The VCF section is a 24 db/oct lowpass filter that removes high frequencies as its cutoff frequency setting is decreased from max, resulting in a dulling of sound. DCO-106 features a super-accurate emulation, with particular attention paid to the original Juno's non-linear self-resonating characteristics - phew, that's a mouthful, but all you gotta know is that DCO-106 sounds just like the real thing!
Freq- Sets the frequency where high-frequency attenuation begins, from 20 Hz up to 24,000 Hz. Though it seems odd to have a cutoff control that goes 4000 Hz above human hearing threshold, remember that all filters have a "slope" to their response, so that extra range guarantees you won't hear any filtering at maximum setting.
Res- Short for “resonance,” this emphasizes sound energy at and around the current cutoff frequency by adding feedback from the filter's output back to its input. At lower settings, this can be used to create mild resonances such as those heard in acoustic instruments. DCO-106’s implementation is fully self-oscillating - at extreme settings, it can be used as a sine wave generator, but be careful because high resonance settings can result in loud, screamy, dog-terrifying (and speaker blowing) occurrences.
Env Slider and Mod Invert switch- The Env slider sets the amount of envelope modulation applied from the envelope generator. With the invert switch in the up position, envelope mod affects the cutoff frequency positively. In the down position, envelope mod is inverted for “reverse” effects.
LFO- Adds cutoff frequency modulation from the onboard low-frequency oscillator section.
KYBD- This is short for “keyboard” and causes the cutoff frequency to increase as ascending notes are played on a keyboard. The idea behind this is, because actual note frequencies rise as higher pitches are played, the Kybd slider adds a rising CV to the cutoff frequency in order to maintain the brightness of notes as higher pitches are played.
VEL- Short for "velocity," this allows keyboard velocity to control the filter cutoff setting. Increasing the amount adds sensitivity; lightly played notes will reduce the filter cutoff setting.