Atmospherics and electrical blips can spoil the reception of analogue signals. These signals can also be interrupted by the close proximity of hills and tall buildings - especially in built up areas. This type of FM interference is called 'multipath'. DAB avoids this problem by transmitting a series of digital bits which can still to be recognised even in the presence of interference. Such interference is essentially ignored by DAB sets with the help of a system known as COFDM.

COFDM stands for Coded Orthogonal Frequency Division Multiplex. This system uses a precise mathematical relationship to split the radio signal across 1,536 different carrier frequencies as well as across a time lag. This ensures that, even if some of the signals are affected by interference or the signal is lost for a short period, the receiver is still capable of recovering the original source and reconstructing it perfectly.

COFDM also enables the same frequency to be used across the whole of the UK. This means that no re-tuning is required when travelling in a car or taking a portable set from one area to another. There are currently 230 frequencies being used by FM stations and existing National stations like Radio One use up an amazing 22 different frequencies. With DAB, the same frequency is used at every transmitter and, in areas of overlapping, the signals strengthen the total received signal on the receiver. This is a clever way of actually using what was previously considered to be interference as an advantage.

Consequently with DAB there are no more frequencies to remember. A single block of frequencies known as a "multiplex" is used to carry not just one service but around 6 to 8. In the same way as you might visit one multiplex cinema and have a variety of films to choose from, you will be able to tune into a radio multiplex and select a whole variety of stations or data services at the touch of a button.

The crystal clear sound that DAB produces is provided by a highly efficient sound compression known as MUSICAM which works by discarding sounds that are not detectable by the human ear. This audio coding process enables the vast amount of digital information to be reduced and make optimum use of the spectrum available. MUSICAM has a commonality with MPEG (often used with sound files on computers), ISDN (method of transmitting high quality sound and data via digital telephone lines), MiniDiscs, CDs and DAT tapes. Consequently, the system is proven, tried and tested and now reaching a new level of sophistication. A further advantage of using the MUSICAM system is the ability it provides broadcasters to vary the bit-rate transmitted on sound sources. For example, a lower bit-rate can be used for telephone chat shows than the highest bit-rate required for a symphony concert. The system is similar to the way sound files can be sampled at different quality specifications on a computer.

Each multiplex can be divided into a mixture of stereo and mono broadcasts and data services. The fewer services carried, the higher the audio quality allocated to each. Large divisions of a multiplex are needed for music and smaller ones for speech or text. 7 UK multiplexes have been allocated by Government on the radio spectrum 217.5 to 230.0 Mhz .

Until now only simple associated text has been possible on FM transmission. The BBC developed Programme Type Codes (PTY) that resulted in RDS which displays the radio station name and automatically re-tunes stations to the strongest signal and traffic announcements. The additional spectrum DAB offers will make RDS seem very basic. The BBC is already using new production software which creates 128 character radio text. This enables an LCD screen on a DAB receiver to display scrolling text which can display the title of a song being played and the name of the artist or even the title of album.

The BBC has run a pilot of this exercise on its Radio 2 DAB test transmission. Eventually it is anticipated that you will also be able to purchase a song being played on the radio at the touch of a button. All of this information is allocated to each programme stream within its allocated "headroom" of bit-rate transmission. Melody FM, one of London's commercial stations, has also developed an alternative text system for DAB known as Programme Associated Data (PAD).

The British Government has stipulated that scrolling text in this country must be immobilised in a car that is moving for safety reasons. The distraction of a busy screen will be disabled either when an engine is switched on or when the handbrake is off. There is further legislation, though, which opens up all kinds of possibilities for static sets. 10% of all available DAB spectrum is to be allocated to data transmission (over and above that already provided for programme associated text). This is a substantial chunk of high speed data capacity which is sufficient to feed animated pictures on a colour monitor. Manufacturers such as Grundig have designed home and hi-fi sets capable of having monitors attached. Another company, Technisat, is putting the finishing touches on a PC DAB receiver podule which will not only automatically facilitate DAB reception of your favourite stations, but also connect PC screens to visual data and other downloadable information. has also developed an alternative text system for DAB known as Programme Associated Data (PAD).

The BBC is likely to make use of this data segment for public service information. Commercial companies applying to run multiplexes during 1998 will be asked to come up with innovative ideas for the data transmission facility. One idea is to have a superfast miniature version of the internet as HTML and JPEG files (used in web design) are ideal for data transmission. It would also be possible to operate a radio version of teletext with far superior graphics etc. Another idea is to enable the download of large audio or video files on demand which can be stored in RAM for later playback. Ideas for downloadable files include traffic news, sports results and weather reports. Such services could be free to the user and contain advertising or use pay-to-view encryption technology. A further different application could be provided for corporate businesses who may require large blocks of data to be shifted around the UK at lightening speed. The possibilities seem almost unlimited.

Below there is tabular breakdown of the BBC's pilot broadcast at the Radio Festival in 1996 which shows how the multiplex was split down into 15 separate services.

This BBC exercise shows how incredibly versatile usage of DAB spectrum can be. Understanding how DAB works may take a little while to absorb, however, the really tough job is going to be dreaming up ideas for all the possibilities of the technology. You can rest assured that by the time you buy your first DAB receiver that a good many concepts will already have been incorporated. What's more, early sets are going to be built in a modular fashion so that improvements and upgrades can be fitted to accommodate what's bound to be an ever growing box of tricks!

Acknowledgements : European Klassik Rock would like to thank the BBC for its co-operation in providing both information and graphics which have assisted us in producing our DAB pages. Other major sources drawn upon have included The WorldDAB Forum, The UK DAB Forum, ASTRA, Bang & Olufsen and Grundig. Images not owned by European Klassik Rock either represent companies actively involved in the promotion of DAB or are assumed to be public domain. Should any inadvertent misuse of images have occurred, we will immediately act to remove offending items on request. All original material is © RadioWorks (Associates) Ltd and EKR Ltd 1998.