Removing the Static From Old Recordings
Lawrence M. Fisher, New York Times
December 21, 1988
A small San Francisco company, Sonic Solutions, has applied digital signal processing technology and artificial intelligence computer programming techniques to create a new system that takes noise out of old recordings.
The system's most enthusiastic users say it can cut the background interference out of a recording without harming the original work itself. Using artificial intelligence, it can even reconstruct short pieces of missing music.
Ever since Edison first recorded sound on a wax cylinder, engineers have sought to make recordings free of noise. Over the years, such new technology as Dolby noise reduction and digital recording has steadily made recordings clearer and more realistic.
But removing noise from existing recordings remained problematical. Techniques ranging from cutting a click or pop from a tape with a razor blade to electronically filtering out the hiss all removed some of the music along with the offending noise.
The new system, dubbed ''NoNoise'' by its inventors at Sonic Solutions, employs two processes, one to remove brief noise like clicks, and another to remove steady noise like tape hiss, guitar amplifier hum or air-conditioners. If requested by a customer, the system can also provide digital equalization, to boost or de-emphasize certain freqencies.
While still in prototype form, NoNoise was used to resurrect a long-lost recording and film of the Doors playing at the Hollywood Bowl in July 1968. Because of a faulty microphone cord, much of Jim Morrison's lead vocal was nearly obliterated by loud clicks and crunching sounds.
''I sent them a digital tape and in three weeks they sent us back a digital tape and it was glorious,'' said Bruce Botnick, co-engineer of the Doors' recording and now president of Digital Magnetics Inc., which makes master recordings for compact disks. ''We wound up saving 12 minutes of the show that would have been unusable.'' Since then, Mr. Botnick has used NoNoise to reconstruct many old master recordings. ''It is quite an elegant process,'' he said.
The NoNoise system is used primarily to remaster old recordings for release on compact disks because customers have come to expect that medium to be noise-free regardless of the original material's age or condition. The system has been used to remaster such artists as Ravel conducting the Bolero in 1932; ''Live Dead and Europe 72'' by the Grateful Dead; the Andrews Sisters, and Andres Segovia. It has also been used for forensic work, rendering a taped murder conversation audible above background noise.
The process begins by making a digital tape recording of the original master, whether it is on tape, film or an old 78 r.p.m. record. The digital audio data are then loaded onto a large-capacity computer disk drive, which is coupled with a computer work station. Sonic Solutions' original system ran on a Sun Microsystems work station, but is now being adapted for use with Apple Computer's Mac II, which is less costly and easier to use.
For de-clicking, the system reconstructs the problem portion of the recording by analyzing the content before and after the click, and interpolating the portion in between. Since the duration of the click is short, the process can sample enough of the surrounding material to make plausible guesses about the portion hidden by the noise.
Removing steady noise, like hiss, is a longer process. The system first samples the noise at 2,000 points in the frequency spectrum, creating a sonic fingerprint used to differentiate between noise and signal. Tests are then run to determine the maximum level of noise reduction possible without altering content. More than 53 million computations are performed to clean up one second of material; to clean up the master for a typical compact disk, the computer runs all night.
The process relies on both software and hardware developed by Sonic Solutions. A proprietary circuit board with four Motorola digital signal processors, disk controllers and additional memory chips handles all the intense computation, leaving the Macintosh's own processor to control the screen display, keyboard and mouse.
Robert Doris, Sonic Solutions' president, founded the company with James (Andy) Moorer and Mary Sauer; all were formerly with the Droid Works, a subsidiary of Lucasfilm Ltd.
Sonic's system would not be possible without Motorola's digital signal processors. Like the Motorola 68020 microprocessor at the heart of the Macintosh, Motorola's 56001 is essentially a computer on a chip. But it is optimized for certain intense kinds of computations rather than the general purpose input and output functions of a standard microprocessor.
Digital signal processors are extremely fast and can process several tasks at once. In addition to sound processing, they are used for the kind of intense number crunching necessary in high-speed modems, high-quality facsimile machines and technical computing applications.
The 56001 is unique among digital signal processors in that it processes information in 24-bit word lengths rather than the 16-bit standard in digital audio. This was essential to NoNoise because adding any kind of processing to the 16-bit information creates the need for more bits. ''In the studio you need a lot of margin so that when you end up with 16 bits on the CD, they are 16 good bits,'' said Mr. Moorer, Sonic's technical director. ''You need at least 4 to 6 bits of margin for the inevitable error.''
Since completing the prototype system in mid-1987, Sonic Solutions has functioned as a kind of dry cleaner for recordings, taking in record companies' master tapes and cleaning them of noise. The charge is $103 per minute of material, with volume discounts available. Now it is entering the systems business by repackaging the Mac II with the Sonic board and software, and high-capacity hard disk drives, allowing the record companies to do their own no-noise processing.
''Since we never claimed to be mastering engineers, the value of NoNoise is greater when it's embedded in a system like this,'' which allows recording engineers to exercise their own judgment in using the process, Mr. Doris said. Sonic's Desktop Audio system allows an engineer to perform mixing, editing, equalization and mastering tasks all in the digital mode, all on a hard disk, which is substantially quicker and easier than doing them manually on tape. With one disk drive, the basic system sells for $44,100; the NoNoise software is an additional $60,000.
For MCA Records Inc., a test site, making master recordings for compact disks with the Sonic system has saved time and maintenance costs on its digital tape decks, said Gene Wooley, vice president of recording and quality assurance. By using the NoNoise function in the work station, rather than sending tapes out to be cleaned, engineers can hear exactly how much noise reduction to use, and determine when some tone control may be necessary to restore old deteriorated recordings, he said.
And because the system also provides a visual representation of the sound spectrum, ''you can optically compare, and see that you have restored, not altered, the equalization or intent of the original,'' Mr. Wooley said. With extended use, he has found that NoNoise is best when used in moderation.
While it is possible to remove all the noise from an old 78, the result sounds unnatural, so Mr. Wooley leaves some noise in. ''It's like restoring an old piece of art,'' he said. ''You don't just strip all the accumulated dirt and grime of 500 years off and assume what you have left is the natural colors.''
While some articles in the audio press have said the NoNoise system removes high frequencies and detail, Mr. Wooley said that was the result of misuse by audio engineers. ''If one really works with it and knows how to use it, it's wonderful,'' he said. ''In the hands of someone inexperienced, it's dangerous.''
REMOVING THE CLICKS AND HISSES: A snippet from the audio track of Jim Morrison and the Doors' July 1968 performance of ''When the Music's Over'' at the Hollywood Bowl is depicted as two wave forms above. The lower wave form shows two clicks and a dropout in the word ''waitin'' caused by faulty equipment at the concert. In the upper wave form, a computer system has removed the interference and reconstructed the missing music by sampling music on either side of the interference. The photo of Mr. Morrison is from the MCA Home Video ''The Doors, Live at the Hollywood Bowl.'' The concert soundtrack was cleaned up for the video.
