I haven't documented the oversampling objects yet, but you can check out the help files. For an optical feedback I would recommend a spectrum analyzer like the free Voxengo SPAN.
These oversampling filters are really cool for center frequencies above 14 kHz - you can create brutal filter sweeps.
I'm not shure though for standard juse - center freq below 14 kHz. I just made a blind test with my classical guitar, filtered by one vcf, standard versus oversampling version. (using the phi/probabiltiy bang as a random switch). It's hard for me to tell the difference.
About the code. Have a look at: dsp_book_Ch16.pdf. Everything is in there. In my table
jho_downsample_coefs_q30 (see file
jho_oversampling.h), the sinc-function is allready windowed (otherwise it would be infinitely long). I simply calculated the table with open office calc.