Original from T. Lossius - ttblue project
Optimized version in pseudo-code.
[! The filter is unstable for fast automation changes in the lower frequency range.
Parameter interpolation and/or oversampling should fix this. !]
The sum of the Linkwitz-Riley (Butterworth squared) HP and LP outputs, will result an all-
pass filter at Fc and flat magnitude response - close to ideal for crossovers.
Lubomir I. Ivanov