Understanding Digital Signal Processing Chapter 1 - 2

Chapter 1

Analog usually means "continuous". The word probably originates from analog computers, where each signal was "analog" to some real-world value.

Discrete signals are usually discrete in both time and value.

Nonlinear Systems lead to intermodulation. (Always?)

LTI systems are commutative. (Always!)

Important LTI system: The time delay.

If the linear system's parameters do not change over time, it does not mean it's LTI. What is important is that it is shift-invariant.

Magnitude = Abs(Amplitude)

Chapter 2

Aliasing: The spectrum of a discrete signal repeats in intervals of the sampling frequency $f_{\mathrm{s}}/2$, from "DC to Daylight".

Always use a low-pass filter with $f_{\mathrm{corner}} < f_{\mathrm{s}}/2$ to prevent aliasing.

Bandpass-sampling = IF-sampling = under-sampling = deliberate abuse of aliasing to look at some signal with $f_{\mathrm{signal}} > f_{\mathrm{s}}/2$. This degrades SNR, as all noise is folded into the $[-f_{\mathrm{s}}/2; f_{\mathrm{s}}/2]$ band.

Apparently, negative frequencies are all the rage. Does he not want to use negative amplitudes? Explanation is promised in chapters 3 and 8.