DAB is a digital technology offering considerable advantages over today's FM radio, both to listeners and broadcasters.

The most obvious benefit to listeners is DAB's ability to deliver CD-quality stereo sound robustly and reliably, even to mobile receivers, using only a simple rod antenna. In moving vehicles in particular, FM reception is often distorted or interrupted by multipath interference (see figure), as reflections from hills and buildings arrive out of phase with the main signal. By contrast, the DAB receiver's powerful processor use these 'shadows' to actually reinforce the main signal.

Listeners will also welcome the ease of use of DAB receivers. They will be able to switch between the eight or more stations carried by every single multiplex (see How DAB Works) without retuning their sets. And since a single DAB frequency can carry the same signal across an entire network, there will be no need for drivers to retune as they cross a country.

Bandwidth Choice

DAB's flexibility will also provide a wider choice of programmes, including many not available on FM. Each multiplex is able to carry up to six full-quality stereo
programmes, but bandwidth can be seamlessly reallocated to a larger number of lower quality services. A single station might offer its listeners a choice of mono voice commentaries on three or four sporting events at the
same time, for example, and then combine the bitstreams to provide high-quality sound for the concert which follows.

DAB can carry text and images as well as sound, and all receivers will be equipped to handle non-audio data. All but the smallest will be able to display at least two 16-character lines of text, allowing programme
selection by name or programme type, and enabling broadcasters to transmit programme-associated data (PAD) such as album title, song lyrics, or contact details. Additional services, such as traffic information and sports,
weather or stock market news feeds, are already on their way. Some even include full colour images.

All of these advantages will help broadcasters attract new audiences. And since DAB can be transmitted at lower power than today's FM and AM services without loss of coverage, they will even be able to reduce costs
by transmitting at lower power or from fewer transmitters.

DAB Transmission

Like digital recording, DAB starts with sampled sound. EUREKA 147 specifies a full rate of 48,000 and a half rate of 24,000 samples per second. Full rate audio sampling generates a bit rate of 1.5 Mbit/s, so the signal must be compressed for broadcast. DAB uses the MPEG Audio Layer II system to achieve a compression ratio of 7:1 without perceptible loss of quality. This system is modelled on the working of the human ear, and retains only the audible components of the sound.

The signal is then encoded at a bit rate of 8-384 kbit/s, depending on the desired sound quality and the available bandwidth. Programme Associated Data (PAD) is incorporated, and the signal is individually error protected and labelled prior to multiplexing. Independent data services are similarly encoded.

Multiplexing fits the labelled packages from each signal into a standard 'outer container' to ensure efficient use of the radio spectrum. Broadcasters can dynamically adjust the composition of the multiplex to carry any desired combination of services, from six high-quality stereo programmes to as many as 20 mono programmes. Information about the current configuration of the multiplex, together with additional error protection data, is also included.

The Coded Orthogonal Frequency Division Multiplex (COFDM) technology, finally, spreads the 2.3 million bits of the multiplexed signal in time and across 1,536 distinct frequencies within the 1.5 MHz band, so that even if some frequencies are affected by interference, the receiver will still be able to recover the original sound in full.

The DAB Network

To avoid severe multipath interference between transmitters, an FM network must use different frequencies in each area. In a DAB network, however, all transmitters operate on a single frequency.

How? Signals from neighbouring transmitters combine, instead of causing interference. Such a Single Frequency Network (SFN) makes DAB's use of the radio spectrum over three times more efficient than FM, freeing more frequencies to carry more services. The extreme sensitivity of DAB receivers, meanwhile, means that DAB can be transmitted at much lower energy than FM without a loss of signal quality.

DAB is designed for terrestrial, cable and for future satellite broadcasts, which may be used to deliver pan-European services, for example, or as a cost-effective means of covering very large rural areas.

The DAB Receiver

Almost as much signal-processing is needed to decode the DAB signal as to encode it, so each DAB receiver is based around powerful, specialised, highly integrated silicon chips.

The receiver reverses the multiplexing and audio encoding applied during transmission, eliminating any transmission errors at the same time. It then converts digital audio back to left and right analogue signals, and passes them to the speakers.

This device requires hardware that has not yet been released, and is currently provided mainly to show how to write a driver. For more information, see http://dab.in.tum.de.