From the top down, the DSP options are:
Frequency: The current receiver index and the filter width are shown in yellow. To change the frequency position the mouse pointer over a digital and rotate the mouse wheel (you can also click in the main waterfall windows).
Playback: Select the playback device from the dropdown list. The audio level is adjusted with the slider, to mute press the button.
Mode: Select the demodulation mode, to change the available modes click the ellipses button.
Filter: Select the filter width, to change the available widths click the ellipses button.
AGC: The standard options - Fast, Medium, Slow, each individually customisable.
CW: A single node peak filter optimised for CW.
Noise Blanker: removes clicks and pops.
Noise Reduction: A variety of algorithms for combatting unwanted interference.
Notch: A powerful automatic notch filter which can remove at least six simultaneous carriers.
Squelch: For FM only.