2.2 What is digital?

To understand what it means for something to be digital, it is easiest first to explain its complement analog. The world around us is filled with analog signals: the temperature of the air around changes continuously,sound is the undulating change in pressure of the air at our ears, and the ocean moves up and down with the curve of waves. An analog signal has the advantage of being able to represent every value possible,however, the disadvantage of this is that any small error will alter the signal. Over time, many small errors can become a large error.When we measure the temperature outside as about 75 degrees, the analog measurement might be 75.433 degrees. We round off to 75 degrees because for many purposes, we don’t care about the extra .433 difference.

A digital signal rounds off all values to a certain precision or a certain number of digits. Thus a digital thermometer might be able to indicate that the temperature was 75.4 or 75.5 degrees but not 75.433 degrees.The advantage of a digital signal is that, because it automatically rounds off, it is much more resistant to errors. A digital signal can ignore the many small errors which, over time, would become a large error in an analog signal.Errors that appear in a signal are called noise. As long as the volume of the noise remains small relative to the difference between two levels of a digital signal,the noise will not affect the digital signal at all. Any amount of noise affects an analog signal.

In computers, the digital signal is either on or off. If a signal’s voltage level is close to 0V then it’s interpreted as off or 0.If the signal is close to the operating voltage of the device, say 3.3V, then the signal is interpreted as on or 1. Voltages in between are rounded off to on or off, but most devices are designed to keep the signals at on of the two extremes.While the computer operates as if the signal were either a 1 or a 0, the underlying voltage is still an analog value like .121V or 3.1.V