This diagram explains how audio is encoded in IRENE images. The images produced from optical scan images are depth images, where gray-scale value in the image correspond to height of the surface, white is highest and black is deepest. This image shows that if one follows a groove around the cylinder the bottom of the groove undulates up and down, in and out of the cylinder surface. This undulation was engraved into the cylinder surface by the recording stylus and induces the motion in a playback stylus that would produce audio. The speed of the stylus directly relates to sound and so to produce audio all that is necessary is to find the data along the groove bottom, which is done using a processing algorithm, and take the derivative along the bottom.
A close up of the probe (the black cylinder at the right of the image) and a cylinder loaded on the machine (the brown cylinder on the left of the image). The probe shines a white light that is 1.8 mm wide (visible as the white line on the cylinder) and analyzes the intensity and color of reflected light to find the height of 180 points on a surface.
This method is meant to illustrate how data collection works for a cylinder. During acquisition, individual measurements of the height at 180 points along a line are taken in quick succession, at slightly different positions, as the cylinder rotates, and then stitched together in analysis software.
After an image has been produced a software algorithm finds the bottoms of the grooves and traces them in blue. The bottom of the groove is where playback stylus would have rested, and dictates the path the stylus would have taken around the cylinder. Therefor the data along the groove bottom is most important in producing audio.