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Chapter 9
suited. The precise amount of magnetization is no longer important, only its direction. This, incidentally, makes it possible to design recording and replay heads rather differently so that a greater number of signals can be packed into a given length of track on the tape. Since the precise amount of magnetization is not important, linearity problems disappear. n
Note This does not mean that all problems disappear. Recording systems cannot cope well with a stream of identical digits, such as 11111111111 or 0000000000, and coding circuits are needed to make sure that all the numbers that are recorded contain both 1s and 0s, with no long sequences of just one digit. This complication can be taken care of using a specialized IC. n
Noise problems are also greatly reduced. Tape noise consists of signals that are, compared to the digital signals, far too small to register, so that they have no effect at all on the digitally recorded signals. This also makes digital tapes easier to copy, because there is no degradation of the signals caused by copying noise, as there always is when conventional analog recorded tapes are copied. Since linearity and noise are the two main problems of any tape (or other magnetic) recording system it is hardly surprising that recording studios have rushed to change over to digital tape mastering. The surprising thing is that it has been so late in arriving on the domestic scene, because the technology has been around for long enough, certainly as long as that of videotape recording. A few (mainly Betamax) video recorders provided for making good-quality audio recordings of up to eight hours on videotape, but this excellent facility was not taken up by many manufacturers, and died out when VHS started to dominate the video market in the UK. The advantages that apply to digital recording with tape apply even more forcefully to discs. The accepted standard (CD) method of placing a digital signal on to a flat plastic disc is to record each binary digit (bit) 1 as a tiny pit or bump on the otherwise flat surface of the disc, and interpret, on replay, a change of reflection of a laser beam as the digital 1. Once again, the exact size or shape of the pit/bump is unimportant as long as it can be read by the beam, and only the number of pits/bumps is used to carry signals. We shall see later that the process is by no means so simple as this would indicate, and the CD is a more complicated and elaborate system than the tape system (DAT) that briefly became available, though at prohibitive prices, in the UK. DAT is now just a dim memory for domestic recorders. The basic CD principles, however, are simple enough, and they make the system immune from the problems of the long-playing (LP) disc. There is no mechanical cutter, because the pits have been produced by a laser beam that has no mass to shift and is simply switched on and off by the digital signals. At the replay end of the process, another (lower power) laser beam will read the pattern of pits or bumps and once again this is a process that does not require any