I have been reading a textbook on the "fundamentals" of signal processing but the author of the textbook has not given any explanation for the triangular graph located at the bottom of the figure.
I understand the general concept of aliasing, but I do not understand what exactly is being shown in the graph featuring the "spectral band of interest". Why do we need it and why is it triangular in the first place? I see that the horizontal lines connect two points, of which one belongs to the graph of the correct frequency and one belongs to the graph of the "alias" frequency. Beyond that I am lost, even as to why there is only one line linking each correct-frequency+alias pair of points but not more.
I would be much obliged for any help in understanding this graph.