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Author Topic: Ambisonics (general discussion, as related to recording)  (Read 840 times)

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Offline kuba e

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Re: Ambisonics (general discussion, as related to recording)
« Reply #15 on: August 26, 2020, 11:51:56 PM »
I rarely find cases where linear EQ is artifact free or sounds better to my ears.  It has it’s place.  Things like vocal plosives, anything that is a spot fix versus overall, it can be more invisible. 

I make a copy of the track, then apply opposite hpf and low pass filter to each with matched frequency, mix back together.  Really a ‘reverse crossover’ approach. 

Thank you for the correction. I think I tried the linear eq on Mid/Side of close spaced pair and it occurred to me that the linear eq sounds cleaner. But I'm not good for post processing, I can easily overhear some artifacts.

Offline kuba e

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Re: Ambisonics (general discussion, as related to recording)
« Reply #16 on: August 27, 2020, 01:09:01 AM »
Where I think your summary in the other thread goes somewhat astray is in considering 2nd order entirely different from 1st order.  On a fundamental level it is not.  It adds complexity by introducing additional spherical harmonics which do not correspond to native 1st order patterns, but the basis upon which it works in B-format is the same, only further extended.  Where it is very different is in practical implementation (mics arrayed across the surface of a sphere), yet in the same way that a 1st order ambisonic microphone made using 4 cardioids on the faces of a tetrahedron (4 mics on a sphere) differs from a "native array" ambisonic arrangement of 3 figure-8's + omni.[/i]

Thank you Gutbucket. You are right, I didn't write it well. The basis of all ambisonic is the same - everything is based on measuring acoustic pressure and pressure gradient.

Maybe it's better to say this:
The 1st order ambisonic (3 fig.8+omni, tetrahedron ..) assumes a coincident capsules.
On contrast, the higher order ambisonic (HOA) assumes noncoincident capsules. Therefore, it is possible to obtain higher orders. If we consider the capsules as a coincident, the HOA would not work.

I'll be happy if someone correct me. The following will probably be familiar to most, but I'd like to make sure I'm looking at it correctly.

The principle of the 1st order ambisonic is simple. The B-format can be obtained directly from 3 fig.8+omni or it is obtained by the summing and subtraction the signals of the capsules e.g. at a tetrahedron. Here is an example of the four coincident cardiods at a tetrahedron (for better understanding, take a look at a picture of e.g. TetraMic):
   W (omni) = FrontLeftUp + FrontRightDown + BackLeftDown + BackRightUp
   X (fig.8. in X direction) = (FrontLeftUp + FrontRightDown) - (BackLeftDown + BackRightUp)
   Y (fig.8. in Y direction) = (FrontLeftUp + BackLeftDown) - (FrontRightDown + BackRightUp)
   Z (fig.8. in Z direction) = (FrontLeftUp + BackRightUp) - (FrontRightDown + BackLeftDown)


But HOA is more complicated. Here is B-format matrix calculation. The positions of the capsules are hidden in the matrix:
   B = W ^ -1. (Y ^ t.Y) ^ - 1.Y ^ t.pa

If we want to derive  the 1st order pattern of HOA at least four signals are needed as in the 1st order ambisonic. Probably the signals from all capsules are used in the 1st order pattern for error correction (capsules position, signal). Therefore, the 1st order pattern of HOA should be more accurate than the 1st order ambisonic.
« Last Edit: August 27, 2020, 01:59:08 AM by kuba e »

Offline Gutbucket

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Re: Ambisonics (general discussion, as related to recording)
« Reply #17 on: August 31, 2020, 09:50:14 AM »
^Yes.  A tetrahedron is the simplest non-coincident arrangement of evenly spaced sensors intersecting the surface of a sphere.  Four points are the minimum required to define a 3-dimensional spherical volume.
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Offline Gutbucket

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Re: Ambisonics (general discussion, as related to recording)
« Reply #18 on: August 31, 2020, 01:19:45 PM »
They have a pair of Line Audio CM3's at the center lip of the balcony, essentially rear of the room.

[snip]

I took a native B format array with MKH30's and an MKH20, and had an AB pair of MKH20's with the treble boost on at about 27 inches on the same stand. I moved that around during soundcheck and show to get samples for review. 


I also put a pair of DPA 4060's in boundary mounts about halfway back in the room, mounted 8 feet up on the side walls. 

[snip]

In reviewing the tracks, mixing with the broadcast mix in place, I actually like the native array set cardioid XY90 and pointed to the BACK of the room, because there's a lot of direct PA in anything forward facing.  Opposite of our normal taper problem.  I also like the AB pair for spaciousness, but only when it's way back in the room to diminish the direct PA contribution.

Received the other tracks, pretty interesting to compare.  The distant spaced CM3 (or 4) pair is WAAAAY back in the diffuse field...only another 20 feet maybe from where I was.   You get reverb, but no definition.   The wall mounted 4060's are immediately exciting sounding and the width adds to that, but after awhile I realize there's distracting timing differences on percussive sounds. 

Using the same mics in the native array pair, I went to mid-side.  You can use the figure 8, or the omni, or both to get cardioid.   You can flip polarity on the omni to make cardioid MS turn around 180º and point to the back of the room.  All achievable with ambisonics, but the comparison process is different and thus gives different perspectives on the results.   

By itself, no board feed, Something between cardioid and figure 8 sounded best.  f8 has the most pinpoint center clarity, but it's all in the middle at the exclusion of some other stuff.  Cardioid was good, but a little too ambient still. 

Putting the board feed back in and going back to the ambisonic plugin path, Blumlein started to stand out, but eventually I moved the pattern slightly toward supercardioid to lose a bit of the rear sound.  From there you can start to envision how placing different sets of mics with different patterns more forward or further back might achieve similar results to the varying ambisonic patterns, ignoring the time delay aspect.

This all rings true with my experience leading up to mucking around with what we've been calling OMT arrays.

Especially with regards to surround recording and playback, I found ambience channels most useful when the microphones are placed around the same distance from the stage as the forward-facing array due to time of arrival reasons. With true surround playback as the goal, time-of arrival becomes a multi-dimensional problem which places increased constraints on the use of delay for time correction.  One one cannot simply delay the main pair to achieve sync with more distantly placed ambient microphones without compromising the imaging of reflections and direct arrival sounds that come from directions other than the front.  That means using level differences generated through directional sensitivity rather than distance to achieve needed reduction of front main sound pickup in comparison to the pickup of ambience and non-frontal direct arriving sounds.   

After some additional experimentation I realized that regarless of specific application, I wanted near-spaced or closer time coincidence of all microphones in the front/back orientation and greater time-of-arrival differences in the Left-Right orientation, and all my later OMT setups evolved with this basic geometry in mind. Wider to either side while relatively compact front/rear.  [edit- and this is why I plan to revisit the use of TetraMic or some other ambisonic-microphone between a pair of wide omnis.. or subcardioids.. as the central component of an easily setup basic OMT array]

At one point when I was using just a pair of wide-spaced omnis for ambience, routing the omnis to the surround channels for playback, rather than also including rear-facing directional mics, I empirically determined that in order to achieve optimal surround channel playback levels without pulling too much front image into the rear I needed about 6dB less sensitivity to front-arriving sound in the omni channels (this with outdoor ampitheater recordings that didn't otherwise have any room reverberation issues).  That 6dB reduction figure corresponds with the reduction in sensitivity to the rear of a subcardioid and if I had a pair I would have used them pointed away from the stage to achieve that, hopefully in a sufficiently transparent way and without overly compromising the low frequency extension I was getting from the omnis.  I even started a TS thread around it at the time to discuss which cardioids and subcardioids sound most natural 180-degrees off-axis, through their region of minimal sensitivity.  Since I have no subcards I instead I went back to using sphere attachments on the omnis and pointing those rearward, which is less effective but is at least doing something in that direction.  Granted, all of this was about attempting to get just the right amount of front bleed into the rear channels simply by arrangement of the microphones and adjustment of level alone, without any post-production cross-mixing.  Achieving enough bleed for good seamless imaging and blend all the way around, but not so much as to begin to noticeably pull the front around to the rear.  Essentially extending the way that a single stereo pair overlap each other just enough, yet not too much, around the rest of the surround playback array.   In mixing things for 2-channel output we have more control, and greater channel isolation is useful when it can be achieved, but the timing still has to work right.

Besides the outdoor amphitheater recordings (discussed in past OMT threads), much of this was determined over the course of a few years at a monthly jazz trio gig with a very quiet audience, perhaps not dissimilar to the theater music livestream/podcast/radio show EmRR describes above, but in a somewhat smaller room (PA not used).  There my primary focus was a 2-channel mix for the artist and for CD's handed out to patrons at the next month's show, the secondary focus being exploration of ambient mic'ing, 3-channel stereo and surround recording techniques.  I ended up preferring two methods for room/ambience channels which might be worthwhile trying at the theater above.  My main array usually consisted of a near-spaced L/C/R supercardioid triplet on stage near the stage-lip.  I found the best arrangement for the ambience channels was either a wide spaced pair of DPA 4060 boundary mounted to the front of the short front wall under the stage-lip facing out into the audience, or a pair of cardioids clamped to the lip so as to hang just beneath it and face out into the audience, sometimes wide sometimes near-spaced.  Either way, the placement achieved sufficient reduction of sensitivity to on-stage sound by using the stage lip as a baffle, even when using cardioids facing the audience (their rear-rejection allowing them to be mounted somewhat higher than the omnis, yet still needing some baffling from on-stage sound diffracting around the stage lip).  At the same time the placement produced a time alignment that worked "natively" as those microphones were only a few feet more distant in the front/rear dimension than the main pair.  Boundary mounting omnis on the side or rear walls required time-alignment and picked up too much direct sound from on-stage.  Rear facing cardioids placed further back in the room still required time-alignment in 2-channel mixes and didn't work as well for surround channels for surround playback.

Fortunately this general arrangement of relatively narrow front/back spacing relying on pattern or baffling for front/back differentiation, combined with more side-to-side array spacing working in combination with pattern for left/right differentiation is something I realized I could also manage from a single stand when recording from the audience as well as on-stage.

There are photos and discussion of that in the primary tread about on stage taping. You've likely come across it. I will find it and link here.  Here's a link to start of that particular discussion in the on-stage taping thread. Discussion of room ambience pair options begins on the next page following that link with a crappy photo of a wide spaced DPA 4060 pair taped to the back wall.

Cool to read the quoted portion of the posts above in which I find confirmation of these observations.. including achieving sufficient reduction of direct PA sound in rear-facing channels, and how that can be what is most needed to support an otherwise well-balanced if dry direct-board recording.
« Last Edit: August 31, 2020, 01:38:49 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

 

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