# Strange coefficient with a RBJ bandpass filtrer.

Question asked by Maxime Dolberg on Nov 6, 2012
Latest reply on Nov 14, 2012 by Maxime Dolberg

Hello !

I'm trying to make a bandpass filter (constant 0dB peak gain) with the Robert Birstow-Johnson cookbook formulae. So I calculated all coefficients to fixe the bandpass filter but I have a problem with a1 coefficient value. Indeed, a1 = -1,9829.

The features of the Bandpass are :

Center frequency : fo = 1kHz

Bandwith : Bw = 1kHz => so octave Bw = 1,38. (Q = 1)

Low corner frequency : 500Hz

Hight corner frequency: 1,5kHz

Sample Frequency : Fe =  48kHz

Here is the step I did to find a1 :

w0 = 2*pi*fo/Fe = 0,13089 rad/s

S = sinw0 = 0,130526

a1 = -2*cosw0 = -1,9829 ???!!

I read the DSP56k family manual and it write : "... In the DSP56K chip family, the extreme values that a word operand can assume are - 1 and + 0.9999998. ... " .

So did I make a mistake with the calculated ?

If no, how can I implement a1 into the code ?

note : I use the dsp56374