To appreciate the importance of linear systems, consider that there is only one major strategy for analyzing systems that are nonlinear. That strategy is to make the nonlinear system resemble a linear system. There are three common ways of doing this:
First, ignore the nonlinearity. If the nonlinearity is small enough, the system can be approximated as linear. Errors resulting from the original assumption are tolerated as noise or simply ignored.
Second, keep the signals very small. Many nonlinear systems appear linear if the signals have a very small amplitude. For instance, transistors are very nonlinear over their full range of operation, but provide accurate linear amplification when the signals are kept under a few millivolts. Operational amplifiers take this idea to the extreme. By using very high open-loop gain together with negative feedback, the input signal to the op amp (i.e., the difference between the inverting and noninverting inputs) is kept to only a few microvolts. This minuscule input signal results in excellent linearity from an otherwise ghastly nonlinear circuit.
Third, apply a linearizing transform. For example, consider two signals being multiplied to make a third: a[n] = b[n] × c[n]. Taking the logarithm of the signals changes the nonlinear process of multiplication into the linear process of addition: log(a[n]) = log(b[n]) + log(c[n]). The fancy name for this approach is homomorphic signal processing. For example, a visual image can be modeled as the reflectivity of the scene (a two-dimensional signal) being multiplied by the ambient illumination (another two-dimensional signal). Homomorphic techniques enable the illumination signal to be made more uniform, thereby improving the image.
In the next chapters we examine the two main techniques of signal processing: convolution and Fourier analysis. Both are based on the strategy presented in this chapter: (1) decompose signals into simple additive components, (2) process the components in some useful manner, and (3) synthesize the components into a final result. This is DSP.