dahnielson wrote:Keeping in mind that both DEF/PRF probably are implemented by the same filter, the documentation tells us that it is supposed to be a formant filter.
D'oh! Of course it
is the same filter. It's a
7th order formant filter, meaning seven setting to interpolate between: four PRF and three DEF, so that you can both transform a
mf sample to
f and do a
glissando with the same filter at the same time. I'm slow sometimes.
dahnielson wrote:After taking a look at the
Format Filtering Example in
Introduction to Digital Filters with Audio Applications, especially the plot at the bottom of the page, I can wage a guess that's similar to what is going on in your frequency response plot
The fact that it's a formant filter makes the description of Fc and Q obvious, now when I've read up on the subject. Fc sets the center frequency for F1 while Q control the "center frequency offset relative to the initial Fc setting" for F2. Q probably control the bandwidth for both F1 and F2. Q0H control the bandwidth of F3 with a fixed center frequency at 7.5 kHz. There is no need to specify gains directly:
"[The] transfer function is an all-pole filter [...]. As a result, there is no need to specify gains for the formant resonators -- only center-frequency and bandwidth are necessary to specify each formant, leaving only an overall scale factor unspecified in a cascade (series) formant filter bank."
The formant filters can be implemented in parallel rather than series:
"In principle, the formant filter sections are in series, [...] [n]umerically, however, it makes more sense to implement disjoint resonances in parallel rather than in series. This is because when one formant filter is resonating, the others will be attenuating, so that to achieve a particular peak-gain at resonance, the resonating filter must overcome all combined attenuations as well as applying its own gain."
(But I still think your plot looks a lot like a sinc function.)
It would be great if you could do additional measurements to find out if my assumptions are correct.