1-RC and C filter

notes
This filter is called 1-RC and C since it uses these two parameters. C and R correspond to
raw cutoff and inverted resonance, and have a range from 0 to 1.
code
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
//Parameter calculation
//cutoff and resonance are from 0 to 127

  c = pow(0.5, (128-cutoff)   / 16.0);
  r = pow(0.5, (resonance+24) / 16.0);

//Loop:

  v0 =  (1-r*c)*v0  -  (c)*v1  + (c)*input;
  v1 =  (1-r*c)*v1  +  (c)*v0;

  output = v1;

Comments

  • Date: 2005-01-13 18:25:57
  • By: yes
input is not in 0 - 1 range.

for cutoff i guess 128.

for reso the same ?

Nice. This is very similar to a state variable filter in many ways. Relationship between c and frequency:

c = 2*sin(pi*freq/samplerate)

You can approximate this (tuning error towards nyquist):

c = 2*pi*freq/samplerate

Relationship between r and q factor:

r = 1/q

This filter has stability issues for high r values. State variable filter stability limits seem to work fine here. It can also be oversampled for better stability and wider frequency range (use 0.5*original frequency):

//Loop:

v0 = (1-r*c)*v0 - c*v1 + c*input;
v1 = (1-r*c)*v1 + c*v0;
tmp = v1;

v0 = (1-r*c)*v0 - c*v1 + c*input;
v1 = (1-r*c)*v1 + c*v0;
output = (tmp+v1)*0.5;

-- peter schoffhauzer