exactly - i've heard tons of sidtunes and never thought about this before today. i'd say just about all those are purely sequenced which is the norm, but the Cynthcart treats the c64 as just a synthesizer that's playable from the keys. all you can do with it is play notes like a usual synth, so they couldn't (or at least didn't) do anything too complicated for the demos. i'm not sure i've ever heard it used like that before, although it's easy enough to do in lots of ways. most everyone just makes happy chiptunes with it.
it's definitely not just the detuning or the graininess to it. i've been listening to c64 tunes, sounds clips, and stuff over the past few days but when i heard this i immediately thought 'holy shit - that's it!' that sound is just too distinct, and it's easy to get detuned sounds like that on the c64. i've got one hooked up right here, but haven't tried to match their example.
i'll try to come up with a track that uses it, i want to say the opening of something off TCH actually. [oops, check out 'Skyliner'... almost no doubt. could be one of a number of SID emulations though.]
[edit: just noticed the extreme detuning in the second example. that's not quite what i was pointing out, but the sound of the synth itself. the first example is pretty much a dead giveaway though. listen to that fizz!

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