The Sound Archive Project

Sound Recovery

Recovery of Sound From Cylinders

Following non-contact surface measurement of a cylinder recording, sound reproduction is broadly carried out as follows:

Pre-Processing and Feature Detection : Surfaces are usually measured in segments, which require stitching together. The groove spacing and position of local groove maxima (ridges) and minima (valleys) are used as features to aid stylus trajectory estimation.

Segment Scans

Linescan Features

Stylus Trajectory Estimation : The path cut by the recording stylus follows a helix. This path can be determined by tracking the positions of groove minima, or by estimating the relative shift between consecutive linescans. A coherent trajectory estimate ensures that no ‘skipping’ occurs between grooves occurs.

Ridges and Valleys

Trajectory Overlaid onto Surface

Audio Signal Estimation : The audio signal is then recovered by obtaining an estimate of the groove depth along the stylus trajectory. Due to the discrete (digital) nature of the surface, a number of different estimates can obtained by selecting/averaging samples across the groove cross-section. The optimal signal estimate is very dependent on the condition of the recording. Examples of recovered sound can be found here.

Depth estimation from Groove Cross-sectionRecovered Sound Time History



Signal Quality Testing

In order to obtain a quantitative assessment of the recovered sound, a signal quality analysis was carried out using a specially made test cylinder. The cylinder was recorded with test tones, (sine-waves), of known frequency. Recovered audio signals, obtained by different groove-depth estimation methods were then compared in terms of signal-to-noise ratio (SNR), total harmonic distortion (THD), and frequency modulation (FM).


Comparisons with Stylus Reproduction

To compare the optical method with traditional reproduction methods, the same test cylinder was transferred by stylus at The British Library Sound Archive. The stylus reproduction was less affected by impulsive and low frequency noise, but suffered from greater harmonic distortion and frequency modulation. Examples of the audio can be found here. A paper describing these findings can be found here.

Recovered sinusoid signals from the test cylinder recording.