[pulseaudio-discuss] New Module: module-lfe-lp

[Alexander E. Patrakov]

2013/3/19 Justin Chudgar <justin at justinzane.com>:
> I've created a module to ensure that only low frequencies are sent to devices
> at the end of an "lfe"/"subwoofer" channel. This module allows the user to
> select the master channel, the low pass cutoff frequency (aka corner freq, -3dB
> freq) and the number of filter poles.

Sorry, I cannot sign off this filter implementation.

First, the filter currently fails the "attenuation must be 3 dB at the
cut-off frequency" test that all Butterworth filters must pass.
Second, Q=sqrt(2) is only valid for a 2nd order Butterworth filter.

So, please, start over and implement only a 2nd order Butterworth
filter (in both lowpass and highpass versions), as that's the only
type of filter you would need for the LP-4 crossover.

Or, go with a simple RC filter and implement an LP-2 crossover from them.

A good article for LP-4 (unfortunately, only incompletely translated
from Russian into English) is at:
http://ac3filter.net/wiki/Bass_redirection_(math)

-- 
Alexander E. Patrakov