Ryan Silva wrote:Why the hell is it called xy?
Paging the professor, paging the professor
Yo!
I have to admit that on the history of the name I've never heard or read any kind of explanation and I never really cared enough to go find one. My best guess would be that Alan Blumlein named it when he patented everything else about stereo recording in 1931, but I don't recall whether he used the 'XY' to describe that technique there let alone whether he explained why that would be the best choice of names for the technique. I always assumed it was a cartesian coordinate reference also, but then again that doesn't really make much sense either. On a 2-channel oscilloscope AB-mode is where both signals are displayed separately and XY-mode is where the two signals are compared on an XY graph to watch the phase coherence, but I don't know if that's where they took the name for the mic placement.
But as for the placement of the capsules, it doesn't really matter just as long as they are as close as possible to one another.
Dual LDC stereo mics like the AKG C-24 & C-422, the Neumann USM 69, and all the similar models out there have the two capsules stacked with at least one capsule able to rotate to adjust the angle. (Remember 90? is just the 'textbook' setup and you could wider or narrower as you like.) The Rode NT-4 is fixed at 90? and stacked. The Audio Technica AT-822 & AT-825 mics are edge-to-edge in an inverted V if I remember correctly, while the new little Sony palm recorder is edge-to-edge in a V (capsules facing each other).
What I usually to do demonstrate the setup in class is I hold up two coins representing the diaphragms of the mics and explain that they have to be as close as possible to the same point in space and set at a right angle (to start with at least). When you do that, the edge-to-edge "V" setup seems much more obvious.
The reflections idea sounds like a pretty reasonable explanation for stacking capsules, but more so on LDCs where the far edges would actually be about 1.5" apart. With SDCs any phase cancellations from reflections would be at a high enough frequency to be inaudible.
Although I would have to say that my best guess at why the capsules are stacked in those dual-LDC mics is because it's easier to build them that way, especially if you want to be able to adjust the angles between the capsules.
Hope that helps.
-Jeremy
