[Fpga-synth] Simple Filter Question

[Nicholas Gregorich]

On Jun 18, 2007, at 1:35 PM, Scott Gravenhorst wrote:

> The terrormouse PDF specifies that the filter used at the end of  
> the delay
> line is a "simple first order IIR".  It then gives this to describe  
> it:
>
>                        -1
>        0.8995 + 0.1087z
> H(z) = ------------------
>                      -1
>           1 + 0.0136z
>
> From what I understand, this makes:
>
> a0 = 0.8995      a1 = 0.1087
>                  b1 = -0.0136
>
> There was no explanation for the values chosen except that they  
> "sound like
> an acoustic guitar string".
>
> What I don't understand is that this matches neither a first order  
> lowpass
> nor highpass.  Lowpass should have only a0 and b1 terms and  
> highpass has a1
> = - a0 (at least).
>
> I am assuming this is supposed to be a lowpass filter of some kind.
>
> When I did my testing, I used the first order lowpass as described at
> www.dspguide.com, in the recursive filters PDF.  It's not the same  
> filter,
> but it works.
>
> A resource that explains this would be most appreciated ... ?
>
>
> -- ScottG
>

Looks like you got some help on this already, but here's my MATLAB  
work on it anyhow. :)

http://homepage.mac.com/nicksdsu/fpga-synth/terrormouse_iir.pdf

Nick.