[sdiy] Looking for cheap lowpass filter algorithm

[Eric Brombaugh]

Seb Francis wrote:
> 
> With the Parks-McClellan Equiripple (aka Remez) algorithm it's possible 
> to get more stop band rejection at the expense of increased ripple.  It 
> seems the big disadvantage with the results this algorithm gives is that 
> the phase response within the passband is always oscillating between 
> -180 and 180 deg with every 'ripple' in the amplitude response.  This is 
> the case regardless of how many taps you use.

True enough. That's part of the reason that there are so many different 
ways to compute taps. PM / Remez happens to be a fairly simple way to 
specify the filter characteristics and get (reasonably) predictable 
results. Using Matlab & Octave you can try others as well, such as 
windowing, etc.

> I tried quite a few different combinations of taps, ripple, stop band 
> rejection and listened to them as well as looking at the graphs.  In the 
> end I settled on one with 9 taps that sounded really nice to my ears.  

That's starting to blur the line between engineering and art: trading 
off the objective for the subjective. It's great to be able to do 
listening tests and find a filter characteristic that sounds good even 
though the hard evidence of the spectral response looks awful. When it 
uses fewer resources as well that's even better.

Eric