Usually free Spectran software is used here to see VLF signals when doing my earth mode tests. However it also makes a very useful add-on to the FT817 as an adjustable width audio filter. Select the NDB settings and then use the filters (adjustable edges with right and left mouse buttons) with the menu on the left set to "BandPass". It is useful on SSB and on CW and allows the bandwidths to be screwed right down to as low as need be. There is also a CW peak function which puts a narrow filter around 800Hz.