Every form of communication, every audio file, every radar reflection, every video stream, every leaked power trace from a chip running AES is a "signal." The math that lets us slice them, filter them, and reconstruct them is "systems theory." This chapter is the language we will use for the rest of the curriculum: DSP, communications, control, even hardware-security side channels.
This is the most mathematically dense chapter so far, and honestly the densest in the curriculum. But none of the math is hard if you take it slowly. We will build up patiently from "what is a signal?" through Fourier (the language of frequency), sampling (the bridge between analog and digital), LTI systems (how circuits transform signals), Laplace (the language of transients), and finally the Z-transform (the language of digital filters).
The chapter is long because the ideas recur everywhere. Fourier shows up when we modulate a radio carrier in Chapter 7, when we filter a power trace in Chapter 24, and when we compress audio with MP3. Sampling shows up at the front of every ADC. Convolution shows up in every digital filter. Take time on each idea; they pay back many times over.
By the end you should be able to look at any signal (an audio waveform, a chip's power trace, a radar return) and reach for the right tool to analyze it.