A nice series of videos by Forgotten Computer explaining computer sound, the recently published episode 5 covers the Speccy's 1 bit speaker and how it can create various types of polyphony.
https://www.youtube.com/watch?v=N5ACJd2LvbY
The latest ZXDB update will be applied this evening at 1800 BST. The update takes around 30 minutes, and the site goes down whilst this happens.