1.2 Non-periodic signals
Communications is all about transferring information. A periodic signal, though, has limited possibilities for conveying information because of its predictability. After receiving a few cycles and establishing what the pattern is, we know the cycles that follow will be exactly the same. The signal may convey important information when it begins, as in the case of a fire alarm, where there is a call to immediate action as soon as the sound is heard. But a signal that never varies in amplitude, frequency, phase or any other aspect conveys little if any further information to a recipient. So in practical communications, exactly periodic signals are the exception. Signals that carry real information, such as speech, music or video, do not repeat endlessly.
Non-periodic signals (also known as aperiodic signals), unlike periodic signals, do not have just one particular frequency. Instead, they are spread out over a continuous range of frequencies. For example, a speech signal ranges from around 100 Hz to a few thousand Hz (for telephone-quality speech, a range of 300 Hz to 3400 Hz is often assumed).