Methods for signal production from mechanical sound carriers, measured via non-contact, full surface mapping

Abstract

Archivists are often faced with a dilemma when transferring audio from early mechanical sound carriers to digital formats. Traditional contacting stylus methods are not always appropriate for recordings such as wax cylinders or coarse groove discs. The recordings in question may be broken, incompatible with replay systems, or too historically significant to undergo stylus forces, due to the risk of causing irreversible damage.
A non-contact surface metrology technique has been developed which enables the full topology of grooved recordings to be stored as a digital representation of surface heights. From this data, it is possible to reproduce the audio signal encoded in the grooves without any risk of damaging the artefact. This method has great potential for archivists aiming to digitise content from recordings that may otherwise be 'unplayable' with a stylus.
This thesis presents the first detailed study into signal reproduction methods from the non-contact full surface mapping method, developed for cylinder and disc media. The methods developed rely on parameter estimation theory, provided by a Fourier analysis of the measured groove profiles. Savitzky-Golay, polynomial smoothing Alters are also used throughout, and are shown to be well suited to enhancing the medial valley structure of the record grooves.
Two methods of stylus trajectory estimation for cylinders have been developed, to enable tracking of specimens in both good and poor surface condition. Trajectory estimation for cylinders in good condition relies on a global estimation of the groove's shift. For damaged recordings, (or for those where the track is poorly defined), a method has been developed based on the tracking of local groove minima.
A number of different methods for estimating the depth signal h-om a discrete groove cross-section are evaluated. In order to quantify signal reproduction, a test cylinder encoded with sinusoids was produced allowing for signal quality metrics (signal-to-noise ratio and total harmonic distortion) to be calculated and compared with conventional stylus replay.
The sampling and resolution of the measurement system are considered with respect to the requirements for digital archiving of cylinder recordings. Methods are also described for overcoming distortions introduced by the measurement process. Preliminary results for reproduction from discs and tinfoil measurements are also presented. Recommendations for future work into non-contact methods of sound reproductions are given.

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Identifiers

ORCID for John W. Mcbride: orcid.org/0000-0002-3024-0326

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