elliptical_eq

NODE

Moves the low end to the centre, and opens the top. Mid/side, with a high pass on the side component.

Parameters

FIELD	WHAT IT DECIDES	RANGE	REQUIRED
high_side_freq_hz	High-frequency Side shelf corner frequency [Hz].	1,000 〜 20,000	no
high_side_gain_db	High-frequency Side shelf gain [dB] (air/width boost/cut).	-24 〜 24	no
high_side_q	Q factor for high-frequency Side shelf filter.	0.05 〜 40	no
mono_cutoff_hz	Low-cut frequency for Side component [Hz] (mono maker cutoff).	20 〜 2,000	no
mono_q	Q factor for low-cut Side filter.	0.05 〜 40	no
width	Stereo width factor (0.0 = mono, 1.0 = original, 2.0 = extra wide).	0 〜 2	no

{
  "high_side_gain_db": 2,
  "mono_cutoff_hz": 120,
  "type": "elliptical_eq",
  "width": 1.2
}

How it is computed

M = (L + R) / √2                S = (L - R) / √2

1. high pass S at mono_cutoff_hz with mono_q        low end becomes centred
2. optional high shelf on S                         high_side_freq_hz / gain / q
3. S = S · width                                    0 is mono, 1 is unchanged

L = (M + S) / √2                R = (M - S) / √2

The decomposition is energy preserving, so at width 1 with no filters the node is transparent. This is the arithmetic behind what cutting engineers call an elliptical EQ.

Using it

It works on difference, not on bass level.
It removes what differs between the channels down low, so the low end becomes centred rather than quieter. If you wanted less bass, that is a biquad.
It is linear but not loudness matched.
Spatial stages are counted as 0 dB in the A/B combination, so if a setting here changes the perceived level, compensate it with a gain node yourself.
Width above 1 amplifies the side component.
That also amplifies everything that was out of phase in the recording, which is what reduces mono compatibility. If a mix has to survive being summed, keep an eye on it.