Non-Contact Surface Metrology for Preservation and Sound Recovery from Mechanical Sound Recordings

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P. J. Boltryk, M. Hill, J. W. McBride, A. Nascè
School of Engineering Sciences, University of Southampton, Southampton, UK

N. Bewley, W. Prentice
British Library Sound Archive, London, UK

Despite careful storage, early mechanical recordings on cylinders and flat disc formats have been identified as at risk from deterioration, caused mainly by material degradation and biological attack from mould growth. There is therefore an urgency to transfer the content of culturally-important artefacts to digital format to preserve the recordings’ content for archival posterity. However, some recordings are too precious to risk playback using conventional stylus methods because the very act of using a mechanical stylus playback system may in some circumstances contribute to further damage to the integrity of the sound contained in the recording’s groove, caused by wear. Other artefacts, such as 78s exhibiting delamination of the shellac from the metallic substrate, may be too damaged for a stylus to be a practical method for transfer.

In recent years there has been a significant quantity of research aimed at developing optical measurement systems for mechanical recordings for non-contact sound recovery. 2-D imaging systems using high-resolution photography have been developed for flat disc recordings where the sound modulations are encoded as lateral undulations of the sound-carrying groove. However, in cylinder recordings and some 78s the modulations are in a vertical plane relative to the groove, in so-called ‘hill and dale’ modulations. To measure these features requires 3-D surface profiling using optical sensors that measure the surface topology by determining the displacement distance between the surface and the sensor.

Systems have been independently developed by the Ukrainian Institute for Information Recording Problems, Syracuse University (US) and Hokkaido University group in Japan for 3D measurement of the sound carrying groove. However, these methods require a tracking system to guide the optical sensor in the nominally helical path around the cylinder to follow the groove. This tracking must be robust at time of measurement, a task which is made difficult by damage and deformation of the artefact’s surface.

An alternative transfer strategy being developed through collaboration between the University of Southampton, the British Library Sound Archive, and TaiCaan Technologies Ltd, uses optical sensors to measure the recording’s surface in its entirety. A significant outcome from this approach is the full high precision digital record of the artefact’s surface form for preservation, which is available for future research. The post-measurement processing of the surface topology data makes use of image and signal processing to reconstruct the audio content of the recording. This aspect of the research is aimed at facilitating access to the audio content of culturally-important artefacts by current generations. In this paper we provide a detailed overview of the scanning process for cylinder recordings, the data processing techniques used to recover the audio from the data and describe the high sensor precision required for measuring the surface for successful audio extraction. We show examples of groove damage thought to originate from repeated stylus playback, and highlight the advantages offered by this scanning strategy for application to damaged or even broken recordings.

John McBride
Professor John McBride received a degree in Aeronautical Engineering from the University of Southampton in 1978. In 1986, he received a PhD for his work at Plymouth University with a thesis on Electrical Contact Phenomena. From 1985 to 1987 he lectured in the Mechanical Engineering Department at Plymouth University and from 1987 he has been a lecturer, senior lecturer and now Professor of Electro-Mechanical Engineering in the School of Engineering Sciences at the University of Southampton. He is chair of the Electro-Mechanical research group, and Head of Research in the School (2001-05). His main research interests include Electrical Contacts, Metrology and Instrumentation.