, receiver and trans- sion bandwidth, a maximum useful bit and better features mitter manufacturers; regulatory and rate must be conveyed. This implies a Radios that will give you programs standardization authorities. high-spectral-efficiency modulation with associated text information, sta- There are presently more than 80 scheme—more than 2 bit/s/Hz. tion name, record title, singer’s name. members in DRM, corresponding to 27 • Robustness against distortions countries. In the USA for example, (multipath, Doppler, noise): This is man- DRM System Description members include: Harris Broadcast, datory, especially in the shortwave With the limited bit rate available, the International Broadcasting bands (which are often severely affected it is important to strike the right bal- Bureau, Continental Electronics by propagation disturbances and inter- ance between flexibility and efficiency Corporation, Sangean America Inc, ference) or in medium waves during the while protecting each bit of information Technology for Communications Inter- transition between day and night. to an appropriate degree. A distinction national. Some other active members • Flexibility: According to the cur- is therefore made between main pay- include: the British Broadcasting Cor- rent broadcasting frequency band, the load data and the various types of data poration (BBC), Sony, Bosch, Thales, frequency separation of different that the receiver needs to help it find RFI, DW, JVC, Telefunken and so on. transmissions, the bandwidth of the and decode the desired program. The standardization process involves transmitter and the total available The main payload is called the mainly the ITU (International Telecom- bandwidth can be adjusted to the Main Service Channel (MSC). Two munication Union) based in Geneva, needs of the broadcasters. In the same subsidiary channels are also provided the European body ETSI (European way, the required protection level is namely the Fast Access Channel (FAC) Telecommunications Standard Insti- not the same in LW, MW and SW and and the Service Description Channel tute) and the ISO (International Stan- in a given band. It can vary according (SDC). These two are key to ensure dard Office). Other bodies such as IEC to the time of day. Moreover, the sys- simplicity of receiver operation and and ARIB are also involved. tem should include operating modes are therefore designed to be reliably Key features of the standard include: that can be used in the transition received in adverse conditions, with • A worldwide standard to allow for phase, where simultaneous broadcast- different forward error-correction unique replacement of the AM format ing (simulcast) of compatible AM is schemes from the MSC. • Compatibility with existing required. Notice that a change in any The FAC is intended to be decoded channelization: The DRM signal is parameter needs no intervention from quickly by the receiver on first acquir- the listener, since the receiver is re- ing the signal (at switch-on, or during 1Notes appear on page 56. mote controlled by the transmitter. scanning). It carries a minimum of 50 Jan/Feb 2003 constantly repeated data that might hardware and software level. that starts with the SDC special group be essential at this stage: informing These definitions are used hereafter: of COFDM symbols and contains the receiver what bandwidth option is The system conveys symbols that are exactly three transmission frames. As in use, what modulation is used for the located at known instants and frequen- decoding the SDC is necessary to start SDC and MSC, which length of cies. A carrier is the set of symbols that decoding the whole bit stream, one can interleaver is used for the MSC, and are located at the same frequency.