THE clearest sounds ever broadcast into the home are being heard these days on a public station here, and it may signal a whole new radio era -- eventually. The three-month experiment in ``digital audio'' needs a TV channel to carry the sound and requires expensive home equipment to be heard. The listenership is tiny, but the results are impressive.
It's as if listeners ``had a compact disc player in their living room,'' says John Voci, operations director of WGBH-FM, the public radio station conducting the test with its sister UHF-TV station, WGBX-TV. ``You don't hear any of the transmission noise or hiss. This is the first time listeners with the right equipment can receive a broadcast digitally in their home. That is what is very different.'' And Dave MacCarn, engineering director for WGBH television and radio, compares the difference to that between FM stereo and AM radio, or between color and black-and-white TV.
Their claims were dramatically confirmed during a recent demonstration at the public station. The voice of music-show host Ron Della Chiesa was heard over powerful speakers on conventional FM. Then the digital signal was switched on, and his voice suddenly became transparent and distortion-free, revealing every lurking tremulo unnoticed on FM. The dead air between his words -- the total absence of background sound -- was startling and a little uncomfortable at first. His words seemed to float separately in space, as though coming from nowhere.
The concert music that followed was no less striking in its acoustic purity and impact. ``It has a much broader frequency response,'' Mr. Voci explained later by phone, ``from your lowest sounds to your highest sounds, and you've got a much broader dynamic range. You're able to hear the quiet passages as well as the very loud passages.''
The test that makes this broadcast quality possible is being conducted -- with the permission of the Federal Communications Commission (FCC) -- a few hours a day until Nov. 1, and primarily uses programs airing simultaneously on WBGH-FM. But the digital process -- which involves encoding standard ``analogue'' sound into separate packets of electronic information -- requires a bandwidth too wide for FM, so the signal is sent out on WGBX-TV (Channel 44 in the Boston area).
People who tune in to the TV channel at home during these special broadcasts see a flickering pattern of black and white lines arranged in seven vertical columns. But if they're among the few hundred people in the Boston area who own a ``digital audio processor'' -- which ranges upward in price from around $800 -- they can decode the digital signal and listen to it on their home stereos as very high-fidelity sound.
It's a sound many listeners are ready for, according to Marc Finer, a consultant for the Sony corporation, which makes the digital audio processors. ``The consumer becomes spoiled very quickly because of the striking characteristics and positive action of the compact disc,'' he notes. ``The FM listener generally listens to quality broadcasts, particularly of concerts, classical music, jazz broadcasts. . . . In this type of medium, it would only be a matter of time before they would notice this improvement as another quantum-level improvement over analogue broadcast.''
This improvement -- digital broadcasting -- could be the harbinger of a whole new system, according to audio writer and consultant Steven Birchall. ``I think it may well pave the way for a network of digital FM stations,'' he said by phone. ``National Public Radio could establish 6 or 10 super stations all over the country to create the live concerts and other programming and then transmit it to stations that would broadcast it locally in digital form on a UHF station.''
Those FM listeners are already benefiting from digital technology, which is currently being used at various stages in the broadcast process. ``Generally when the Boston Symphony Orchestra is in town, we will transmit the signal [digitally] from Symphony Hall directly to our transmitter,'' says Voci. ``But then it's converted back into regular analogue.''
Digital broadcasts can also be transmitted by satellite. ``We've done satellite from Europe on a couple of occasions,'' Voci recalls, ``and we assisted a Japanese broadcaster to send a performance of the Boston Symphony to Japan. Last Halloween, we did a concert from Kresge Audtiorium at MIT [Massachusetts Institute of Technology] and transmitted it back to our facilities here digitally. We sent it on to our television uplink and distributed it to about 12 other collocated public television and radio stations around the country. They took the digital signal and retransmitted it in the standard way. In that case, it was the first point-to-multi-point digital audio broadcast.''
In addition to this satellite method, digital sound can be sent on cable TV. ``We could deliver a digital signal to a cable company's main office,'' says Voci, ``and they could redistribute that to viewers and listeners.''
What does WGBH hope will happen now? ``We hope we will get a flood of positive response from the public,'' says Mr. MacCarn. ``We know that the sales of audio processors are small and that it's a limited audience at this point, but so was color television. We're hoping that as more processors will be made, more broadcasters will want to broadcast this kind of information, and that the FCC will allocate a specific spectrum for this use.''
Meanwhile, there are many unanswered questions. ``For us, the interest is to really analyze . . . how it works for people at home,'' explains Voci. ``Say, somebody lives in a fringe area of Channel 44 and they get ghosts and all the traditional problems of bad television reception. What are the limitations of the digital audio system as a result? That kind of information doesn't really exist, so it's a key element. That's the kind of feedback we want.''