I've just recently come across the Optimum Cardioid Triangle technique. I've been thinking I might want to try that out, though it violates my policy of not arranging my mics to keep a 2ch "out" that I can always fall back on.
[snip]
Gut -- what was your experience with OCT?
Didn't want to forget you Todd before I shut up.
In its original form OCT was designed around 3 channel L/C/R playback and surround recording if additional channels are recorded, but not 2 channel stereo. The interesting thing about OCT is that it is designed to minimize unwanted crosstalk between the front three channels by using both the directivity of the mics and the spacing between them. In that sense it was optimized specifically for three channel L/C/R recording without any concern for 2 channel compatibility.
There is a newer revised version called OCT2 which is supposedly adapts the original setup for 2 channel compatibility. OCT2 moves the center mic farther forward and adds a short delay to it to compensate. I think the idea there is the whole multiple near-spaced mics correlation thing again. If I understand what’s going on with correctly with the OCT2 changes, moving the center mic physically forward decorellates it from the side facing pair enough (decorellates the diffuse field sound) to allow the three channels to be mixed to two without problems, while the delay brings the direct sound arriving from the forward direction back into the same timing relationship as OCT. Head spin that one. So if 2 channel output is the goal, then OCT2 is theoretically the one you’d want. I’m not sure the difference is significant or not for what we do.
I’ve run OCT/OCT2 a couple times on-stage as an experimental thing when I already had a standard setup going and could just throw the side facing supercards up too. The array dimensions were accurate, but the setup was a bit bastardized. I used 24” spaced ADK TL’s in supercardioid as L/R and had two different mics I could use as the center which were already setup in my alternate rigs- one was a Tetramic at about the OCT center placement, only vertically higher (which probably decorellated it as much as moving it forward). I could decode that to a forward facing cardioid. The other was a 4060 omni boundary mounted on the stage surface farther forward at about OCT2 center placement. I listened in three channel (and surround using two other room mics) to get a feel for what it could do and really liked the separation and angular precision of the forward image. I never bothered mixing the three channels down to two or applying the OCT2 delay for those listening tests. My feeling was that the setup is great if ultimate imaging accuracy in three channel playback is the goal, and can see that it may work well for two channels as well (given the added complexity of adding the delay, if it’s even needed). I also listened to just the spaced side facing supercards alone in two channel to see what they sounded like, and was surprised that it wasn’t nearly as bad as I figured it would be- wide and side sensitive, and mostly more diffuse and less direct for onstage sound.
In the end I decided to use other techniques for that on-going gig because precise angular imaging was NOT what was most important for me in that scenario. The gig is a jazz trio with upright bass on one side of the drum kit and electric guitar on the other. The stage volume and projection of the guitar amp can overwhelm the delicate jazz drum work and the bass if recording from a perfectly centered stage location, so I usually setup directly in front of the bass on the snare side of the kit and arrange the mics to look across the kit towards the more distant guitar to balances things in both level and in image. I can’t easily physically arrange the OCT setup to do that, and if I could the left supercardioid would face the audience more than I’d like. So I choose not to use it there specifically because I want a form of imaging distortion that OCT was designed to eliminate. I’d like to use it more in the right situation where I’ perfectly centered, the room is good, and want to capture the imaging exactly as it is in the mic location, but all that is rare for us.
I might try to do a split A-B pair of forward facing cards, with one card as the center on the OCT, and then the side facing supercard pair of the OCT. That way I can mix the OCT attempt and still have a forward-facing A-B cardioid pair to use as a 2ch recording if I don't like the OCT.
That’s a great idea, and is sure to completely confuse any other taper with the two offset mic pairs. I’d move the forward facing cards a bit forward, and wouldn’t worry much about micro-delaying things unless you want to.
OCT though does seem interesting to me, in that the side-facing supercard pair seems to do what the split omnis do: make an exaggerated stereo spread, with I hope would help with the typical center-panned PA system.
Exactly, and that’s something of what I heard in listening to the supercardioid pair alone, but interestingly it didn’t really suffer from over-wide-omni hole in the middle. I thought it interesting that the null of each side facing supercardioid mic points more or less at the opposing edge of the Stereo Recording Angle- similar to Blumlein, and the backwards facing opposite polarity lobe of each introduces low level opposite polarity information for any sounds arriving farther to the sides. Pretty ingenious and elegant arrangement.
I love Blumlein while also generally prefering mic setups that are not coincident, so OCT really intrigues me. Though it seems a bit perfectionist in general for concert tapers.
Whew, another essay!