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Imagine a Compact Disc scaled up to about the size of AFL park in Melbourne. It would be over a metre thick, but the tracks would still be only 2 mm apart and the bump height about the thickness of paper!
The polycarbonate base of the disc is 1.2 mm thick and is either stamped with a "son" of the master recording, or injection molded. The disc is then coated with an extremely thin layer of aluminium, by vapourisation in a vacuum chamber. Finally, a protective laquer coating 30 um (micro metre or millionth of a metre) thick is applied, the centre hole precisely punched, and the label printed. The disc is read through the transparent base material and the information layer appears as a spiral track of bumps on the mirrored aluminium surface.
The disc has a diameter of 120 mm with a 15 mm centre hole. The track extends between the 46 mm and 117 mm radii. The track pitch is constant at 1.6 um, making 22188 tracks across the radius of the disc, for a total length of 5.7 km. About 60 CD tracks will fit in the width of a typical vinyl microgroove. The program starts at the inside of the disc and extends outwards. In fact, if you look carefully at a disc you should be able to see how much of the 78 minute audio recording time is used. * The track is read at a Constant Linear Velocity of about 1.2 m/s, meaning the disc rotational speed varies from about 500 rpm, down to about 200 rpm when reading near the rim of the disc.
* [Herbert von Karajan demanded this figure, so that Beethoven's Ninth could fit onto one CD, I have heard... Peter Allen, Editor, MAC News]
The bumps have a width of 0.5 um and a height of 0.13 um, which is a one quarter the wavelength of the laser light source. The spot beam is about 1.2 um across at the aluminium coating, and the partial reflection from the bumps and the mirror-like surface either side of them results in destructive interference of the coherent laser beam. The intensity of the reflected beam can therefore be used to sense the bumps.
The length of the bumps vary from about 0.8 to 3.1 um, and are a property of the modulation system used to record the signal. I will discuss this in the next installment.
Originally published in MAC Audio News No. 158, January/February 1988, pp 23-24.