Extract From Byte Magazine - Nov 05 - inside Digital Radio

[Steve Bildermann]

As Byte is now a subscription service I thought some people might enjoy reading some extracts.

Radio is the oldest of the broadcast media: FM radio was invented in the 1940s, AM in the 1920s. Efforts to bring radio into the digital age began in the mid 1970s, when BBC Research engineers investigated the possibility of providing a digital radio service based on a NICAM stereo signal. They set up a small transmitter and a reception vehicle which could receive and decode the signals using both a directional antenna on a tall pole, and a standard whip aerial. They found that the reception on the directional antenna was generally very good, but that the vehicle's rooftop antenna was not satisfactory over a significant proportion of the service area. The problem was generally caused by multipath reception the reception of two or more signals from different directions due to reflections which resulted in considerable interference to the digital signal. <continued>

[Steve Bildermann]

In 1992, the World Administrative Radio Conference reserved the frequency band 1452 1492 MHz (the L band) for the Eureka 147 project, a European consortium of industry and governmental interests which went on to develop the DAB (Digital Audio Broadcasting) standard. While the L band spectrum is ideal for DAB, other frequencies, including VHF and UHF frequencies, can also be used. In the Eureka 147 system, each carrier occupies 1.5 MHz of radio spectrum and provides between 1.2 and 1.5 Mb/s of error corrected data. By using MPEG Audio Layer 2 or 3 compression, with each stereo stream requiring 192 to 256 Kb/s, between five and eight such programs can be conveyed per carrier. Such bit rates are often claimed to offer "CD quality," though while the results may be indistinguishable to most listeners, the algorithms certainly do degrade the sound measurably and perceptibly in good listening conditions.

So what's so great about digital radio? Well, there's the matter of perfect reception. DAB "blankets" wide geographical areas with an even, uninterrupted signal. Once full services are up and running, a driver will be able to cross an entire country while staying tuned to the same station, with no signal fade, and without altering frequency. And DAB has the potential to be very cheap, since broadcasters can provide a wide range of material simultaneously on the same frequency.

DAB technology also brings new possibilities for program associated data. Digital radios might display text information such as program background facts, a menu of future broadcasts, or complementary advertising information. Receivers attached to a small screen could display visual information as diverse as weather maps and CD cover images. Services from sources other than the broadcasting station can be included within the same channel for the user to access at will. These include news headlines, detailed weather information or even the latest stock prices. And because digital technology allows a massive amount of different information, specific user groups can be targeted with great accuracy because each receiver can be addressable. Beyond this, there are other, more futuristic possibililies for digital radio: for instance, personal receivers that could correlate GPS signals in order to provide location based services.

DAB trials have been conducted in many countries. Transmissions began in Belgium in 1997 using VHF frequencies. Six stations in Singapore now broadcast programs using DAB technology, while DAB trials are in progress in places like Australia, India, China, Malaysia and Taiwan.

In Japan, a decision has been made to pursue a different technology: ISDB T, which broadcasts both digital radio and television at both narrow and wide bandwidths. It utilizes COFDM signal coding technology, the same system that was developed for DAB. ISDB T services are expected to be available in Japan in a few years' time.

In the United States, there is continuing opposition to Eureka 147 from established broadcasters and manufacturers. Ostensibly the subject of debate concerns available frequencies, but it's widely thought that the current industry is simply opposed to new competitors with low cost structures. This has lead U.S. broadcasters to continue with the technically flawed in band system using the existing frequencies of the incumbents, and to develop a different technology: MMBS (Multimedia Broadcasting System), which would operate in the UHF band, using principles similar to Eureka, but remaining incompatible with it.

Transmission of a DAB Signal

The modulating and demodulating signals of the digital radio are digital pulses rather than analog waveforms. DAB uses a modulation technique called COFDM in order to overcome the problems associated with traditional FM/AM transmissions, namely interference. COFDM stands for Coded Orthogonal Frequency Division Multiplex. This system uses a 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. A single block of frequencies known as a "multiplex" is used to carry not just one service but between 6 to 8.

Each multiplex can carry a mixture of stereo/mono broadcasts and data services. The fewer the number of services carried, the higher the audio quality that can be allocated to each service. The DAB multiplex is made up of 2,300,000 "bits" which are used for carrying audio, data and an in built protection system against transmission errors. Of these, about 1,200,000 bits are used for the audio and data services.

The sound that DAB produces is provided by a sound compression method known as MUSICAM (Masking Pattern Universal Sub band Integrated Coding And Multiplexing). It is the algorithm used in MPEG encoding that removes audio data imperceptible to the human ear, while retaining the high quality audio. The MUSICAM system allows broadcasters to vary the bit rate transmitted on sound sources: A lower bit rate can be used for talk shows, and a higher bit rate for a symphony concert. The system is similar to the way sound files can be sampled at different quality specifications on a computer.

The number of channels in a DAB ensemble can vary. For instance, on Saturday afternoon a stereo channel could be divided into a special sports channel and a further mono channel. The overall data rate available in DAB can be flexibly distributed over various program channels, and the number of program channels per DAB ensemble is flexible.

On the receiver side, the DAB ensemble (multiplex) is selected in the analog tuner. The digitized output is fed to the OFDM demodulator and channel decoder to eliminate transmission errors. The information contained in the fast information channel (FIC) is passed to the user interface for service selection, and is used to set up the receiver appropriately. The main service channel (MSC) data is further processed in an audio decoder to produce left and right audio signals, or in a data decoder.

DAB was developed with the aim of improving radio reception. But DAB, as a digital transmission system, can transmit other data as well as audio. In principle any type of information can be transmitted by DAB, provided simply that it is available in digital form and does not exceed the maximum available DAB data rate (approx. 1.7 Mbit/s). Examples of such additional services would be images or animations, stock listings, or even software updates. In contrast to multimedia applications via TV or cable, DAB's additional services can also be received in car and with portable equipment.

DAB Receivers

The future DAB receiver will consist of a tuner, speakers, a graphics screen, a processor, and a large amount of memory. There will be all types of receivers, from home and car receivers, to walkman like portable receivers with optional displays. One of the first available consumer oriented digital radio tuners is Arcam's Alpha 10.


Thanks to the ISO frequency programs, the receiver needs to be preset only once. Because of the associated program metadata, it's possible to select your program according to subject e.g. news, sports, classical music, soft rock or by the name of the station, rather than by a difficult to remember frequency. The name appears on the digital display of the DAB receiver and one can push simple "up" and "down" buttons to scan through all the programs that are on the air. More sophisticated sets will have large LCD screens to provide more of a graphical user interface.

Digital radio can be received with a telescopic or disk antenna on a car, and with a whip or throw out antenna on a portable radio. The antennas can be smaller than the ones we're used to now, as DAB uses a shorter wavelength than VHF FM.

A home computer could potentially become a DAB receiver, if equipped with a card capable of receiving the L Band DAB signal. Modular Technology has launched a DAB PCI radio card; it goes inside your PC like any other PCI expansion card. The DAB card has an external aerial that looks like the type of mobile phone aerial you would mount on the roof of a car, connected via a thin cable to the CPU. It uses your PC's sound system, and you can record radio programs directly to your hard drive. The user interface not only displays full program information and the genre of each radio station, but it also can schedule recordings ahead of time from electronic program guides. Recordings can be stored as MPEG 2 files or in MP3 format for direct downloads to MP3 players.

Modular's card is based around the TI DRE200 chip, and Radioscape's software. There are no Linux drivers available and Modular won't be releasing any, but the company won't comment on whether it will release enough information for third parties to write some.

Psion produces a device called the Wavefinder, which is another gadget that is able to receive and decode digital radio transmissions using the power of PC. The digital antenna unit connects directly to a spare USB port on your PC, giving access both to DAB digital radio stations and new multimedia data services.

Why You're Not Listening to DAB

Despite all its advantages, DAB has seen relatively slow uptake, in part because of the high cost of receivers. A digital radio runs $300 and upwards, as compared to $50 for a reasonable quality analog radio tuner. An FM car radio might cost between $300 and $800, but a DAB car radio is priced between $1,500 and $2,000. And so a sort of "chicken and egg" situation has developed: DAB radio sets are pricey because only small numbers of them are produced. High prices in turn discourage radio stations from investing in DAB broadcast. Since there's little compelling content, few consumers invest in digital radio sets; and so manufacturers don't produce the sets in bulk.

Digital radio holds great promise for the future, but even though broadcasts on the Eureka system are available in many parts of the world, penetration of digital radio tuners into the home market has been minimal. It will be interesting to see how the introduction of the In Band On Channel system in the U.S. fares, and whether the two digital radio formats will continue to co exist. Either way, even if digital radio becomes a success, it will probably take five to ten years possibly even longer before conventional analog radio broadcasts are suspended, if ever.

[Big Booger]

DAB sounds like the future. The price indeed is rather steep, but once it comes down I suspect that it will become the future standard.

[Taro Toporific]

DAB sounds like the future. The price indeed is rather steep, but once it comes down I suspect that it will become the future standard.

The equipment price of digital radio that worries me. The will be general migration to digital broadcasting no doubt in the next decade. The PROBLEM is in the States and Japan all digital broadcast services use satellites or cable, which costs a monthly fee $9-12 in the US and about 1,100yen in Japan. BAH.

XM Satellite and Sirius Satellite radio are fighting it out. However, in addition to these satellite-based digital radio services, there's HD RadioTM, a land-based digital broadcast service. The two satellite services mentioned above are both pay services, but HD RadioTM is a free service funded by advertising, and so I expect that FREE service will grow quickly. Now in Japan.... bend over...