**I've had numerous requests for Elliptic Equalizer filters to use with the Stereo Width Controller.**

**Update November 2017. Construction information for the EEQ-12 Elliptic EQ is here:**https://www.proaudiodesignforum.com/for ... ?f=7&t=911

**Update October 14, 2017. The Elliptic Equalizer design I finally settled on is here:**https://www.proaudiodesignforum.com/for ... 8&start=46

Conventional Elliptic Equalizers are usually first-order with vertical to lateral crossover and left to right crosstalk curves that are both 6 dB per octave.

When a second-order (or greater) low pass filter is inserted into the Stereo Width Controller's side chain, which subtracts side to produce mono, an interesting thing happens.

The vertical to lateral crossover is 6 dB per octave but the Left to Right crosstalk curve is 12 dB per octave. (Or the filter order.)

While the low frequency benefits of a second-order filter are the same as a first-order, the undesirable effect on the mid and high frequency stereo image is less when the filter is second-order or greater.

Sometimes, in addition to EE, an increase in midband or high frequency width is also needed.

A broadband width control can narrow or widen width, but not do both at the same time.

This thread explores a second-order Elliptic Equalizer combined with Width Enhancement.

Remember that Lateral modulation is Mono, Sum or Mid, while Vertical modulation is Difference or Side.

**The first family of curves show the relationships between Lateral, Vertical, Left and Right with Left-only modulation and a 300 Hz second-order Elliptic Equalizer.**

*Curves showing the relationships between Lateral, Vertical, Left and Right with Left-only modulation and a 300 Hz second-order Elliptic Equalizer.*

The lateral mono sum is always unity.

The vertical difference signal has a 6 dB per octave slope.

The Left and Right channels begin to blend below 300 Hz. Below about 50 Hz they are fully mono.

The Right channel's crosstalk from the Left falls at an approximate 12 dB/octave rate in the midrange.

**By using two parallel filter paths it's possible to Elliptically Equalize, which collapses low frequencies to mono, and expand or reduce width.**

In the example plotted above the EE low pass filter was set at 300 Hz.

The Right channel crosstalk curve shows the -3dB point of the filter.

The -3 dB point of the Vertical modulation appears to be an octave lower...

(See this thread showing that the filter cutoff and vertical cutoff are an octave different viewtopic.php?f=6&t=622&start=52)

**A second high pass filter, operating in parallel, can expand or reduce width above the EE frequency.**

To prevent width expansion from undoing the work of the EE filter, a guard band exists between them.

A high pass filter at 1 kHz was chosen to separate the work of the two filters.

**The following set of curves show the Lateral and Vertical responses for Left-only modulation when both the 300 Hz Elliptic EQ is engaged and the Width above 1 kHz is expanded to 200%.**

Lateral and Vertical responses when a 300 Hz Elliptic EQ is engaged and the Width above 1 kHz is expanded to 200%.

Lateral and Vertical responses when a 300 Hz Elliptic EQ is engaged and the Width above 1 kHz is expanded to 200%.

The Vertical gain above 1 kHz is 200% (6 dB greater) than the Lateral showing Width enhancement of 200%.

Below 300 Hz the signal blends to mono.

**It is possible to implement both Elliptic EQ and Width by having a split filter path.**

The advantage of higher-order EE filters is that there is less blending in the midrange and high frequencies providing more accurate imaging.

The advantage of higher-order EE filters is that there is less blending in the midrange and high frequencies providing more accurate imaging.

I'll show the circuit in my next post...

Using the Precision MS Matrix for Mono Crossover LF Blending https://www.proaudiodesignforum.com/for ... ?f=6&t=622

A Simple Elliptic Equalizer Using the Width Controller https://www.proaudiodesignforum.com/for ... ?f=7&t=736

Frequency-Dependent Width Using the Stereo Width Controller https://www.proaudiodesignforum.com/for ... ?f=7&t=788

In this post I argue for third-order 18 dB/octave filtering: https://www.proaudiodesignforum.com/for ... 2&start=52

**Related topics:**Using the Precision MS Matrix for Mono Crossover LF Blending https://www.proaudiodesignforum.com/for ... ?f=6&t=622

A Simple Elliptic Equalizer Using the Width Controller https://www.proaudiodesignforum.com/for ... ?f=7&t=736

Frequency-Dependent Width Using the Stereo Width Controller https://www.proaudiodesignforum.com/for ... ?f=7&t=788

In this post I argue for third-order 18 dB/octave filtering: https://www.proaudiodesignforum.com/for ... 2&start=52