[PD] Understanding Oversampling

[David F. Place]

On Nov 11, 2008, at 1:35 PM, pd-list-request at iem.at wrote:

> Date: Tue, 11 Nov 2008 09:49:55 -0800
> From: Miller Puckette <mpuckett at imusic1.ucsd.edu>
> Subject: Re: [PD] Understanding Oversampling
> To: pd-list at iem.at
> Message-ID: <20081111174955.GA5244 at imusic1.ucsd.edu>
> Content-Type: text/plain; charset=us-ascii
>
> I looked, and it's very confusing, sorry.

Thanks for the clarification.

> There's an error, too :)
>
> The filters are incorrectly set to 7500 Hz.

Instead, we would want them set to 15kHz as the comment says?


> The main patch is assumed to
> be 44100 Hz. so the subpatch runs at 16x44100.  So the cutoff in  
> radians
> per sample is beta = 2*pi*7500/(16*44100) = 0.017518.
>
> The rest is as in chapter 8 section 6 (designing Butterworth Filters)
> with r = tan ( beta/2) = 0.033405, theta = 0 and +/- pi/3 , e.g.,
>
> (1-r^2)/(1 + r^2 + 2*r*cos(pi/3)) = 0.96555
>
> (that's one of the numbers that shows up in J07).

It would be so nice to have an exegesis of each number and object in  
the patch.  As you say in your book, digital filter design is a huge  
subject.  I'd love to be able to honestly say that I would like to  
devote myself to learning it, but truthfully I just want a cookbook  
for creating a low pass filter to do the application of oversampling.

>
> To change the value of 7500 or 16, adjust beta accordingly and repeat.
>
> cheers
> Miller

_____________
David F. Place
d at vidplace.com