Crossovers

[themickster]

How do you make Robin Schmidt's Crossover plugin work? I can't make it do anything.

[steve]

I don't think that it can be used in Audacity.
In Audacity, there are always the same number of output channels as input channels, whereas that plug-in is splitting each input channel into 4 output channels.

[themickster]

Thank you, Steve. That explains it. I'll stick with duplicating the track and putting one through a low-pass and the other, high-pass at the same setting.

[steve]

I'll stick with duplicating the track and putting one through a low-pass and the other, high-pass at the same setting.

It would be better to make a matched pair of filters using the Equalization effect. The High Pass and Low Pass filters are Butterworth style filters, so the phase response is non-linear, which means that they will give a "bumpy" response in the cross-over band. The Equalization effect on the other hand is an FFT filter, so the crossover band will be smooth.

(There is an alternative approach, but I'm going out soon - will add more later).

[steve]

There is an alternative approach

The alternative approach is to use either a high or low pass filter for one of the tracks (half of the crossover), and the inverse of that filter (1 - filtered audio) for the other track. Doing it this way ensures that the sum of the two halves is equal to the whole (which is not the case when using a regular high pass and low pass filter).

As an example, the way to do this manually is:

1) Duplicate the track.
2) Make a third copy of the track.
3) Apply a high (or low) pass filter to the first track - that is one half of the crossover.
4) Apply the same filter to the second track (this can actually be done at the same time by selecting the first two tracks and applying the filter)
5) Invert the second track ("Invert" effect turns the waveform upside down)
6) Mix the second and third tracks. In effect, this "subtracts" the second track from the third track to provide the other half of the crossover.

The easier way to do this is with "Nyquist commands". Nyquist is a scripting language built into Audacity. Nyquist commands may be run directly in the Nyquist Prompt effect (http://manual.audacityteam.org/o/man/ny ... rompt.html), or may be written as text files and run as plug-in effects. The High Pass and Low Pass filters are Nyquist Plug-ins.

The basic code for (say) a 1000 Hz first order (6 dB per octave) high pass filter is:

Code: Select all

(hp *track* 1000) 

To create the inverse of this filter, we can calculate the difference between this and the original track audio:

Code: Select all

(diff *track* (hp *track* 1000))

Similar simple commands are available for 12, 18 or 24 dB per octave filters.

There is more information about Nyquist here: http://wiki.audacityteam.org/wiki/Nyqui ... _Reference
but if you just want one specific crossover filter, please feel free to ask and I can give you the exact code.
If you want to take it a bit further, it would be fairly simple to make the code into a plug-in so that you can apply it easily without typing the code each time.

[themickster]

Thank you. I'll try the invert way. I'm not yet ready for Nyquisting, though.