I have several times taught a class in a subject I did not already know well. This is always exciting, and is even sometimes fun. It depends on how well you cope with discovering all your notes for the coming week are gibberish to yourself and will need a complete rewriting. One of the courses I taught in those conditions was on digital signal processing. This was a delight, and I’m sorry to not have more excuses to write about it. In the Summer 2015 A-to-Z I wrote about the z-transform, something we get to know really well in signal processing. The z-transform is also related to the Fourier transform, which is related to Fourier series, which do a lot to turn differential equations into polynomials. (And I am surprised I don’t yet have an essay about the Fourier transform specifically. Maybe sometime later.) The z-transform is a good place to finish off the spotlights shone on these older A-to-Z essays.