Japan is about to launch the world's first nationwide digital radio broadcasts.

It will radically improve the fidelity of radio sound, finally matching the quality of compact discs, laser discs and digital audio tape decks.Digital, or pulse code modulation (PCM), broadcasts will be immune from static and the constant background noise heard on standard AM and FM bands.

A symphony broadcast, for example, will be free from the distortions that mar performances on conventional radio. If no sounds are being broadcast, the listener will hear nothing, rather than the usual low hissing noise.

Full services are scheduled to start next April, following test broadcasts via satellite this month.

Digital radio transmissions - which encode sound as a series of ones and zeroes rather than an analogue of sound waves - are not new.

Satellites and microwave relay stations distribute radio signals digitally. A handful of cable TV networks in the United States have offered digital radio, and satellite TV has delivered digital soundtracks for several years.

But nationwide digital radio will be a first, says Japan's first PCM radio station, SDAB (Satellite Digital Audio Broadcasting). It is investing millions of dollars and building new studios near Tokyo's trendy Harajuku district.

Inaugural programs will feature "relaxing but not sleepy" music, mostly American, along with natural sounds such as bird singing and ocean waves. There will be no commercials and a minimum of disc jockey chatter.

SDAB is the first of some 19 PCM radio channels, all of which could be on the air late next year.

But despite unprecedented sound quality the success of the new PCM stations is far from assured, and some critics doubt they will win enough listeners to make a profit.

They will not succeed on fidelity alone.

"There will be a revolution in the quality of programming. Otherwise, we'd be just another radio station with a fancy transmission system," said Steve Tanaka, vice president of SDAB.

Critics say high costs will prevent the new stations from attracting enough listeners to make a profit.

Kotaro Wakui, director of the New Electronic Media Division of Dentsu Inc., Japan's biggest advertising agency, reckons SDAB will find it hard to achieve its goal of break-even in two years by signing up 700,000 subscribers.

The subscription fee is low at $4.40 a month, but the installation fee for the decoder is nearly $200.

Listeners must also invest several hundred dollars in a satellite antenna and receiver. Then, of course, to appreciate digital radio fully the listener needs a fancy stereo system.

About 3 million households have already installed satellite antennae and receivers to view satellite TV channels.