Extracting the individual filters in a crossover filter

I have set up a crossover filter like so:

cof = crossoverFilter(1, 5000, 48, 44100);

I know I can visualize the two constituent filters like so:

visualize(cof);

But for publication purposes, I would like to change the visualization quite a bit. That's why I'm looking for a way to extract the individual filters from the crossover filter in some way, but I cannot seem to figure it out. I even tried going through the code of the visualize function, but I am having a hard time making sense of it.

Am I missing something? Or is there really no easy way to get to the filter coefficients?

Answer 1

[jibrahim]

Hi Simon,

There is no documented way to get the individual filters. However, there is a hidden function that should help you:

numCrossovers = 2;
cof = crossoverFilter(numCrossovers, [300 5000], 48, 44100);

% Get the SOS filter coefficients for the 3 filters
[b1,a1,b2,a2,b3,a3] = getFilterCoefficients(cof,numCrossovers);

% Use fvtool to visualize
fvtool([b1,a1],[b2,a2],[b3,a3])

% Visualize using dynamic filter visualizer
fv = dsp.DynamicFilterVisualizer('SampleRate',cof.SampleRate);
fv(b1,a1,b2,a2,b3,a3)

[Simon Stone]

Brilliant, working like a charm! Exactly what I needed and a huge help. Thank you!

[Soli]

Hello, Thank you so much for this interesting thread. I am also willing to extract coefficient from a Matlab CrossOver Filter object. I have tried the routine you wrote above and it works. However I would like to have more information on how the function getFilterCoefficients() works especially what are the possible outputs (SOS coefficents, a b coefficients etc.). I can't manage to find the documentation for this function in Matlab. Is there any on this function ? If yes could you send me a link to there?

Thank you

[jibrahim]

This method is not documented as of yet.

[b1,a1,b2,a2,b3,a3] = getFilterCoefficients(cof,numCrossovers);

b1 represents the numerator coefficients of the filter filter. a1 represents the denominator of the first filter. Each filter is a SOS filter, so b1 is N-by-3, where N is the number of sections in the SOS filter. a1 is N-by-3.

b2 and a2 represent the numerator and denominator coefficients of the second SOS filter, and so on.

[Soli]

Hi, perfect ! Thank you very much for this info! :)