[PD] Notch filter

[Claude Heiland-Allen]

yvan volochine wrote:
> Could someone explain me how I can make a notch with biquad~ (or with 
> any other native pd objects) please ?

A notch filter is basically this in a z-plane plot:

         |
         |    o         cos(a),    sin(a)
         |   x        r*cos(a),  r*sin(a)
         |
--------+--------
         |
         |   x        r*cos(a), -r*sin(a)
         |    o         cos(a),    sin(a)
         |

Where:

  o = zero on unit circle
  x = pole just inside unit circle

Parameters:

  a = 2*pi*frequency/samplerate  (zero+pole angle)
  r = just less than 1           (pole radius)


The closer r is to 1, the narrower the notch is.


Pd objects:

  samplerate~
  czero~
  cpole~
  expr~  -- for accuracy
  cos~   -- for speed, note cos~ expects 0..1 instead of 0..2PI


See also:

  http://crca.ucsd.edu/~msp/techniques/latest/book-html/node144.html


Hope this helps,


Claude
-- 
http://claudiusmaximus.goto10.org