Purists already have begun complaining, but the latest refinement in recording technology apparently is rolling full-steam ahead.

It's called the "NoNoise System," and it's reaching record stores in the form of 15 new classical compact discs from Philips (under the "Legendary Classics" banner). Each carries an instantly recognizable logo: the sign commonly used for "no smoking," with the word "noise" crossed diagonally.So isn't a "no noise" sign redundant on a compact disc, which is supposed to offer pure, noiseless sound reproduction?

Not exactly. In this case, Philips has dug deep into its archives for vintage performances, most of which had pops, clicks, tape hiss and other surface noises typical of early 20th-century recordings. To make these musty performances (by artists such as Sergei Prokofiev and Maurice Ravel) more appealing to today's sound-conscious consumers, Philips invested in the new NoNoise technology, which had been developed within the last two years by Sonic Solutions in San Francisco.

Essentially, the NoNoise system strips away most of the extraneous sound. To prove the point, Philips has produced a sample CD (distributed only to critics), which offers various vintage selections "before" and "after" the NoNoise treatment.

To this listener, the "before" and "after" comparisons seemed striking and impressive. A particularly noisy segment from Beethoven's Symphony No. 5, for instance, was shorn of noticeable tape hiss; the music that remained was clearer and easier to perceive as a result. A choral passage from Mozart's "Requiem" revealed a performance with far more precision of voicing and balance once the distracting noises were removed.

How is it done? Basically, the record company digs up the original master tape, makes a copy onto digital audio tape and sends it to Sonic Solutions. Technicians at Sonic Solutions then feed the digital audio data on the tape into a computer. And by analyzing the digital data on computer printouts, the folks at Sonic Solutions say they can pinpoint what's noise and what's music. They then instruct a computer to eliminate the digital information that represents the pops and clicks.

"The important thing to understand," says Mary Sauer, vice president of Sonic Solutions, "is that in the old days, if you wanted to get rid of a click, you literally had to go into the tape and cut out the click. And that meant you would be losing everything on that piece of tape.

"But we don't physically cut out the click. We simply have the computer isolate the click, and we have the computer look at the characteristics of the signal before and after the click. Then the computer simply refigures or redraws the portion of the tape where the click used to be."

As to tape hiss and other intrusive noises (such as the hum of a fan that might have been on in the studio during the original recording session), Sauer insists that the NoNoise technology has no trouble locating and excising that, either.

"Once the recording has the clicks removed, we address the problem of background noise, tape hiss or surface noise," she says. "First we take a `fingerprint' or `sample' of pure noise. This could be from a pause between movements or a moment before or after the performance. We don't need much - just a couple tenths of a second, so we almost always can find it.

"Once we find our sample, we run various analytical tests on it (in the computer) to determine its exact makeup. And once we know its digital makeup, we simply program the computer to get rid of that information. It's a little like micro-surgery - we look for a tiny amount of material and precisely peel it away.

"Then we send the newly revised digital audio tape back to Philips, and the project is finished."

Well, almost. For even though the recording has been "de-noised," as Sauer puts it, there's the matter of public response and acceptance to the new procedure.

A recent letter in Gramophone (the respected British records magazine), for instance, protested that certain performances on Philips' Legendary Classics series (available in Europe for the past two months) "are seriously compromised by the bleached sound of the transfer. Everything is very clear and there is indeed very little background noise, but also lost is the vital ambience that placed the original sound in a perceptible space and blended the various instruments together into something musically homogeneous.

"I'd rather have some background noise and the atmosphere of the original recording venue," the letter-writer said.

Classical collectors are finicky about their recordings, which, of course, comes as no surprise to Sauer.

"First of all, we should distinguish between atmosphere and tape hiss," says Sauer. "You're absolutely right - people don't want to see the life sucked out of the hall.

"We're trying to get rid of hiss while preserving the atmosphere, and we've recently refined our technique so that the noise estimate does not contain any room ambience. Thus we're now really hitting only the tape hiss and not any ambient characteristics of the hall in which the recording was made.

"And we believe that the next batch of Philips' `Legendary Classics' series (10 more discs to be released toward the end of this year) show the benefits of the perfected technique."

Though the technology obviously is still in its early stages, Sonic Solutions has had no shortage of work (the company declines to specify the fees it charges clients; the Legendary Classics discs are mid-price, meaning they generally go for slightly under $10). Various labels have employed the firm to clean up pop and jazz records from the '60s, such as vintage Grateful Dead discs - "We're kind of like a dry-cleaning service for records," says Sauer.

Recently a prosecutor's office in Texas enlisted Sonic Solutions' help in a murder trial.

"I can only describe this in general terms," says Sauer, "but about a week ago, we cleaned up a murder confession tape. The tape had been sent to the FBI, and they were able to clean it up quite a bit, but there still were a number of key sentences that were not real intelligible.

"So we cleaned it up further, and then it was admitted as important evidence in a trial.

"It's important to note that the surveillance market has a completely different interest in our services than the music-recording market. The recording industry wants the utmost fidelity, so we don't touch the music at all. The surveillance people don't really care about fidelity - they want intelligibility."

It will be fascinating to watch how classical listeners will respond to the new technology, since most of the major labels have archival material that, until now, has been too fuzzy for wide commercial distribution. The future of vault recordings likely will be affected by the success or failure of the NoNoise recordings.

Regardless of the outcome, Sauer stands by her guns.

"All of the record companies have expressed interest in getting this technology into their own shops," she says, "because they want to make the ultimate decisions about noise reduction.

"As for the purist who absolutely wants to have no changes to their recording - that's fine, I respect that decision.

"They always have the option of listening to their 78s."