In The Groove
The history of recorded music is a long and storied one that is worth preserving for future generations. Unfortunately, the earliest examples of the recording arts are difficult if not impossible to hear anymore. Many wax cylinders and shellac discs are crumbling in archives, unable to be played because any physical contact with a stylus would cause irreparable damage. Even those that can be played often suffer from lots of surface noise and scratches that cause clicks and pops. And many are broken, making even the most careful stylus-based playback impossible.
Recently, an ingenious solution to these problems appeared from an unexpected field of studyparticle physics. Carl Haber and Vitaliy Fadeyev, physicists at the Lawrence Berkeley National Laboratory, realized that the optical scanning techniques they use in precision detector arrays and the statistical methods they use to analyze the tracks of particles emerging from collision experiments could be adapted to extract the audio information from old discs and cylinders.
From the earliest recordings of Thomas Edison to the monaural 78rpm discs of the 1950s, audio signals were mechanically encoded directly onto the medium. On discs, a spiral groove was cut into the surface, and audio information was represented by lateral displacements in the groove from its nominal path. In other words, the groove wiggles from side to side of what would otherwise be a simple spiral path. On a cylinder, the groove forms a helix that wraps around it from one end to the other, and the displacements that represent audio information are vertical, forming small hills and valleys along the groove rather than lateral wiggles.
To play the audio, a stylus rides in the groove while the disc or cylinder turns at a constant rate. As the stylus follows the wiggles, its velocity changes in a pattern that is analogous to the original audio waveform, which is reconstructed by the playback device.
After hearing about the problems of preserving mechanical audio recordings, Haber and Fadeyev realized they could optically scan the surface of a disc or cylinder and process the resulting data much as they do to reconstruct the paths of particles created in beam collisions. This would avoid all physical contact with the medium and allow them to remove the effects of scratches, dust, wear, and surface noise using sophisticated data processing.
Their initial experiments were conducted on 78rpm shellac discs from the 1950s. Using a computer-controlled, high-resolution optical scanner, they imaged small sections of the disc surface as shown in the example above. (The depicted section measures 1.39 x 1.07mm, and the groove is 160 microns wide.) Each image typically generated about 1MB of data, which means an entire 10-inch 78rpm disc would occupy 100 to 1000GB before processing. The computer stitched together the images and analyzed the groove wiggles, deriving the stylus velocity while removing spurious data and reconstructing the audio signal as 44.1kHz/16-bit WAV files.
These experiments were a big success as a proof of concept, but the equipment was not optimized for the task, so the process was quite slow, taking 40 minutes to yield one second of audio. However, Haber and Fadeyev believe that a dedicated system could dramatically reduce the time frame to somewhere around five to 15 minutes for a 10-inch 78 rpm disc.
In addition, their 2D imaging cannot be used to scan cylinders; for that, 3D scanning is required to measure the hills and valleys of the groove’s displacement. Haber and Fadeyev have already started experimenting with such a system, and the initial results are most promising. Clearly, this work could help rescue our musical heritage from oblivion, allowing future generations to learn from the past.