 One function that did disappear from the OB-X voice architecture was cross modulation, or frequency modulation of the first VCO with the second VCO.When done with analogue circuits, it's a unique sound made famous by the Sequential Circuits Prophet 5 and its poly-mod section which is considerably more flexible and dynamic than the basic implementation offered in the OBX.In place of this feature on the OB-Xa front panel is Filter Envelope modulation for oscillator 2, which allows the pitch to be modulated by the envelope.