The ES1 sub-oscillator is used to make your sound richer. Its pitch matches the frequency of the main oscillator. You can choose from the following sub-oscillator waveform options:
A square wave that plays one or two octaves below the frequency of the primary oscillator
A pulse wave that plays two octaves below the frequency of the primary oscillator
Variations of these waveforms, with different mixes and phase relationships, resulting in various sounds
White noise, which is useful for creating percussion sounds as well as wind, surf, and rain sounds
OFF, which allows you to disable the sub-oscillator
EXT, which allows you to run an external channel strip signal through the ES1 synthesizer engine, by using a side chain
Set the Sub knob to EXT.
Choose the side-chain source channel strip from the Side Chain pop-up menu in the upper-right corner of ES1.