10th December 2002 - Digital Radio

The BBC has been at the forefront of digital radio since starting DAB transmissions in 1995. In the last seven years, there have been many changes in digital radio, with transmission now possible on the back of the various digital television systems, and Internet streaming. The BBC's policy is to transmit radio on all the available systems. This has two purposes. Firstly it allows a greater audience to receive digital radio, and secondly it acts as an insurance policy against any system not becoming popular.

Each of the three transmission systems used have different strengths and weaknesses. The Internet is probably the most powerful, being able to carry rich media content, is available worldwide and can supply programmes on demand (the BBC store programmes for seven days). However, the Internet suffers from poor sound quality, isn't portable and broadcast costs can be relatively high as they are charged per listener. Digital Television is currently available in eight million houses either via satellite or terrestrial broadcasting.

Digital Audio Broadcasting (DAB) allows portable receivers that are easy to use and can offer better sound quality than most people currently get with AM or FM receivers. However, the average household has five radios and coverage is still limited. The coverage issue is currently being addressed though, and it is hoped that current negotiations with broadcasters will result in coverage increasing to 85% of the population by Easter 2004, by building approximately five transmitters a month.

The DAB system has been designed for rugged reception. Coded Orthogonal Frequency Division Multiplexing (COFDM) is used as the basic transmission standard. It uses 1,500 carriers per multiplex and long guard intervals between symbols to prevent problems with signals bouncing off buildings and hills etc, leading to multipath distortion (the fading and hissing you can often hear on analogue radio while listening in a car). In fact DAB uses the multipath distortion to its advantage, allowing a whole network of transmitters to be built operating on a single frequency. The signals from adjacent transmitters add together producing a stronger signal for the receiver.

No discussion on DAB can go by without mentioning bit rates and audio quality. The BBC is no stranger to the quality versus quantity debate that rages continually. Lindsay pointed out that the people who insist on ultimate quality are only a very small minority and the majority of the public are more interested in content. A compromise has to be reached that best serves the license fee payers as a whole. A DAB multiplex can carry 2.3 Mbits/s of raw data. After error correction, the bandwidth available for data is approximately 1.1 Mbits/s. The BBC has one national multiplex on which it now transmits 10 stations - the "normal 5" plus five new digital-only stations. DAB allows dynamic bit allocation between different stations on a multiplex and the BBC are constantly reviewing new coders to try and improve sound quality.

Despite the name, DAB can do more than transmit audio. 5/6 kbits/s are used to transmit "vision radio". This is a rolling carousel of pages from Ceefax. The pages are updated every five minutes or so and are automatically generated by a computer script which converts Teletext pages to simple HTML. Currently this can only be viewed on a PC, but this may change with future portable devices. Another 5/6 kbits/s are used for an electronic programme guide (EPG). This contains full listings information for the next seven days. If a radio has data storage, recording programmes of a particular genre and pausing live programmes etc becomes possible. The EPG uses XML so device manufacturers can choose how they wish to display the information available.

As with any new format, DAB needs affordable receivers to become popular. New DAB receivers are now starting to enter the market at £99 and are being sold out in a matter of weeks. Manufacturers are now working on DAB receivers built into all manner of products, including mobile phones and MP3 players.

Seven years after the BBC started transmitting, the rest of the world is starting to catch up!

Now the eventual replacement of FM seems to have found its feet, a consortium of broadcasters and manufacturers have started to look at whether the same can be done for AM. In China, 1998, the DRM consortium was founded to develop a world standard for transmitting digital radio below 30MHz (LW/MW/SW). Its objective to design a system that provides better sound quality than analogue, is available over wide areas and is future proof. DRM is designed to fit in the current analogue spectrum using 9 & 10kHz channels. The same building blocks developed for DAB are used in DRM, but with a different set of compromises. The narrower transmission channel requires less processing than DAB, but more efficient audio decoders require more processing. Like DAB, broadcasters can trade off between ruggedness and capacity.

Developments in encoder technology have meant that it is now possible to transmit audio at 20-24 kbits/s. DRM uses MPEG layer 4, AAC with Spectral Block Replication. No one is pretending that it is High Fidelity, but the quality is far better than even the best analogue AM transmissions. Lindsay demonstrated the audio quality of different test transmissions, one transmitted 736km from England to Germany at 25 kbits/s on MW, and one transmitted 3645km from Portugal to Cyprus at less than 16 kbits/s on SW. Despite problems with the replay equipment, the difference between the analogue and digital transmissions were startling.

It is still early days for DRM. Although it has been recognised as an international standard by the ITU, there are no permanent transmissions and no receivers available for consumers. Only time will tell if this promising format has a future.

Howard Farrar