234 Chapter 12 corresponding stage in the production of black vinyl discs) and the recorded surface is coated with aluminum, using vacuum vaporization. Following this, the aluminum is protected by a transparent plastic which forms part of the optical path for the reader and which can support the disc label. The system is illustrated in outline in Figure 12.4. Summary The CDs are produced by using photoresist material on a flat glass plate. This is then recorded using a laser that will harden the photoresist so that after processing a master can be produced. This is used to produce plates which will stamp out plastic copies. The End Result All of this encoding and decoding is possible mainly because we can work with stored signals, and such intensive manipulation is tolerable only because the signals are in the form of digital code. Certainly any efforts to correct analog signals by such elaborate methods would not be welcome flat glass plate