DIGITAL radio could soon arrive in the US and unlike their European counterparts, American listeners will be able to pick up the digital broadcast of a station on exactly the same frequency as the normal, analogue broadcast. The news follows advances announced by two companies at last week鈥檚 Consumer Electronics Show in Las Vegas.
The BBC launched the first digital radio service in 1995, but the new transmissions are on completely different frequencies. Broadcasters in the US, however, have always insisted that digital radio services must be transmitted on the same frequencies as their existing analogue stations so as not to confuse listeners. But any attempt to transmit digital information on the analogue channel has always resulted in audible interference. Now two companies say they have solved the problem.
USA Digital Radio of Columbia, Maryland, says its tests showed that the reception of the analogue signal on the same channel was 鈥渧irtually unaffected鈥� by the digital signals. And a similar system developed by Lucent Digital Radio of Warren, New Jersey, is also said to be proving successful.
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Digital radio promises better sound quality than FM broadcasts, but spare radio frequencies are scarce. Britain assigned digital radio to the former black-and-white TV band of 217 to 230 megahertz, far above the 88 to 108 megahertz band used for FM radio in most of the world.
But broadcasters in the US have held out for transmitting digitally on analogue frequencies using an idea called in-band on-channel (IBOC) transmission. 鈥淚BOC makes a lot of sense. If we had been able to make it work, we would have done it,鈥� says Peter Shelswell, a researcher at the BBC in London.
The BBC found that destructive interference from IBOC signals bouncing off buildings made reception erratic. Instead, British broadcasters chose to combine several stations in a 1.5-megahertz-wide digital signal. But researchers in the US continued working on IBOC technology.
Now Lucent and USADR say they have succeeded in getting IBOC to work by placing the digital signals above and below the central frequency of the analogue signal, instead of right on top of it as they had tried in the past. Once a listener tunes into the central frequency, radios pick up the rest of the 200-kilohertz-wide channel, so the regions at the edges of the channel can carry digital data.
The two companies use different techniques to control interference. USADR broadcasts the same information on both digital bands, arguing that one of them will work most of the time. If, in extreme conditions, both fail, reception slips back to the analogue signal.
Lucent breaks its digital signal into four separate time-shifted 32-kilobit streams (see Diagram). The loss of some signals degrades but does not prevent reception. At 128 kilobits, Lucent says that its system offers 鈥淐D-quality鈥� sound.
The two systems will be compared by the independent National Radio Systems Committee and the Federal Communications Commission. Digital radio broadcasts could begin in the US by 2002.