Topic: Understanding Computer Sound. ZX Spectrum (youtube)
I've just found this. I'm watching it now:
Another episode of Understanding Computer Sound. We dig into the details of square waves produced by the Z80 CPU and Ferranti ULA chip, and find out fascinating things... we also show a few great demos by masters of the chiptune and demoscene. For many more details details, see the accompanying article: http://forgottencomputer.com/retro/sound/
https://www.youtube.com/watch?v=N5ACJd2LvbY
(sorry wrong topic, it should go to the Sinclair part, of course...)