In this book, we will primarily be dealing with the processing of real-world signalsusing both analog and digital techniques. Before starting, however, let's look at afew key concepts and definitions required to lay the groundwork for things to come.Webster's New Collegiate Dictionary defines a signal as "A detectable (ormeasurable) physical quantity or impulse (as voltage, current, or magnetic fieldstrength) by which messages or information can be transmitted." Key to thisdefinition are the words: detectable, physical quantity, and information..