A-110-4 is being modulated with a sine wave from the wmd Spectrum vco.  the Spectrum is going through a vca before it goes into the A-110-4's LFM input which the vca is being modulated with an envelope from Maths triggered with every note.  In the first half of the track, both the A-110-4 and the Spectrum are getting the same v/oct sequence. The track starts with just a sine wave from the A-110-4 and then the LFM knob is turned up to about 8 to bring in through zero fm modulation from the sine wave from Spectrum. after every time the sequence plays through i push the frequency of the Spectrum sine wave lower.  I'm just turning the knob up a little each time to a random lower note. eventually when it's at the lowest point, in the middle of the recording, i pull the v/oct cv out of the Spectrum so only the A-110-4 is getting the sequence so the Spectrum is no longer tracking with the A-110-4.  at this point i start pushing the frequency of the Spectrum up after each time the sequence plays through. 

there is some reverb and delay in this recording.

Doepfer A-110-4 QZVCO Quadrature Thru Zero VCO "Bell Sounds" Demo

BLogic