Hi,

I implemented a 2nd order allpass filter in both the direct form and as a state variable filter. 

SVF implementation from: https://mural.maynoothuniversity.ie/18520/1/VL_imp... 

Direct form from: https://thewolfsound.com/allpass-filter/

I notice i can get the exact same frequency response from both filters, but in order to have the same frequency response, i have to set different values for the bandwidth. It seems like the bandwidth of the direct form stays constant with changing cutoff frequency but the SVF bandwidth changes when changing cutoff frequency. For example, as cutoff frequency goes up, i need to increase the SVF's bandwith value to have the same response as the direct form. 

I am thinking about how to scale the bandwidth of one of the two filters so it will always have the same frequency response as the other., but have not got a good mapping formula yet. I am a bit lost at how to approach this problem, anybody have tips?

I'm confused about what you're trying to do since things like frequency response and cutoff frequency shouldn't be applicable to an all-pass filter, which by definition has unity gain for all frequencies.   Is this just a tool problem where you are unable to specify that you're trying to implement an all-pass filter?

Thanks for you reply.

A 2nd order allpass has a ‘center’ frequency where it has a phase response of pi. That’s what i mean with cutoff frequency. With frequency response i mean the phase response since it indeed has unity gain.