IN EARLY June, electronics giant Texas Instruments unveiled the future of radio: a tiny receiver that took several years and millions of dollars to develop. Eagerly awaited by broadcasters and manufacturers alike, it isnât designed to pick up radio broadcasts from satellites or the internet. Instead the new chip will tune in to the old-fashioned AM bands â the short, medium and long-wave signals pioneered by Guglielmo Marconi more than a century ago.
At first glance this would seem as likely as Boeing resurrecting the biplane. Audiences have been turning their backs on AM for decades. Medium and long-wave broadcasts sound rotten and are easily swamped by interference. And although short-wave broadcasts can reach around the world, if youâre on the move you must constantly re-tune your receiver if you want anything more than white noise.
Meanwhile FM radio has spread almost everywhere, and the internet can give you perfect reception for hundreds of stations based all over the world. High-frequency digital audio broadcasts and satellite radio coverage are also expanding fast. So why would Texas Instruments â and the global radio community, for that matter â waste time and money on defunct wavebands?
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This chip is anything but a backward step. It is in the vanguard for a global broadcast standard called Digital Radio Mondiale that is designed to stuff the AM bands with a swathe of new digital transmissions. Using the latest coding and compression tricks, DRM will squeeze high-quality stereo sound into the narrow AM bands and send these signals over greater distances and with less interference than ever before. The technology promises to breath new life into ailing AM stations the world over. Convert a medium-wave transmitter to DRM, for instance, and a station that once struggled to reach 10,000 listeners could embrace a hundred times as many.
But DRM is more than just a way to please advertisers or to make Beethoven sound better in the bathroom. Along with music and chat, signals will carry a stream of digital data and multimedia information that could provide everything from text and pictures to news headlines, educational material for schools in remote communities, or even software upgrades for your car or washing machine, delivered and installed while you sleep.
âRadio signals could even carry software upgrades for your car or washing machineâ
If youâre thinking that the airwaves are already awash with digital radio signals, youâd be right. Many FM stations broadcast a signal called radio digital system (RDS), a digital add-on to the analogue signal that contains limited amounts of information, such as the station name or a digital tag for traffic reports that allows radios to tune in to them automatically. Some broadcasters in the US are switching to a system called In-band On-channel (IBOC) that can send digital and analogue signals on the same band. Meanwhile, European engineers have pioneered a new form of transmission called Digital Audio Broadcasting.
DAB takes an analogue music stream and digitises and compresses it before transmission. The idea is to give listeners clean crisp stereo sound along with extra information, such as the name of the music track being played or contact details for the station. This data stream is beamed out alongside the main signal and the information can be displayed on the screens built in to many DAB radios. The BBC began DAB transmissions in the mid-1990s, and the system is now widely used across Europe, in parts of Asia and in Canada.
But these systems have distinct drawbacks. Some broadcasters claim that IBOC creates interference on neighbouring bands. Meanwhile DAB frequencies are allocated nationally, not internationally, so a DAB radio designed to pick up UK stations is useless in Germany or Canada. Also, DAB operates at megahertz or gigahertz frequencies, in the VHF or UHF region of the radio spectrum (see Graphic). These relatively high-frequency signals donât travel far â they are more or less restricted to line of sight â so broadcasters need expensive networks of transmitters to achieve national coverage. The BBC, for instance, has more than 70 DAB transmitters, yet the signal covers just 85 per cent of the UK.
Whatâs the frequency?
Of course if itâs good coverage you are after, you canât do better than conventional AM radio, in use since the 1920s. Its long and medium-wave signals can follow the curvature of the Earth for long distances, so in Europe, for example, a small network of transmitters can broadcast across several countries. And since short-wave radio transmissions reflect off the planetâs ionosphere, they carry even further â which is why travellers around the world have traditionally tuned in to short wave to hear news from home.
Yet in the past 20 years, audiences have abandoned AM in droves, retuning their sets to FM or switching to portable CD or MP3 players or internet radio. The main issue is sound quality. AM signals are vulnerable to interference from electrical noise from motors or atmospheric disturbances like lightning, and from multiple reflections that create periodic changes in volume or obscure the signal entirely. Short-wave radio listeners face an additional complication: they must keep retuning their sets to follow broadcastersâ complex transmission schedules.
In an attempt to rescue the AM wavebands from these problems, in 1998 the International Telecommunication Union agreed on a new standard for broadcast technology that could use AM frequencies for digital transmissions. The result is Digital Radio Mondiale, the only digital system that can be used on any AM frequency anywhere in the world.
The DRM Consortium consists of a hundred companies from 30 countries. So far, over 20 broadcasters have begun DRM test transmissions, including BBC World Service, Deutsche Welle and Radio Luxembourg.
The BBCâs research and development labs at Kingswood Warren in Surrey, recently showed broadcasters how DRM works. An analogue recording is filtered, digitised and then compressed, just as with DAB and MP3, but using technology that had not been invented when those earlier formats were conceived. The result is that DRM can squeeze high-quality stereo sound into a single AM band (see High fidelity).
Unlike AM radio receivers, DRM sets can use digital error correction to mathematically subtract interference, just as a CD player can deliver perfect sound from a dirty or scratched disc. As long as the noise or interference is not stronger than the transmission itself, it is ignored. Like DAB, but unlike old AM radio, several DRM transmitters can share the same frequency: receivers will always lock onto the strongest signal and ignore the rest.
Along with music, news or chat, each DRM transmission can also carry a supplementary data stream at a rate of up to 40 kilobits per second â around two-thirds of the speed of a standard telephone modem but several times as fast as RDS on FM. Fast enough to transmit a lot more than just the radio stationâs name.
To show how this might be used, BBC engineers demonstrated a prototype DRM receiver that stores the data stream while your radio is tuned in to a programme. This could give listeners a ânews on demandâ service, for example: your radio automatically downloads a regularly updated bulletin of news headlines over about 10 minutes or so, and then at the push of a button plays the bulletin from the top. Or if the data is multimedia information rather than news audio, you could download it to a PDA and view weather maps or traffic reports, say, as a web page.
Unlike DAB, DRM makes use of longer-wavelength signals, so it carries much further, just like traditional AM. For example, DRM software developer Radioscape based in the UK recently detected test broadcasts from Thailand. Similarly, radio enthusiasts in New Zealand have picked up test signals from across Europe and Asia.
Whatâs more, the digital receivers are far more sensitive than older AM units, so converting an existing radio transmitter from analogue to DRM means its transmission power can be reduced to just a fifth â from 100 kilowatts to 20 kilowatts, for instance â without affecting its coverage. âThat reduces the electricity bill, which is a big consideration for broadcast stations continually pumping out many kilowatts of power,â says Gerald Moser of Coding Technologies, a German-based company that developed the compression system used for DRM. Alternatively, keeping the transmitter power the same would dramatically increase the coverage area. Estimates suggest that converting a modern transmitter from analogue to DRM will cost around âŹ30,000.
RTL, the broadcast group that owns the old Radio Luxembourg station, sees the technology as a way to bring back the glory days of AM in the 1950s, when millions regularly tuned in. RTLâs financial officer Thomas Rabe describes DRM as the âessential cornerstoneâ to a new radio revival. âWith DRM, one long-wave transmitter in Luxemburg can reach 50 million listeners in France, Belgium, the UK and Germany,â says Rabe, âand with a medium-wave transmitter we can reach 100 million.â
The BBC, Texas Instruments and Radioscape are already planning the next step. Lindsay Cornell, who steers the BBCâs policy on DRM, describes their ideas as âbeyond radioâ. She sees DRM becoming an essential broadcasting tool, and points out that AM radio is already one of the only ways to deliver programmes into inaccessible regions. âSome countries like China censor the internet. Others donât allow satellite dishes. Many people rely on services like the BBC and Deutsche Welle to get independent news.â DRM should certainly help here, improving the sound quality and reliability of broadcasts.
âThe first DRM radios will go on sale later this year and are expected to cost around $200â
And when disaster strikes â be it a major earthquake or a terrorist attack â radio can suddenly become vital, as phone lines often fail and internet news sites become swamped. Police and rescue services already use short-wave radio to communicate from accident scenes or disaster zones. DRM should be more reliable, and the extra data channel will allow rescuers to send maps, images or other useful information back to base. The Red Cross has already joined the DRM Consortium.
Village schools in Africa, India or Latin America could benefit too, Cornell believes, since DRM could reach areas without a phone line or cellphone service. To help with lessons, radio sets could download speech in more than one language, switching from one to the other at the push of a button. A DRM radio could also download and store text and data for use in later lessons. In the Australian outback or on the plains of Mongolia, farmers could download news and weather forecasts overnight, then call them up on demand in the morning. Bus timetables at remote stops could be updated. DRM could also be useful for environmental monitoring, by broadcasting local weather conditions from automated monitoring stations on an Arctic ice floe, say.
DRM could even deliver computer software automatically. Texas Instruments and the BBC have begun talking to a major European car manufacturer about using DRM to deliver the software updates required by modern engine management systems. These are usually installed during servicing. Instead, a vehicleâs radio could save data from DRM broadcasts and store it in memory while it sits outside your house. When all the data has been downloaded and the receiver has checked there are no errors, the update is triggered automatically. The same technique could work with things like washing machines too. âIt doesnât matter that the data rate is low,â says Cornell. âIt can take a day or a week to deliver.â
Whether all these ideas succeed depends mainly on the eventual cost of DRM receivers. So far engineers have built fewer than a thousand radios and computers modified to receive the broadcasts, and these have been expensive. But Texas Instrumentsâ new receiver module should help bring prices down, and it shouldnât alienate fans of older technology: as well as DRM, the new module can pick up traditional AM, FM and DAB. âThis unit is the breakthrough that DRM has been waiting for,â says Peter Senger, DRM Consortium chairman and chief operating officer of Deutsche Welle.
When the first DRM radios go on sale towards the end of this year they are expected to cost around $200 each. Prices should begin to drop soon after. But if Cornell and the consortium are serious about getting AMâs new incarnation out into the developing world, these radios will have to match the price of the crackly old sets they want to replace.
This isnât as implausible as it sounds. The ITU is already planning to extend DRM transmissions across North and South America and has extended it to FM frequencies. The potential audience for DRM is skyrocketing and that can only push the price of these sets downwards. âThe technology is developing fast,â says Andrew Buckhurst, a spokesman at RTL. âWatch this space.â
High fidelity
IN CONVENTIONAL AM and FM radio transmissions, the audio signal is encoded by modulating the amplitude and the frequency of the carrier wave respectively. The quality of the sound is related to the bandwidth of the signal: each AM channel is just 10 kilohertz wide, while FM channels are around 20 times as wide. Digital Radio Mondiale, on the other hand, uses the latest digital compression tricks to squeeze FM-quality stereo into 10-kHz AM channels.
To create a DRM broadcast, the analogue sound is first filtered using a system called spectral band replication to separate the high and low frequencies in the signal. Then the two frequency bands are separately converted into digital code using a method called advanced audio coding. More bits are allotted to the essential low frequencies than to the subtle highs, which helps to keep the overall number of bits lower than if the high and low frequencies were handled together. The two bands are then broadcast separately and recombined by the receiver to rebuild the original.
To help DRM broadcasts avoid interference, DRM uses a system called coded orthogonal frequency division multiplex that is already used for the DVB digital TV system and for Digital Audio Broadcasting radio transmissions. The radio signal is split into many parallel streams, all within the same 10-kHz AM channel. Even if one stream does not get through, most of the others will.
In addition, error correction bits are added so that the receiver can correct digital glitches mathematically, just as a CD player can correct bad bits read from a dirty disc.
Instead of audio, each 10-kHz channel can also carry a mix of sound and data signals at up to 40 kilobits per second. If a DRM broadcaster has access to several channels, they can be harnessed together to produce higher fidelity sound, or faster data rates.
