Digital Radio or DAB (Digital Audio Broadcasting), as it is sometimes called, is a method of broadcasting and receiving radio signals in the digital domain, as opposed to the traditional method of analog transmission by AM and FM systems. While digital television signals are now routinely received over cable and satellite (DSS) systems, reception of digital radio broadcasts is still fairly uncommon.

Digital radio works similarly to satellite TV, except that, instead of being transmitted from a satellite, it is normally broadcast from transmission towers just like standard radio. Two companies, Sirius Satellite Radio, Inc. and XM Satellite Radio Holdings, Inc., plan to offer satellite-based subscription radio services (something like DTV), but for the purposes of this article, we are concentrating on the more widely available conventional broadcast version of digital radio.

In contrast to analog radio, digital radio signals are immune to distortion from multipath, adjacent stations, overly weak or overly strong signals, etc. Additionally, digital radio signals can carry extra information such as text that can provide station call signs, artist and song titles, weather, or even traffic information.

Eureka!

The digital radio system currently being used around the world is known as Eureka 147. This system broadcasts at much higher frequencies than standard radio transmissions, which operate in the FM band, from 88 to 108MHz, and the AM band, which is 0.525 to 1.705MHz. While the actual frequencies used for digital radio depend on the availability for that particular country, Britain utilizes Band III, which covers 174 to 240MHz while Canada uses part (1452 to 1492MHz) of the L-Band, which is 300 to 1500MHz. In comparison, satellite television utilizes frequencies starting around 4GHz and going up to almost 20GHz.

The Eureka system broadcasts multiple stations and services over a single frequency in something called a multiplex. Bandwidth within the multiplex can be allotted to the various stations as needed. For instance, a high-quality stereo station can be given more bandwidth than a news or talk radio station that may be broadcast in mono. Stereo programming is typically broadcast at 192 kilobits (kbit). Digital radio utilizes a method of digital compression (similar to that of MP3) so that many services can be combined onto one signal for broadcast over a single frequency.

The sound quality of digital radio has been described as close to or as good as CD. With a bandwidth of 192kbit for a stereo broadcast, the sound quality of digital radio, which utilizes MUSICAM digital compression should be quite good, but will probably not be true CD quality. The main advantage of digital radio is that it will not have the usual distortion associated with analog radio such as hissing, popping, phasing etc.

What about the United States?

Unfortunately, the US has decided not to adopt the Eureka system for digital radio and is developing its own system, In-Band On-Channel (IBOC), which is compatible with the current analog radio formats. This method will utilize the existing AM and FM bands by attaching a digital side-band signal to the standard analog signal so that the system is compatible with any existing tuner. Thus, if a station is currently located at 99.9 on the FM band, it will remain at 99.9 FM whether in analog or IBOC digital radio. Due to the limitations of the bandwidth of AM frequencies, digital AM broadcasts are said to have FM-like sound quality while digital FM broadcasts will have near-CD sound quality. IBOC utilizes a form of digital compression called PAC (Perceptual Audio Coder), originally developed by Lucent Technology. Another limitation of IBOC is that AM and FM frequencies are not suitable for satellite transmission if extraterrestrial transmission is desired in the future.

Although IBOC seems like a good idea and is a promising format, it has yet to be introduced to the public. Utilizing the existing AM and FM bands has both advantages and disadvantages, but the biggest disadvantage of the IBOC system is its incompatibility with the Eureka digital radio systems in use throughout the world.

What is the current state of digital radio?

Digital radio is a reality in much of the civilized world in the form of the Eureka system. As mentioned, much of the UK receives the BBC multiplex as well as something called the Digital One Network. Canada has digital radio broadcasts in major centers such as Toronto, Vancouver, Montreal, and Windsor with plans for expansion into several other areas by early 2002. Testing of the IBOC system in the United States has been completed and rollout of the format is expected sometime in 2002. Companies such as the Harman Consumer Group and Alpine Electronics have already committed to producing digital radio tuners utilizing In-Band On-Channel technology. Actual products should most likely be available by 2003.

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 US 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.

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