Optical Audio Reconstruction for Stereo Phonograph Records Using White Light Interferometry
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10th International Society for Music Information Retrieval Conference (ISMIR 2009) OPTICAL AUDIO RECONSTRUCTION FOR STEREO PHONOGRAPH RECORDS USING WHITE LIGHT INTERFEROMETRY Beinan Li Jordan B. L. Smith Ichiro Fujinaga Music Technology Area Music Technology Area Music Technology Area Schulich School of Music Schulich School of Music Schulich School of Music McGill University McGill University McGill University beinan.li@ jordan.smith2@ [email protected] mail.mcgill.ca mail.mcgill.ca ABSTRACT 2. EXISTING OAR APPROACHES Our work focuses on optically reconstructing the stereo In this section, four previous OAR approaches are audio signal of a 33 rpm long-playing (LP) record using a described. Although they operate on recordings of white-light interferometry-based approach. Previously, a different formats, most OAR frameworks follow the same theoretical framework was presented, alongside the high-level three-step procedure for reconstructing an primitive reconstruction result from a few cycles of a audio recording: first, the grooves are scanned; second, stereo sinusoidal test signal. To reconstruct an audible the groove undulations are isolated and extracted; third, duration of a longer stereo signal requires tackling new these undulations are converted into audio. Approaches problems, such as disc warping, image alignment, and vary significantly in terms of the hardware used, some eliminating the effects of noise and broken grooves. This using a general-purpose commercial product such as a paper proposes solutions to these problems, and presents confocal microscope, others using a custom installation. the complete workflow of our Optical Audio Recon- The hardware, in turn, affects how the grooves are struction (OAR) system. scanned and thus how groove undulations must be extracted. By contrast, the audio conversion step (which 1. INTRODUCTION may include post-processing, such as equalization) depends solely on the record production procedures that OAR has proven to be an effective contactless approach were used for the particular item being scanned. This step to digitizing monophonic phonograph records [1] [2] [3] almost always includes filtering the signal to undo the [4]. Furthermore, it is an available solution for restoring RIAA equalization used in production and obtain the broken records. Li et al. previously presented a audio. theoretical framework for optically reconstructing audio The systems developed by Iwai et al. and Nakamura with a white-light interferometry (WLI) microscope and et al. use a ray-tracing method to obtain the groove image processing [5]. A few cycles of stereo sinusoidal contour of a phonograph record [6] [7] [8]. The groove is signal, extracted from a small number of images, illuminated with a laser beam, and the groove undulations illustrated that their approach is capable of extracting are measured by detecting the angle at which the beam is stereo signals from LPs. To reconstruct a few seconds of reflected. In this way the laser functions as a simulated audio, however, the scanning region must be scaled up to stylus—a replacement for the mechanical stylus—and a much larger disc area, resulting in thousands of images. can output an analog audio signal directly. A sophisticated image acquisition and post-capture Unfortunately, since such systems must trace out the processing workflow is thus desired to tackle the grooves, they are unable to handle broken records. In challenges that emerge from large-scale scanning: e.g., addition, two types of errors limit this approach: the disc surface warping, image alignment errors, groove errors caused by the finite laser beam width, which leads damages, and unwrapping the grooves into a one- to echoes and high- and low-frequency noise in the dimensional audio signal. extracted audio signals, and the tracking errors that may In Section 2, we review previous OAR systems. Our occur when the beam misses the groove entirely. system to acquire record groove images is introduced in Fadeyev and Haber built an OAR system for 78-rpm Section 3, followed in Section 4 by our image processing records based on confocal laser scanning microscopy [1]. procedures for extracting audio from the scanned images. With the help of a low-mass probe, they built another one The reconstructed result is illustrated and discussed in for wax cylinders [2]. Their system is capable of scanning Section 5. the record in 3D with a vertical accuracy of around 4.0 microns. However, in their work on 78-rpm records only 2D imaging is emphasized, at a resolution of 0.26 x 0.29 Permission to make digital or hard copies of all or part of this work for microns per pixel. It takes their system 50 minutes to personal or classroom use is granted without fee provided that copies scan about 1 second of recorded audio, corresponding to are not made or distributed for profit or commercial advantage and that 0.5–5 GB of image data. The groove bottom is obtained copies bear this notice and the full citation on the first page. using 2D edge detection on the pixel illumination data, © 2009 International Society for Music Information Retrieval 627 Poster Session 4 and the groove undulation is defined as the radial either the top or bottom of the groove. Ray-tracing is deviation of the groove bottom with respect to an used to create a 3D image of the entire record groove unmodulated trajectory about the centre of the record. surfaces, including the sidewalls. The stylus movement Lengths of the groove are skipped when dust and across the grooves is represented by the optical flow debris occlude the image and no edges are detectable, but derived from groove image intensity derivatives. The no solution to data restoration is provided; such skipping sidewall orientations are obtained by using dense depth may thus cause a loss of data. Fadeyev and Haber maps and projecting complex surface onto the groove compared their optically reconstructed audio sample, a cross-section plane. The microscope in use has a lateral turntable-digitized audio sample, and a remastered CD resolution of 1µm per pixel. Tian’s optical flow approach sample of the same recording. The quality of the requires 1390 x 36 images of the FOV 640 x 480 pixels reconstructed sample was judged to be better than the to represent a two-second audio signal, although on turntable-reconstructed version, but poorer than the CD which groove revolution the reconstruction is performed version: while the OAR system produced fewer clips and is not reported. It takes three days for four workstations pops than the turntable, and had a lower continuous noise to generate the image representation of a three-second level, it also contained background hissing and low- audio. The image acquisition time is unclear in the frequency modulation absent from the CD version. literature. The equivalent audio sampling rate in their Stotzer et al. created a system that performs a multi- experiment is about 2404.71 Hz. step optical reconstruction for 78-rpm records [3]. First, a Although laser turntables can serve as a solution to 2D analog camera is used to rapidly photograph the optically retrieve audio from phonograph records, they records and the images are preserved on film. This film is require emulating the exact groove-following behavior of then scanned and digitized into 2D digital images, which a turntable. We would like to find a general image- are further analyzed to extract the audio. During the acquisition-based preservation solution without scanning, the film is placed on a rotating tray with an mimicking turntable behavior to derive digital audio overhead stationary camera that carefully captures the directly from images. We also wish to obtain 3D groove images passing through its field of view (FOV). information: although Fadeyev and Haber claimed their The rotation of the tray simulates the rotation of the system can be adapted to a 3D groove profile, they did record during playback, and effectively unwraps the not implement it, while the system of Stotzer el al. does groove segments to become uniformly oriented in the not retrieve 3D information. Tian’s system is 3D-based, resulting images. Similar to Fadeyev and Haver’s but his experiments are not performed on stereo LPs, the strategy, edge detection is then used on these images to target considered here. Thus, in contrast to these works, extract the groove undulations, which are simply our research focuses on optically reconstructing a digital described by the edge that separates the groove valley stereo audio signal from LPs by extracting the lateral and from the space between grooves. vertical groove undulations from 3D groove information. In this system, the imaging resolution is compro- mised by shading blur, motion blur, and sampling blur 3. WLI-BASED IMAGE ACQUISITION introduced by the illumination and the rotating scanner. The blur is estimated to be roughly 24.6 microns along WLI is a powerful scanning technique based on physical the direction of rotation. The system also suffers from optics, as opposed to the geometrical-optics-based various acquisition artifacts, such as the very low- approaches that include confocal microscopy and ray- frequency noise caused by the off-axis placement of the tracing. In WLI, a broadband light source simulates an film. ensemble of narrow-band interferometers to make high- In an effort to restore damaged grooves, smoothing precision measurements. We used a Wyko NT8000 WLI and corrupted-pixel-map-based enhancement are per- microscope equipped with both Michelson and Mirau formed. The robustness of the damage detection is interferometers. Adjusting the vertical focus of these nevertheless questionable due to the simplifying allows one to perform vertical scanning interferometry assumption that damages such as scratches are solely with a vertical resolution of better than 1 nm [9]. (The perpendicular to the grooves. The blurring described amplitudes of the groove depth are typically on the order above also makes it difficult to reliably detect scratches of 25–100 µm [3].) In the current experiment, only the in the grooves.