I've done some further digging around in my boxes of old E&MM mags, and found out that the ultra minimal ZX81 polyphonic sequencer was a four part article series, and luckily enough I've got all of them. I posted up part 3 a few days back which covers the CV out interface & some polyphonic sequencer machine code, and its been pretty popular, so I am scanning the other 3 parts.
Here is part 1 which gives a good introduction to those unfamiliar with ZX81 and will help with understanding specifics of the other 3 parts (little machine code monitor, keyboard reading business & ZX81 hardware).
Note that at the end of part 3 there is an errata for this article series!! (nothing too heavy, but you best check it out first!). Part 2 is here.
This article is scanned from my copy of Electronics & Music Maker, Dec 1981. Article written by Peter Maydew.
disclaimer : i dont know who owns the copyright these days, at the time it was Maplin Publications ltd.