[iiwusynth-devel] Filter: solved

[Markus Nentwig]

Hello,

good news: I have figured out the solution for the filter problem (sound
fond specs). As so often, it is all rather simple.
In a nutshell:

-> The Q mentioned in the specs is given in dB! Therefore, a soundfont Q
of 0 corresponds to an 'ordinary Q' of 1
-> A standard resonant low pass filter behaves exactly as described
regarding the Q: A Q of x dB results in a resonant peak of Q dB.
-> There is an additional rule: The total gain of the filter (linear)
has to be divided by Q (linear). Therefore, a Q of 10 dB lowers the
total filter gain by 5 dB, and adds a 10 dB resonant peak.

Some plotted curves for filter frequency responses, and more
explanation:
http://prdownloads.sourceforge.net/millisampler/SoundFont_FilterSpecs.pdf
A complete implementation with all equations, ready for implementation:
http://prdownloads.sourceforge.net/millisampler/Audio-EQ-Cookbook.txt

I'll try to find the time this afternoon to modify the existing filter
(shouldn't be too difficult).

Regards

Markus

PS: I hope that you don't have to turn your monitor in order to read the
file. Should I use .ps instead of .pdf?