Analog and Digital Conversion/Analog vs Digital
Signals
What is a "signal", exactly? A signal in the sense that we will be considering in this book is changing value of electric voltage or current through a transmission medium. There are two general types of signals: periodic and aperiodic. Periodic signals repeat themselves after a certain period of time—after they have cycled through one period, following periods don't contain any new information. Aperiodic signals, on the other hand, don't repeat themselves, and therefore can contain information. Signals also can be analog or digital signals, and we will discuss them both below.
Analog
Analog signals equate levels of electric voltage or current to amounts of information by applying some rule. Consider for instance, an analog clock, where the passage of time is displayed as the motion of the clock hands. In electric signals, a certain amount of voltage corresponds directly to a measured physical phenomenon. For instance, on an accelerometer, the amount of acceleration, measured in g's will correspond directly to volts. So at one g, we have 1 volt output, at 2 g's we have 2 volts output, etc. Analog signals have an advantage that they can represent any fractional quantity, by outputting an equivalent fractional quantity of voltage or current. This fractional output, or put in a different manner, being continuous in value, is the distinguishing characteristic of an analog signal.
Uses of Analog
Analog signals have a number of uses. AM and FM radio, for instance, are signals that are transmitted in analog. Telephones (at least simple, older telephones) use analog signals to transmit voice data to the phone central office. Many electrical components, such as sensors, will output analog data, because of the accuracy that can be obtained from analog signals.
Digital
Digital signals are different from analog signals in that there are only a countable number of different levels. A digital stopwatch, as found on a digital wristwatch, for example, has a finite resolution.
Binary mathematics will be useful in working with digital signals. A binary signal assumes a value of "high", or 1; and "low", or 0; this is a bit, or BInary digiT. Multiple bits may be combined to form larger binary numbers.
Uses of Digital
Because bits can only be a 0 or a 1, digital transmissions don't have the same amount of accuracy as analog signals. Also, digital systems need to have complicated digital circuitry to read and understand the signals, that can cost more money than analog hardware does. However, the benefit is that digital signals can be manipulated, created, and read by computers and computer hardware.
Examples
One of the best examples of digital signals are the control signals and data that are in use on your computer. Computers are almost completely digital, except for the sound card (which produces analog sound signals), and maybe a few other peripherals. Cellphones now are mostly digital, and the internet is a digital network.
Continuous Time and Discrete Time
Another concept that is important in analog and digital conversion is the notion of continuous time and discrete time signals. A continuous time signal may change in value at any time, while a discrete time signal may only change at specified times.
Continuous time and discrete time signals are separate concepts from analog and digital signals. An analog signal fed into an ideal comparator would turn into a digital signal (high or low), but would be continuous in time. An analog signal fed into an ideal sample-and-hold clocked at a fixed rate would be an analog signal, but discrete in time (the sampling rate of the clock).