craz@tonyblack.org says and then deftly demonstrates:
You can use the internal filter of the ATC to process external audio signals. In addition to this.. the envelopes of the ATC still work!!! This is awesome. Just have a listen to this demo I made with just a TR-909 and the (SEM i think) filter of an ATC. I switched off all sound from the ATC's internal oscillators for this (I made a separate patch '99' just for this purpose).
Enjoy!
CraZ
Editors Note: Not technically an ATC-X demo but I was unsettled about it getting "snowed under" on the already overstuffed ATC-1 audio page! Besides, it is of a recent vintage, AND most phat.