In Hard Sync, the sync jack acts as an input only. In Soft Sync, it acts as both an input and output simultaneously. The harmonic richness that results from Hard Sync, in which the pitches of the oscillators are locked together, is part of what gives an analog synth its characteristic sound. On the other hand, Soft Sync, a rare innovation that first appeared on the E-mu modular synth, latches two oscillators together when they are close in frequency but allows for drift when the frequencies are farther apart.