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Various Biquad filters

References : JAES, Vol. 31, No. 11, 1983 November
Linked file : filters003.txt

Notes :
(see linkfile)
Filters included are:


Added on : 19/10/08 by neolit123[ AT ]gmail[ DOT ]com
Comment :
I'm kinda stuck trying to figure out the 'pointer' 'structure pointer' loop in the presence EQ.

Can someone explain:


  *a0 = a2plus1 + alphan*ma2plus1;
  *a1 = 4.0*a;
  *a2 = a2plus1 - alphan*ma2plus1;

  b0 = a2plus1 + alphad*ma2plus1;
  *b2 = a2plus1 - alphad*ma2plus1;

  recipb0 = 1.0/b0;
  *a0 *= recipb0;
  *a1 *= recipb0;
  *a2 *= recipb0;
  *b1 = *a1;
  *b2 *= recipb0;


void setfilter_presence(f,freq,boost,bw)
filter *f;
double freq,boost,bw;
  f->cy1 = -f->cy1;
  f->cy2 = -f->cy2;

How can this be translated into something more easy to understand.

Input = ...
Output = ...

Added on : 28/10/08 by neolit123[ AT ]gmail[ DOT ]com
Comment :
Managed to port the presence eq properly. And its sounds great!

Altho I did some changes to some of the code.

changed "d /= mag" to "d = mag"
"bw/srate" to "bw"

There results I got are stable within there parameters:

freq: 3100-18500hz
boost: 0-15db
bw: 0.07-0.40

Really good sound from this filter!

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