Ok, back to discussion of the configuration of the L/R near-spaced pair and its role. I started this post before the discussion of the Improved 3-point PAS at the end of the previous thread, but pushed it off to the start off the new thread, since it sort of recaps a lot of how OMT developed up to this point. I intend to start a new thread on Improved 3-point PAS to split it off from this more esoteric OMT discussion, or may just append the existing 2-point Improved PAS thread(s).
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Ah, I understand now! I was misinterpreting +/-45° to mean 45° ± Nº (and therefore wondering what range of N to expect) but I realize now you mean +45° on one side and -45º on the other for a total angle of 90° fixed. Thank you.
Yes, that's it. A few more thoughts on that in my next post..
Several things going on there. The imaging relationship across L/C/R mic positions is one. An improved sense of clarity, proximity, and "upfront impact and bigness" (to my ear, for lack of better description) is another. The problem is that fully optimizing for one becomes at odds with optimizing for the other. Over time I've worked on improving the second without overly compromising the first, and I feel this points to a potential convergence between Improved 3-point PAS and OMT I mentioned at the end of the previous thread.
Where this came from (bear with me though a bit of historical progression of OMT)-
I found I gained a lot of flexibility using three mics instead of two, largely because it allows for three new forms of balance control unavailable with just two mic channels: Level of the center verses both sides, slight panning adjustments of the center without affecting the sides, and when needed, the ability to push the L/R energy balance to one side while panning the center the opposite way to compensate. All things that can be very helpful in the real world of concert taping. I gained the ability to separate the control of energy distribution and center image position to a limited but useful extent. Originally this was 3 omnis in mounted in spheres, later a directional mic between two omnis, sometimes 3 directional mics when indoors. That's 3 ignoring the rear-facing channel or mono SBD in making full use of a 4 channel recorder.
I later increased channel count from 4 to 6 channels by adding Left/Right directional mics between the center mic and the wide omnis, and looked into a number of ways of doing that, trying a few different approaches at my favorite outdoor amphitheater. Günther Thiele's OCT (Optimum Cardioid Triangle) arrangement made a lot of sense to me and I found it worked best for playback over three front speakers, which was my primary focus at the time, but also it worked better for 2ch playback. Ok good.
The imaging of OCT really does work astoundingly well over three front speakers, which it was originally designed specifically for by minimizing the overlap between the Left and Right mic patterns as much as possible, while retaining just the right of overlap with the Center pattern (not too much, nor too little) so as to hand off smoothly from one segment to the other without conflicting. It does that by using a cardioid in the center and leveraging the supercardioid pattern in such a way that the null of the fully sideways pointing (+90°) Left supercard is pointed directly at the Right edge of the recording angle, which is where the Right speaker will be during playback, and vice-versa. Pretty cool. In addition to that, sounds arriving from outside of the recording angle window are picked up by the rear-lobe of the opposite mic channel and therefore will be in antiphase, which is sort of like Blumlein. But unlike Blumlein, as the angle of arrival shifts farther around the sides and on toward the back, the signal into the opposite channel is picked up with much reduced less level due to the the reduced sensitivity of the rear lobe. That, along with the arrangement capturing phase / time-of-arrival information by being near-spaced, is unlike and arguably perceptually superior to Blumlein. Its a very clever 3 microphone arrangement.
So my initial OMT array incorporating six channels used an OCT center arrangement with fully sideways facing supercards. The primary difference between that OMT6 and OCT was the wider spaced omnis and the single rear facing channel. This worked really well for me outdoors, and much better than most tapers might expect indoors. I think the directly forward facing center mic anchoring everything, and the other mics being purposefully arranged to work well with it is a big reason why.
At some point I no longer had a good surround playback system setup, and was listening primarily in stereo. I still wanted to support excellent surround playback (done right its arguably better used for the replay of live concerts than for anything else), but was really listening most critically in stereo. First thing I tried was making the center mic more directional, then switched it to a Mid/Side pair. The change to a center coincident pair was all about optimizing 2ch stereo, it wasn't needed for surround. Recording using a 6 channel recorder required sacrificing the single rear-facing channel to do that. The addition of the coincident center pair worked great for all the reasons discussed at length in previous threads, but I missed the rear channel. I went back and forth about what was more important. Really, I wanted both. Recording 8 channels felt extreme but allowed me to do that.
Also, all this is primarily about general concert taping, and not so much classical music recording done in gorgeous sounding halls where the microphone array can be put in just the right place and shifted slightly until positioned just right. I wanted to better accommodate taper realities, while hopefully also improving things further in already good acoustic situations like nice sounding outdoor amphitheaters where it was already working well.
To do that I wanted the L/R directional pair angled more forward, rather than pointing directly sideways at either side wall (the omnis or side facing subcards cover that just fine). That change would impart some forward sensitivity bias into the geometry of the array itself, and not just have that forward bias entirely reliant on the level of the center channel in the mix. To achieve that that I shifted to using a more narrow supercardioid pattern in all three positions and used some additional L/R spacing to try and maintain a somewhat equivalent recording angle to OCT: Supercards in all three positions, a bit more spacing, and L/R angled reasonably forward. Left/Right at +/-45° is the result of this. Switching to a supercard center also helped a bit with clarity and a bit more reduction of audience noise in the center, arriving from elsewhere.
All of that was about increasing forward sensitivity in the L/R directional imaging mics while trying to preserve good imaging. Upon making the change I found the imaging remained good when listening in stereo. I began to wonder if I was getting too "belt and suspenders" with phase-correlated imaging being contributed by both the coincident center pair AND the near-spaced triplet. I found I could use either in the mix, and each provided somewhat different qualities. The L/C/R tended to be more focused and precise image-placement wise, with cleaner transients, but flatter and less dimensional. The M/S pair more phase-correlated, yet at the same time more ambient in its stereo width. I now typically balance both separately in the mix first, then go back and tweak the level of the Side channel while listening to everything in play. With the near-spaced L/R pair in there providing a lot of the L/R imaging information, I'm using the Side channel as sort of an "ambient width special sauce" type of thing. It may be more directionally accurate without it, but the right amount of it brings "the sound" that makes me smile and pulls me into the recording.
