The abacus could be thought of as the first computing device. It was developed in China more than two millennia ago and is used in some remote areas of the world to this day. Mechanical computing devices that worked on an analog basis eventually followed. The computing devices we use today are digital. The difference between analog and digital is shown in Figure 2-1. Analog signal voltage could slowly change with time, as displayed on the vertical y axis, with reference to time on the horizontal x axis, and analog signals could have a curved or a ramped wave shape, whereas digital signals rapidly jump between two discrete voltage levels. One of the digital levels is considered to be a low (0) and the other a high (1).