The delay between our ears is about .5 milliseconds, and that certainly has an audible effect in the sense that it impacts how we locate a sound. So it would stand to reason that an even greater delay, i.e. .8 milliseconds, should have an audible effect, right?
Well yes, maybe. Consider that this delay is not between Left and Right, but between center and sides. So it's sort of a Mid/Side thing rather than a Left/Right thing. Also there are three channel pair relationships to consider which are all superimposed upon one anther rather than just a single one. We don't have just the left+right relationship, but the left+right, left+center, and center+right relationships all interacting. That interaction is complex. It is interacting in the electronic realm (mixing) in the air (speakers to ears) and perceptually in our ear/brain system. Taking just the perceptual part of that, the Left/Right microphone channel relationship remains fixed while the left/center and center/right relationships change on the move from OCT to OCT2. Perhaps the left/right part perceptually "anchors things" leaving more freedom for the mid/side part to not be perfectly aligned. Or perhaps the Mid/Side difference is perceived as depth, or as first arrival presence emphasising "centeredness".
The OCT 2 approach improves downmix quality further with its decreased inter-channel correlation of diffuse-field pickup.
^
Good to have my memory/presumption of what that is intended to achieve confirmed, thanks.
To step back for a moment for folks following along, correlation
between adjacent channel pairs is desirable for sharp imaging, while low correlation ("increased decorrelation") is desirable across all channels for the ambient, diffuse-field sound arriving from all directions. How to achieve both of those things at the same time, and to what extent one should go in order to achieve it is what is at issue.
OCT2 goes to more a involved extreme to achieve this than OCT(1) does. That's because OCT(1) was originally designed for 3 speaker Left/Center/Right playback where the center signal remains discrete and is not electronically mixed with the Left and Right channel signals until the signals pass through the speakers and "mix in the air of the room". Phase interactions between correlated signals manifest more strongly when signals are mixed electronically, than when they mix together in air after being reproduced by speakers in a typical listening setup of speakers set some distance apart from each other in a non-anechoic room.
To make this real-world, consider polarity-inversion. Take a signal and duplicate it to two channels. You have a 100% correlation between those two channels. Invert polarity of one channel, mix them together without changing gain and you get total, or near total cancellation. But play both signals thorough a stereo system and invert polarity ahead of one of the two speakers and you don't get total cancellation in the room. You may notice attenuation of low frequency energy, because that remains more correlated in the room than higher frequencies. This difference between mixing in the electronic realm verses in air is what OCT2 is intended to improve for mixing the 3-channel microphone feed down to a 2-channel stereo signal. To further extend this real-world example- If the signals in both channels are
not correlated, they won't cancel when summed electronically, regardless of the polarity inversion of one channel. Think uncorrelated pink noise in two channels. It sounds the same when one channel is polarity inverted. If the two channels are correlated at low frequencies but not at high frequencies then only the low frequency part will be attenuated.
We can decorrelate the diffuse pickup by using a microphone setup geometry that introduces differences of pickup pattern + angle and/or by differences in time-of arrival (spacing). Using microphones with a tighter directional pickup pattern combined with increasing the angle between the microphones will decrease inter-channel correlation of diffuse-field pickup. But with 180 degree oriented L/R supercardioids, pattern and angle have already been maximised as much as possible, so increasing the spacing between the L/R pair and the center microphone is the only remaining way to do that using setup geometry. OCT2 pushes the center microphone forward then introduces a delay to compensate. The compensating delay realigns wavefront arrival for direct sound coming from the forward direction (it keeps the signal correlated for that angle of arrival). For sound arriving from other directions it effectively increases time of arrival non-alignment and therefor decreases correlation.
Can we do the same by changing the microphone setup geometry in some other way? There are a couple options- we can push the L/R supercardioid pair wider, yet that will narrow the SRA. We can move the center microphone up or down out of the horizontal plane, keeping time of arrival more or less the same for horizontal arriving sound, but increasing the distance between the center mic and all others for sound arriving from all directions other than the horizontal plane. Neither of those are very practical, although I have seen some odd 3-channel arrangements using a standard near-spaced-stereo center pair plus a center microphone positioned a couple meters higher on the same stand. The 3 positions remain more or less closely aligned for horizontal traveling plane waves, but not for sounds arriving from above or below the horizontal plane.