Legend claims in 1981 Bill Gates said, "640K ought to be enough for anybody." Truth is he never said it and more storage is always better.
In this chapter of the series "Data transmission and storage," consultant John Watkinson introduces key design concepts used to both detect binary signals and conserve space in data storage systems.
In the previous article (Data Recording and Transmission: Channel Coding) the concept of channel bits was introduced. Channel bits completely control the recording process. In most formats, a channel bit zero causes nothing to happen, whereas a channel bit one results in a transition on the medium. That transition could be a change in voltage on a cable, a reversal of magnetic direction on a disc or tape, a change from a pit to a land (or vice versa) on an optical disc or some change in electromagnetic radiation in an optical fibre or radio link.
One of the key performance factors in data storage is the linear density; the number of bits per unit track length. At a given relative speed between the medium and the pickup, all media offer a certain amount of bandwidth. The art of channel coding includes finding ways to increase the data rate as much as possible whilst not exceeding the available bandwidth.
Figure 1 shows some channel bit sequences and the record waveform that results. When arun of channel bit ones occurs, the highest frequency, a fundamental of half the channel bit rate, is generated. However, if channel bits alternate between one and zero, the frequency is halved. To be more precise, if adjacent channel bits can never both be ones, a fundamental frequency of one quarter the channel bit rate cannot be exceeded.
Figure 1. At a) the channel bits are unconstrained and if they happen to be all one, as here, the highest frequency is created in the channel. At b) if there must always be at least one zero between ones, the channel frequency is halved. At c) with at least two zeros between ones, the channel frequency is divided by three. Click to enlarge.