[sdiy] Weird Vocoder project

[Ian Fritz]

At 09:22 AM 5/28/2007, Magnus Danielson wrote:

>A typical vocoder analyzer channel will use a dual-resonance (4-pole) bandpass
>filter where as a graphical EQ uses typically a single-resonance (2-pole)
>bandpass filter. The dual-resonance peaks are spread such that you acheive an
>approximation of a flat pass-band with a little responce-dip inbetween. The
>slopes at the resonances are steeper than the bandpass roll-offs would allow,
>and since the non-resonant gain is set to damping, those slopes is 
>considerably
>below the detection level. Those, response-wise they are quite different
>animals.

Yes, this is a very important point.  If you use hi-Q but low-order filters 
you may run into a problem because the low-order filter response has fairly 
strong tails away from the resonant frequency.  When you have 30 of these 
piled up you get a significant background signal.  I learned this from my 
36-stage fixed frequency filter bank. :-(

   Ian