Topic: Oddball microphone technique (OMT) - part 3

[checht]

And now for something completely different:

For OMT4, how far ahead of the split omnis should the center coincident pair be? I've seen 9" stated a few times, wondering if there is consensus on a distance or a formula to calculate it?

TIA

[Gutbucket]

^ Hey there checht.  Precise forward spacing of the center position is probably less important with wide omnis than a more near-spaced directional mic triplet, but between 6" and 9" forward seems to be optimal depending on the L/R spacing. 

That's based on playing around with Image Assistant, which is able to model imaging across 3 mic arrangements, and also to some extent the Michael Williams multichannel array designs (the Stereo Zoom guy).


Alright, this is probably a good opportunity to let you all take a look at something I was working on last summer, but don't feel is really ready for full release yet, which I lost steam on and haven't really revisited since.  Attached is a PDF which covers the extension of the Improved PAS Table from 2 to 3-microphone positions.  A stereo arrangement of 3 positions will consist of an L/R pair of mics with either a single mic, or a coincident pair in the center.  The basic idea is the same as with 2 ch improved PAS.  The wide pair is pointed directly at the PA speakers, and the ideal spacing between that pair (in regards to imaging, Stereo Zoom like) is determined by the PAS angle.  The difference in comparison to 2-channel Improved PAS is that there is now a microphone position in the center interacting with the other two, which changes things.  Its OMT3 or OMT4, and because of that is going to be applicable to more tapers than any of the higher channel count OMTs. 

The other difference is that to make it easier to read, rather than providing a list of angles and spacings in a table, I drew this up more like an OMT PDF using diagrams showing dimensions and angles. The first two pages of the PDF are introduction and overview.  The actual diagrams are all on page 3.

This really needs verification by testing before full release into the wild, but the basic idea behind it is sound. At wider PAS angles corresponding to more narrow L/R spacings, I suspect it will work better with more highly directional mics than omnis. Take the actual spacing dimensions as preliminary for now.  If anyone tries this out please let me know how it works for you!

[Gutbucket]

I hope that my comment was taken how it was offered - as encouragement towards a possible learning experience for everyone following along - and not criticism. 

We all learn differently. I learn best through critical listening and comparison.

Thanks. I totally get that and agree!

[al w.]

Alright, this is probably a good opportunity to let you all take a look at something I was working on last summer, but don't feel is really ready for full release yet, which I lost steam on and haven't really revisited since.  Attached is a PDF which covers the extension of the Improved PAS Table from 2 to 3-microphone positions.  A stereo arrangement of 3 positions will consist of an L/R pair of mics with either a single mic, or a coincident pair in the center.  The basic idea is the same as with 2 ch improved PAS.  The wide pair is pointed directly at the PA speakers, and the ideal spacing between that pair is determined by the PAS angle.  The difference in comparison to 2-channel Improved PAS is that there is now a microphone position in the center interacting with the other two, which changes things.  Its OMT4, and because of that is going to be applicable to more tapers than any of the higher channel count OMTs. 

The other difference is that to make it easier to read, rather than providing a list of angles and spacings in a table, I drew this up more like an OMT PDF using diagrams showing dimensions and angles. The first two pages of the PDF are introduction and overview.  The actual diagrams are all on page 3.

This really needs verification by testing before full release into the wild, but the basic idea behind it is sound. At wider PAS angles corresponding to more narrow L/R spacings, I suspect it will work better with more highly directional mics than omnis. Take the actual spacing dimensions as preliminary for now.  If anyone tries this out please let me know how it works for you!

Oooh, this looks awesome... can't wait to try it out!

[Gutbucket]

Thanks Al. It will be very helpful to hear your thoughts when you do.. and any recordings made using that approach that you can make available.

Last spring I was working steadily on revising all the OMT PDFs, which is well over due and still not complete.  At some point early last summer I shifted that effort over to this 3-point PAS project because I felt it would be more helpful to far more tapers, mostly because it requires only 3 or 4 mic channels.  Unfortunately I've not had time to return to either project since.

It took a lot of gong back and forth changing variables and checking different things in Image Assistant, and there are some assumptions in it.  I massaged the data somewhat to try and keep the spacings toward the less-wide side of things to hopefully make it more practical and attractive to tapers.  As mentioned, best to take the suggested spacing dimensions as preliminary, particularly at extremely narrow and wide PAS angles.  However I feel its verified as correct at PAS angles around +/-45°, since the values for that are close to what I've used for OMT4 indoors with good results (done in the same venue as the Oteil & Friends recording), and is very close to the central triplet portion of my current OMT8 array.  Specifically for a PAS angle of +/-45° it calls for a total L/R spacing of 25" to 28" apart, and placement of the center mic or pair forward of that by 7" to 8".  That equates to a PAS angle of 90° total, fitting into a Stereo Recording Angle of 120° overall (its the 4th setup down from the top of page 3).

Also as mentioned in the previous post, this can be seen as "Improved 3-4 channel OMT", and I foresee it converging with the rest of OMT at some point as a way of optimizing the directional mic triplet at the center of the array - the four channels that form the center of most higher channel count OMT arrays.

..so in that way, not really something completely different after all!

[Gutbucket]

We're now past 25 pages in this thread.   I'll leave it open through the weekend at least to allow some time for discussion of the freshly posted stuff here, where its most accessible and appropriate, then close it up and start a new one.  Plenty more thoughts about all this for the new thread..

[Gutbucket]

New thread continued here: Oddball microphone technique (OMT) - part 4

Edit- Looks like the option for the thread-starter to lock the tread has been removed.  Please post in the new thread. Thanks.  I'll ask the admin to lock this one up.