Various Biquad filters

  • Author or source: JAES, Vol. 31, No. 11, 1983 November
  • Created: 2002-01-17 02:08:47
  • Linked files: filters003.txt.
notes
(see linkfile)
Filters included are:
presence
shelvelowpass
2polebp
peaknotch
peaknotch2

Comments

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;
{
  presence(freq/(double)SR,boost,bw/(double)SR,
           &f->cx,&f->cx1,&f->cx2,&f->cy1,&f->cy2);
  f->cy1 = -f->cy1;
  f->cy2 = -f->cy2;
}


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

Input = ...
Output = ...

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!