Hi Derek, thanks for your help, but my problem is related with the answer given by Charles Henry. Thanks anyway! Libero

On Jan 22, 2008 7:43 PM, Derek Holzer derek@umatic.nl wrote:

Is what you are looking for a resonant comb filter? I.e. a delay line with a resonant frequency which "rings"? Check the archives for the "resonant comb filter series" thread. Me, Davide Morelli and Ian Smith-Heisters worked out a bunch of stuff there. I wouldn't know off-hand how to convert a band pass filter to a comb filter, though. I only how to do it with delay lines right now. Some Googling on filter theory could fix that pretty fast, though...

best, d.

Libero Mureddu wrote:

...

Hi, I have a simple patch with a [click~] object connected to a [reson~] filter. I'd like to know how it is possible to convert the bandwith parameter of the filter to decay lenght (in milliseconds). In SuperCollider there is a particular version of the "resonz" filter called "ringz", here is the description: Ringz.ar(in, freq, decaytime, mul, add)

-- derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista ---Oblique Strategy # 117: "Make it more sensual"

Hi Charles, thanks, this is exactly the solution I was looking for. Thanks a lot for the detailed math explanation too. ciao

Libero

On Jan 22, 2008 9:12 PM, Charles Henry czhenry@gmail.com wrote:

It depends upon the order of the filter... by resonant filter, I assume you mean a two-pole bandpass filter. Here's the math for converting between bandwidth and exponential decay

Take a function, g(t)=1000^-(t/r) where r is the -60 dB ring time. I used a base of 1000, because a factor of 1000 = 60 dB

g(t)=e^-(t*ln(1000)/r)

Now we take a one-sided fourier transform of this function:

G(f)=integral(0, inf; e^(-2*pi*i*f*t)*e^-|t*ln(1000)/r|) G(f)=integral(0, inf; e^((-2*pi*i*f-ln(1000)/r)*t) G(f)=1/(-2*pi*i*f-ln(1000)/r) * e^((-2*pi*i*f-ln(1000)/r)*t), eval at t=0, t=inf

G(f)= - 1/(-2*pi*i*f-ln(1000)/r) G(f)=1/(2*pi*i*f+ln(1000)/r)

G(f)=r/ln(1000) / [2*pi*i*f*r/ln(1000) + 1]

now, it's just a simple matter of finding the -3 dB points to find bandwidth of this function and interpreting the result of this function...

|G(f)|^2=(r/ln(1000))^2 / [1 + (2*pi*f*r/ln(1000))^2]

0.5 = 1 / [1 + (2*pi*f*r/ln(1000))^2] (2*pi*f*r/ln(1000))^2 = 1

f = +/- ln(1000)/(2*pi*r) bandwidth bw= 2 * ln(1000)/(2*pi*r)

conversions: bw = ln(1000) / (pi*r) r = ln(1000) / (pi*bw) r is in seconds and bw is in Hz

Try out these relations and see if they work.

Chuck

On Jan 22, 2008 12:11 PM, Libero Mureddu libero.mureddu@gmail.com wrote:

...

Hi, I have a simple patch with a [click~] object connected to a [reson~] filter. I'd like to know how it is possible to convert the bandwith parameter of the filter to decay lenght (in milliseconds). In SuperCollider there is a particular version of the "resonz" filter called "ringz", here is the description: Ringz.ar(in, freq, decaytime, mul, add)

This is the same as Resonz, except that instead of a resonance parameter, the bandwidth is specified in a 60dB ring decay time.

My knowledge of filters is too little to be able to convert the between the two parameters by myself, any help appreciated!

thanks,

libero

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