there's more examples for sure, but what are your thoughts on track number 16 off 'a few old tunes'? seriously. this is a pretty obvious example, but there's a lot of tracks that 'meet the criteria'

to have been done on a C64. there's also a lot which utilize only two or three voices, which is a limitation of the C64 having only three oscillators to work with.
another thing is something i noticed in the Remix Magazine interview. it says:
"...Eoin and Sandison learned to play various musical instruments when they were children. Sandison formed a band and began making experimental tracks with old synths, drums and tape decks in 1980, when he was only nine years old. Influenced by television documentaries and soundtracks, particularly those by the National Film Board of Canada, Sandison named his band Boards of Canada. When Eoin became the band's bassist in 1986, Boards of Canada were mixing real instruments with computer effects and found sounds from radio and television broadcasts. Sandison dabbled with Super 8 home-movie visuals for the band early on, and by the late '80s, BoC were making full-length films accompanied by their own soundtracks."
first, how many 'old synths' do you think were available to a nine year old in 1980? there weren't even that many 'new synths' at the time, and this stuff wasn't cheap by any means. six years later they're working with computers. (please correct me if i'm way off here)
i would think that around this time the C64 was probably the most advanced and accessible computers for music at the time. not sure what other options there were at the time, but the C64 seems the most likely. an Atari 1040ST was about $1000 at the time, which could be pretty steep for a fifteen year old and his little brother. they may have been big into the C64 music scene around this time, seems it was even more popular in europe than over here.
again, could be a long shot but who knows... and really, what's more 'BoC' than this?
