Rescuing recorded sound from silence

By W. Conard Holton

Imaging technique helps recover sound from old record disks.

While listening to National Public Radio (Washington, DC, USA; www.npr.org), Carl Haber, a physicist with Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA, USA; www.lbl.gov), learned that librarians at the Library of Congress (Washington, DC, USA; www.loc.gov) were faced with a problem. Much of the library's audio collection is recorded on wax cylinders and shellac and lacquer disks, many of which are now too fragile to play in their original format.

"I knew sound was stored mechanically and realized images could be used to recreate the sound," says Haber. Since he already had access to a machine that made high-resolution digital scans, he realized that an optical scanning system could solve the library's problem.

Haber used a beam of light to illuminate a record's surface. Flat bottoms of the grooves and the spaces between tracks appeared white, while the sloped sides of the grooves, scratches, and dirt looked black. The image was then computer analyzed. The program found the edges of each groove by focusing on areas of high-contrast and corrected areas where scratches, breaks, or wear made the groove wider or narrower than normal.

On antique monaural disks, sound is recorded in horizontal wiggles of the record groove. On cylinders, sound is recorded in the vertical plane--the depth of the groove. "With disks, we used a camera to image the disks in two dimensions. Once we understood how cylinders were recorded, we needed to measure the third dimension." Haber says.

With the library's urging, Haber produced a dedicated disk scanner, IRENE, that was installed at the library last summer. The current 3-D scanning process takes 20 hours to record one minute of sound. But a new version of the confocal scanner, developed for the dental industry, should reduce this time to about 10 minutes. Sound reproduction from the scanner can be heard on-line at Irene.lbl.gov.