I would like to be able to implement an All-Pass filter with a frequency-dependent group delay. I need a maximum group delay at low frequency of about 20 samples (for Fs = 44.1kHz), and it needs to fall to zero(-ish) at the Nyquist frequency. Ideally, I would like to specify a corner frequency and a rate at which the delay falls off either side of it. How the heck would I design such a thing?
I'm afraid I'm not sufficiently competent to follow the design approaches I can find on the web that plunge straight into poles, zeros, transfer functions and complex conjugates. What I can do, though, is implement the darned thing efficiently in code once it is designed.
I am able to use Matlab to design FIR and IIR low-pass filters, because it has a built-in tool that spits them out. But it doesn't have an All-Pass filter design tool (at least not one I can find).