Hi forum,

I was wondering if there is anything wrong with the definition of the first-order filter in the help of SigmaStudio.

I found the following definition for the first-order filter:

However when I am trying a high pass filter with a cut-off frequency at 1000 Hz then get the coefficients that appear below:

Now, if the definition of the implementation is right, then for a gainLinear operator of 1 (meaning gain 0), for a highpass filter, I should get a1=b0=-b1.

But clearly this is not the case as the 3 coefficients that are being produced in SigmaStudio are not all the same.

Could you please enlighten me on if I am missing something?

thank you

Hello dimitris,

The formulas above are OK for lowpass, but Steven W. Smith's DSP book shows different ones for hi-pass (it also swaps the "

a" and "b" coefficients compared to SigmaStudio's convention, besides flipping the sign of the "a" coefficient). Here's how to calculate the coefficients (these formulas cover only the case of gainlinear = unity):fo(cutoff freq) = 1000 Hzfs(sample rate) = 48000 HzFirst calculate

w = 2*pi*fo/fs= 0.1308997Then, calculate

x = e^(-w)= e^ (-0.1308997) = 0.8773Finally figure the coefficients:

b0 = (1 + x) / 2= 0.98365b1 = - bo= -0.98365a1 = x= 0.8773One of the

General Second-Orderfilters allows you to type in coefficients as shown above -- this is handy when troubleshooting coefficients.Best regards,

Bob