I can confirm Len's assertion that the Tetramic produces a well behaved virtual figure-8 with an unusual degree of low-frequency extension. And I also find the Zoom F8 not providing gangable gain control across multiple channels a disappointment.
Nothing prevents one from using these mics in various combinations with standard microphones, or with additional ambisonic mics as long as sufficient recording channels are available.
I developed bluround about a decade ago when I was quite frustrated about the not so precise "source positioning" playback of all the multichannel techniques I tried. Also I found that pretty much all the multichannel techniques are missing definition when a source is coming from the center back and needed to find a way to have hat point locked and very clear. [snip]
Let me try to explain how to set it up:
You build a ORTF with two cardioid microphones facing forward. This will be your front end. Add a MS facing backward keeping the same distance (17cm) from the front end. This will then be your rear end. This equilateral triangle is fundamental to keep clarity of the source positioning. You can change the distance for the microphones if you like but is important to keep always the same triangle. You can also move the front end and rear end, this will work with a bit of compromise of the clarity. [snip]
If you then need more channels you need to draw an "ideal circle" containing the triangle vertexes. You then add the needed mics. For instance if you need a center front you will add the center mic half way from the LR ones and on the circumference of the "ideal circle".
You do the same to add two more mics for the 7.1 multichannel configuration. The same to add Z axis for three dimentional recordings, where you can add a mic on top an done below if needed.
Thanks for the explanation. Discussions of this sort are rare around here and I look forward to them. I believe I understand the process you describe above, and expect it has worked satisfactorily. I hope you won't mind if I point out what I see as a few issues with the thinking behind it which run counter to my understanding of optimal multichannel surround microphone arrays- either those intended to be symmetrically uniform across all (horizontal) directions, or those intended for optimal recording and reproduction of music on asymmetrical playback arrays such as more common home cinema setups.
Perhaps somewhat similar to what you are doing, in that they are essentially combinations of front and back facing 2-channel stereo microphone arrays, are setups commonly used for recording location sound and ambiences. The sound from these is immersive with 4-channel playback and translates well to non-surround 2-channel playback systems. Implemented with attention to the spacing and angle relationship between all four microphone pairs, these are essentially 4-channel quadraphonic surround arrays along the lines of the standard 4-channel ambience recording techniques
IRT-cross and
ORTF-surround. The Schoeps implementations of those are informative: the
IRT-Cross is either four cardioids with a 90 degree angle between each, spaced either 200mm or more optimally 250mm from the next, or four supercardioids, 90 degrees apart, spaced 140mm apart.
ORTF-Surround places two 2-channel ORFT arrays back to back(110 degrees, 7cm), with the longer distance between the two determined by the 70 degree angle between each mic pair on either side. These are somewhat similar to your technique of a forward-facing ORTF plus a rearward-facing Mid/Side pair, optimally spaced and arranged with regards to each other.
Notice that in each of these three cases, the spacing between microphones is optimized with respect to the microphone pickup pattern, angle between mics, and the behaviour of the adjacent 2-channel array to either side of the sector in question. This is sort of in the Michael Williams camp of multichannel microphone array design, and is a logical extension of his Stereo Zoom approach which relates pattern, angle and spacing to a
Stereo Recording Angle which correlates to what images between the playback speakers. In his mulitchannel arrays, the edges of the Stereo Recording Angle of each sector is designed to mate up seamlessly with it's neighbors on either side. Each microphone is a member of two stereo pairs, of the four total pairs which makeup the entire the array- the sector to the microphone's left, and the sector to it's right.
That can be further extended to more than four channels by introducing additional microphones, playback channels and sectors, but the spacing and angle between each pair then needs to change as well. It does not work (optimally) to simply add additional microphones on the circle described by the placement of the existing microphones. At the very least, the circle must increase in size as the channel count rises.Basically with the addition of each channel, either the microphone patterns need to become tighter, and/or the the spacing between each of them needs to be increased. Otherwise there is too much overlap between sectors. Note that with four microphones working together as an array this way in one plane, using the most unidirectional 1-st order pickup pattern available (supercardioid), the optimal spacing between each as realized in the Schoeps mount for this is 140mm. In other words,
not coincident.That's sort of an indirect way of understanding how any coincident 1-st order microphone array (Double Mid/side, an ambisonic mic, or any other with mic spacings which attempt 0mm) is limited to synthesizing no more than 3
optimally interacting virtual microphones in the horizontal plane. Blumlein requires
2 figure-8 microphones crossed at 90 degrees, which is why ambisonic mics can do Blumlein so well. First order coincident arrays can virtualize 3 supercardioids arranged at equal 120 degree increments just as well, but that's the upper limit for the number of optimally interacting virtual microphones in any one plane, imposed by their 1st order capabilities.
Topic: New inexpensive ambisonic mic out of India (Brahma) (Read 11672 times)