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The DGX is not one of the supported models by Smart Pianist.
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The adjustment you seek is accessed via the Function button, Page 60 of the Manual explains what to do, it is under Voice Editing.
For each of the Main, Dual and Split Voices, you can edit the parameters such as volume, octave and depth of the Reverb and Chorus, allowing you to create new, custom sounds that best suit your musical needs.
1 Press the [FUNCTION] button.
2 Use the CATEGORY [r] or [f] buttons to call up the desired item.
Main Voice parameters are indicated as “Main ***,” Dual Voice parameters as “Dual ***,” and Split Voice parameters as “Split ***.” For infor- mation about each parameter, refer to the Function List on page 73.
3 Rotate the dial to set the value. Check the sound by playing the keyboard.
4 Repeat Steps 2–3 above to edit various parameters.
5 If necessary, memorize the settings to the Registration Memory (page 50).
The Voice-related settings are reset when a differ- ent Voice is selected. If you want to store the cre- ated sound for future recall, memorize the settings to the Registration Memory.