The shelve filter is enabled by tapping the button on the XY-pad.
The above image shows the various frequency responses at different positions.
The X-coordinate specifies the cutoff frequency.
The Y-coordinate specifies the balance between the low spectrum
and high spectrum. At the bottom, the lower spectrum is allowed
through and the higher spectrum is stopped. Going up the high
frequencies are allowed through a bit more. I the vertical middle, the
low and high spectrum are passed equally loud (= no shelve). Then
going further upward, the low spectrum attenuates until it is a pure
high pass filter at the top.