Digital Signals 171 mechanical movement of a stylus or any pickup mechanism, and no contact with the disc itself. Chapter 12 is devoted to the CD system and later developments in digital processing. n Note DVD uses the same principles as CD, but crams more information on to the disc by using a shorter wavelength of laser light, two layers of recorded information, and in some cases both sides of the disc. We will look at the Blu-ray development of DVD later. n As with magnetic systems, there is no problem of linearity, because it is only the number of pits or bumps rather than their shape and size that counts. Noise exists only in the form of a miscount of the pits/bumps or as confusion over the least significant bit of a number, and as we shall see there are methods that can reduce this to a negligible amount. Copying of a CD is not quite so easy as the molding process for LPs, but it costs much less than copying a tape, because a tape copy requires winding all of the tape past a recording head, which takes much longer than a stamping action. CDs are therefore more profitable to manufacture than tapes. A CD copy is much less easily damaged than its LP counterpart, and even discs that look as if they had been used to patch a motorway will usually play with no noticeable effects on the