Dust Detection by Colour Analysis in an Optical Method of Phonographic Discs Digitisation

A contactless optical playing device for 78 rpm and 33 rpm lateral modulation phonographic discs, using structured colour illumination, is presented. An area of the disc is illuminated by a beam of rays, with colour depending on the direction of incidence, that are reflected by the groove wall towards a camera. In contrast with standard methods that measure the velocity of the groove at a single location, access to the audio signal is directly obtained here from pictures through colour decoding, and the whole height of the groove wall is exploited for interpolating damaged signal. The pixel colour saturation level is related to the diffuse or specular reflection and allows for the detection of occluding dust, useful for automated interpolation.