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Old 16th July 2011 | Show parent
Lives for gear
daniel c's Avatar

Hi Lee,

what's the bandwidth of one? and how can it be maximised?
An ideal all pass filter by itself doesn't have a limited bandwidth, it has a flat magnitude (amplitude) response at all frequencies in the audio range and beyond (in an ideal circuit).

can stages be put in parallel to cover a wider range?
Interesting idea. You could bandpass the signal (like a 2-3 way crossover) and put an all pass filter in each band with the "tuning frequency" optimised for each band. (the tuning frequency for a single stage/first order APF is the frequency where there is a 90 degree phase shift when the output is compared to the input) This might get a bit messy though, it's a lot of filtering.

I recently had to program an APF in Matlab and I had to do a lot of research on analogue filters, but I am by no means an expert on them.

My advice is to head down to one of the Uni engineering libraries and look in the 700-800 section. Now that's like 3-4 long rows of shelves but if you search the catalogue for "filter" it will give you a long list of titles. (I think most of them are around 725 and 728). All pass filters don't occupy much space in most of the texts but you can collect a lot of information about them. A lot of the references are for radio frequency APF's, but the principle is still the same.

To be honest, from my experience you have to start with a theoretical understanding how a basic filter works (i.e. poles and zeros and where they sit in relation to each other when you plot them) before you can get into designing an all pass filter.

Have fun!

EDIT: Section 16.4 will get you started