I should add; I am familiar with how the other controls work just not Center Dist.
However; also unclear is whether the differential Low shelf filter in LMF Spread is phase compensated or not.
There have been 2 developments in the past that use such a filter, with different target applications.
With Blumlein shuffling, group delay in the difference is desirable (90deg at approx 700hz) to re-align signals derived from spaced microphone, aka phase rotator.
A more recent, stereo-sonic shuffler was adapted for pan potted mono stereo signals . With it, linearity was the rule, rather than the exception.
One can only assume that BASE, Behringer and AntiPoP products are of the non-linear sort.
As such these products can be used in conjunction with spaced mikes, even though not their intended purpose. One would need to work out the corner frequency of the filter and go from there, because in blumlein shuffling, boost and frequency are coefficients that correspond to certain spacing; Once the fixed corner frequency present in the products has been determined the LMFSpread dial can be marked to denote corresponding spacing.
Im not all that good at maths but I'll try to explain the principle (
alternative explanation here);
Imagine a sound coming from the right. Low frequency sound will arrive at both mics with a small delay between mic1 and mic2, with little or no difference in amplitude. The purpose of non linear shuffling is to add a phase shift at mid-high frequencies and enhance LF amplitude differences. The spacing between the mikes corresponds to a wavelength in meters, given speed of sound in air one can work out a frequency. It is at this frequency a 90deg phase shift is introduced between the channels by means of RC filter theory, recall; a phase shift by means of filter in the difference . The left and right channels are phase shifted but in a frequency selective way, with that shift rising above hitting at the target frequency. Once played back through speakers the sound is more correct.
However it would be questionable to add such shift to recordings that haven't been recorded in that way - With BASE & Co. One would get an illusion of space but of the gimmick variety considering no attention has been payed to the underlying theory its based on and depends hugely on the source material,i.e. if its mono panned / multi track or not.
In the original design, blumlein used a 6db/oct LPF in the difference, and attenuated the mono accordingly, a low shelf as seen in BASE etc does more or less the same thing, with the mono attenuation being made redundant.
As for the Waves S1 plugin, it has a shuffler of the Linear sort - as such its more appropriate for general use although its effect not as pro found as using the non-linear method via spaced microphones. Lexicon had/has a similar filter in their effects units as well.
