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ORTF + Central Omni
Old 3 weeks ago
  #1
Here for the gear
 

ORTF + Central Omni

Hi folks,

I've encountered something a bit unusual recently, I'm keen to hear your thoughts:

So - a chap who records all the major orchestras and ensembles in my city uses a rather odd configuration (described below) : I asked him about it a while back, and he swears by it as being the most accurate and 'perfect' (his word) method for recording large ensembles.

What he does is to have an ORTF pair (well, almost - they're supercardioid), and an omni in the middle (elevated by a couple of inches so there's room for the physical mics and cabling). Then both ORTF mics are EQ'd with a high-pass, and the omni with a low pass - the crossover is somewhere about 200-250Hz. I've attached a picture of the setup - it's the only one I could find - for context the mics are perhaps 6m (20ft) from the stage.

I do like his logic, it makes a lot of sense as we cannot localise bass frequencies- if they're being lost anyway by the cardioid rolloff why not use an omni and distribute equally left/right. It's similar in concept to the idea of having a central subwoofer and L/R speakers in a hi-fi system. But, the cynic in me can't help wondering, are there any disadvantages to this type of setup that other techniques would fix?

Thoughts? If it's fantastic, why has it not been used earlier? A similar sort of thing was described in this thread in the context of mono compatibility but there doesn't seem to be a lot of discussion on this specific idea... I'd be keen to hear what you think

Cheers!
Freddo
Attached Thumbnails
ORTF + Central Omni-threewaymics.jpg  
Old 3 weeks ago
  #2
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Following the Sengpiel visualisation it woud make perfect sense for the orchestral angle of 70 degrees, what means he has to e quite far from the ensemble I think. From the photo it's not easy to define the distance and hight of the mics, but it looks like it's not far from the orchestra and not high above. Or is it?
Old 3 weeks ago
  #3
Gear Addict
Well, it's very logical, and makes perfect sense, especially to have just one mount and stand. It just proves there are many ways to skin a cat, and there are no cast iron rules. It's a very economical solution.
Old 3 weeks ago
  #4
Im not crazy about the idea, but if I was going to execute it, I’m not sure I see the need for a high pass on the ORTF. The mics have a predictable rolloff (which is what you’re using the Omni to compensate for in the first place).... I think I’d just low pass the Omni to match that. But I’ve never tried this before so, maybe that’s where he started, and found this worked better.
Old 3 weeks ago
  #5
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It seems to be a 'simplistic' variant on the Faulkner/boojum/norman "4 mics on a bar array", except that instead of 2 more widely spaced omnis, you have one in the centre.

The high and low passing is a crude method of bass 'steering'...ie trying to pull it towards the centre. Which is ok if you;'re going to make a vinyl record of it, and you want the bass to be summed to the centre to stop needle excursion jumping out of the groove....however, I doubt this is his modus operandum

So, in an orchestral recording, I'd expect the double basses to make a dramatic jump from far right to centre every time their contribution to the mix becomes audible, and I'm not sure that's something I'd appreciate hearing ?

I'd rather hear a non-passed recording, with the cardioids and and omni both operating full range...at least as an A/B comparison with his 'passed' version.

I suspect if you simply want the single centre omni to convey a sense of weight and spaciousness (at precisely the zone where the cardioids typically fall short in this regard), he'd be better off making the crossover point at around 80-120Hz instead ?

The theory about subwoofers is that you can place them anywhere in a domestic listening room situation, as long as the crossover (with respect to the satellites) doesn't initiate the ear/brain's ability to localize them...and I'd guess that his 200-250 would be a bit too high to allow localizing of the positional source to go un-noticed ?

This is to be taken in the context of the 3 mics being used to record the entire ensemble/orchestra...and not just as a section/spot mic array, where any such spatial anomalies would be far less evident (if at all)

I wonder if the net result would be all that different than having a M-S array, with an omni as the mid mic .....?

Last edited by studer58; 3 weeks ago at 06:45 PM..
Old 3 weeks ago
  #6
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using filters is not 'crude' but the most simple and elegant way of cutting what's not needed or wanted!

i haven't been using this exact setup and can't see much of a point using it for celli (as they are not really full range) or actually on any spot/section mics but i've quite frequently been using an additional blm or pzm for orchestra, organ, piano or marimba and even harp recording in 2.1 or 5.1 even though mixes might be stereo or 5.0.

i wouldn't necessarily go as low/high with filters and imo they do not need to be symmetrical but i'm using filters on my desk's default setting across almost all channels: quite helpful for more precise localisation within the stereo or surround field...
Old 3 weeks ago
  #7
I believe Da-Hong Seetoo (who posts here occasionally) has mentioned something about doing something like this on occasion to help the bass extension on an ORTF pair. He a highly-decorated classical engineer, and someone who definitely worth paying attention to, so I'd not dismiss the validity of the idea.

Maybe he'll see the post and chime in.
Old 3 weeks ago
  #8
Following the high-pass / low-pass: when I recorded a Fiocco requiem disc (on which I commented here), I used two main pairs with musicians all around:
one DPA4006TL with APE50 aimed at the strings, but the double bass (centered behind the main pair) was slightly unfocussed.
I added a second pair of Schoeps MK2 aimed slightly closer to the double bass and prefered that pickup.
I blended both pairs with high-pass on the DPA and low-pass on the Schoeps.

Call it a variant on spot-miking, but it worked like a charm and I got the best of both worlds.
Old 3 weeks ago
  #9
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There's no doubt that the strengths of both ORTF and pressure omnis can complement each other, my main reservation was the potential for pulling the lowest bass into the centre....but if this turned out not to happen at all (or was inaudible as an artifact) then I don't see any reason not to employ it as a valid approach !
Old 3 weeks ago
  #10
Quote:
Originally Posted by king2070lplaya View Post
Im not crazy about the idea, but if I was going to execute it, I’m not sure I see the need for a high pass on the ORTF. The mics have a predictable rolloff (which is what you’re using the Omni to compensate for in the first place).... I think I’d just low pass the Omni to match that. But I’ve never tried this before so, maybe that’s where he started, and found this worked better.
I've experimented with this somewhat but only with a pair of wider omnis on the same plane (as in the "Faulkner array"). I still ended up blending both pairs without filters. What I keep forgetting to try is ORTF using a Strauss packet and using a low-pass on the omnis that counteracts the natural rolloff of the cardioid/sub-cardioid. Still feels a bit gimmicky, as I tend to always use widely-spaced omni flanks for large ensembles - and I get my LF response from there.

Also, every time I listen to nice omnis with a low-pass filter, I get sad. Such a waste of a great mic.
Old 3 weeks ago
  #11
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Thanks for your replies!

Quote:
Originally Posted by Stradivariusz View Post
...it looks like it's not far from the orchestra and not high above. Or is it?
Yep, I think it's about 6m back from the orchestra, and around the head height of the conductor.

Quote:
Originally Posted by studer58 View Post
I'd rather hear a non-passed recording, with the cardioids and and omni both operating full range...at least as an A/B comparison with his 'passed' version.

I suspect if you simply want the single centre omni to convey a sense of weight and spaciousness (at precisely the zone where the cardioids typically fall short in this regard), he'd be better off making the crossover point at around 80-120Hz instead ? ... ...and I'd guess that his 200-250 would be a bit too high to allow localizing of the positional source to go un-noticed?
I'll be trying this out with a small chamber orchestra over the weekend, and will definitely try a few different crossover points and see what happens! Will post reference recordings of all the mics and the 'crossed-over' versions for comparison - watch this space.
Re MS: I don't have any suitable mics in my locker but will see if I can scrounge any before then to compare.

Quote:
Originally Posted by studer58 View Post
The high and low passing is a crude method of bass 'steering'...ie trying to pull it towards the centre. Which is ok if you;'re going to make a vinyl record of it, and you want the bass to be summed to the centre to stop needle excursion jumping out of the groove....however, I doubt this is his modus operandum

So, in an orchestral recording, I'd expect the double basses to make a dramatic jump from far right to centre every time their contribution to the mix becomes audible, and I'm not sure that's something I'd appreciate hearing?
Re the bass 'steering' - please do correct me if I'm wrong, but won't just about any technique have a fairly centred bass 'image'? Unless there's a huge difference in L/R levels (which won't be the case in most large venues) they should be picked up fairly evenly by most mics.

Listening to a few of his recordings, the basses stay pretty well off to the right: even though their fundamental and first couple of harmonics will be centred, all of the timbral information is in the upper frequencies above 250Hz, which (as far as I can tell) is the part that we latch on to for localisation.

Quote:
Originally Posted by king2070lplaya View Post
...I’m not sure I see the need for a high pass on the ORTF. The mics have a predictable rolloff (which is what you’re using the Omni to compensate for in the first place).... I think I’d just low pass the Omni to match that. But I’ve never tried this before so, maybe that’s where he started, and found this worked better.
Good point - I'll try it both ways over the weekend - with an omni EQ to match the ORTF rolloff, and with a few 'higher' steps. Looks like there are going to be quite a few variants of this mix once it happens on Saturday!
Old 3 weeks ago
  #12
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Quote:
Originally Posted by Freddo View Post

Yep, I think it's about 6m back from the orchestra, and around the head height of the conductor.

...

Good point - I'll try it both ways over the weekend - with an omni EQ to match the ORTF rolloff, and with a few 'higher' steps. Looks like there are going to be quite a few variants of this mix once it happens on Saturday!
Wow...that's not what the theory would say. Distance seems ok, but the height not really. Normally you want to be higher to get the back of th orchestra better balanced with the front rows, but probably in this case it worked out great!

Will be very curious for your recordings, do that!
Old 2 weeks ago
  #13
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Quote:
Originally Posted by Stradivariusz View Post
Wow...that's not what the theory would say. Distance seems ok, but the height not really. Normally you want to be higher to get the back of th orchestra better balanced with the front rows, but probably in this case it worked out great!

Will be very curious for your recordings, do that!
I hear you! It seems very low - if I was the one doing the recording for the orchestra in the picture I'd have put the mics in line with the percussion or just above (who are about 4m higher out of shot, on a tiered stage). Listening to the audio, the winds are 'fuzzy' sounding, as are the percussion while the soloist, first violins and celli sound very crisp - which is probably a function of height rather than the three-way setup.

Last edited by Freddo; 2 weeks ago at 07:21 AM.. Reason: Modifying a sentence for clarity.
Old 2 weeks ago
  #14
Quote:
Originally Posted by Freddo View Post
Hi folks,

I've encountered something a bit unusual recently, I'm keen to hear your thoughts:

So - a chap who records all the major orchestras and ensembles in my city uses a rather odd configuration (described below) : I asked him about it a while back, and he swears by it as being the most accurate and 'perfect' (his word) method for recording large ensembles.

What he does is to have an ORTF pair (well, almost - they're supercardioid), and an omni in the middle (elevated by a couple of inches so there's room for the physical mics and cabling). Then both ORTF mics are EQ'd with a high-pass, and the omni with a low pass - the crossover is somewhere about 200-250Hz. I've attached a picture of the setup - it's the only one I could find - for context the mics are perhaps 6m (20ft) from the stage.

I do like his logic, it makes a lot of sense as we cannot localise bass frequencies- if they're being lost anyway by the cardioid rolloff why not use an omni and distribute equally left/right. It's similar in concept to the idea of having a central subwoofer and L/R speakers in a hi-fi system. But, the cynic in me can't help wondering, are there any disadvantages to this type of setup that other techniques would fix?

Thoughts? If it's fantastic, why has it not been used earlier? A similar sort of thing was described in this thread in the context of mono compatibility but there doesn't seem to be a lot of discussion on this specific idea... I'd be keen to hear what you think

Cheers!
Freddo

What happens to the bass frequencies and location if the basses/tuba etc are seated to the right - I wonder where they end up in the final image?

Without trying this out, and certainly not wanting to be a damp rag, it seems to me that there are likely to be better imaging solutions than this involving M&S with central omni and shuffling.
Old 2 weeks ago
  #15
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Quote:
Originally Posted by Geoff Poulton View Post
What happens to the bass frequencies and location if the basses/tuba etc are seated to the right - I wonder where they end up in the final image?
Most of the localising information that we receive is in the upper partials and the 'air' - so the timbre of the instruments; what that means is that even though the lowest few frequencies will inevitably be centred, the higher frequencies within those sounds will appear to come from the right (or from wherever the players are seated), which are the ones we use to pinpoint location. That's the theory at least
It's similar to the idea of playing an orchestral recording through a 2.1 system (if my terminology is correct - L/R + subwoofer), you still have a 'true' image as the detail in the sound comes from the L/R speakers, or in this case the ORTF pair.
Old 2 weeks ago
  #16
I'm not cutting vinyl, and I'd much rather use a widely-spaced pair of omnis in the interest of LF decorrelation. If they are on the same bar as the ORTF, then a cross-over makes sense, but you don't need to do it if they are placed farther back in the hall.

If one needs more definition on bass or tuba, then a co-located omni isn't going to do it. That's what spot mics are for. The problem in many halls is that incrreased reverberance and low frequencies shortens the "critical distance" or "room radius". An example is Schubert's piano quintet in A: You can occasionally get everything perfect with a single pair, except that the double bass sounds like it's playing off stage. More commonly, you have that problem in a quartet when the cellist sits on the right.

David L. Rick
Seventh String Recording
Old 2 weeks ago
  #17
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For the purpose of experimentation, this pair (OM-1 plus Superlux ORTF stereo mic with 4 mini cable ties) should suffice?
Attached Thumbnails
ORTF + Central Omni-2666e18a-1c9e-493f-99b9-57786655fb9b.jpg  

Last edited by studer58; 2 weeks ago at 03:04 PM..
Old 2 weeks ago
  #18
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adding an omni to an ortf will get you more ooomph in any situation but as david mentioned, it may not necessarily be the best option: in my experience, it's very rarely the case that one actually needs more lf level but more often, it's about better definition (and/or localisation) - for this, one better uses spot mics.



p.s. a wider spacing of mics does affect phase correlation and hence the stereo (or surround) width but localisation of lf within the soundfield is only frequency dependent and gets worse the lower we go (as can easily be experienced by listening to an orchestra or to a playback on a speaker system with a single subwoofer)
Old 2 weeks ago
  #19
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Quote:
Originally Posted by studer58 View Post
For the purpose of experimentation, this pair (OM-1 plus Superlux ORTF stereo mic with 4 mini cable ties) should suffice?
I made a concert recording using this mic configuration tonight, using mic array as pictured 2 posts earlier than this ie post #17 (ORTF plus central OM-1).... without any spot mics to confuse the situation.

Omni is low passed and ORTF high passed.... around 160 Hz in each case.
ORTF pair : 0dB OM-1: -7.7dB
No added reverb, just the church sound au naturel

I'll post a few audio segments edited together, to display how it captured the acoustic and performance ...see below
Attached Files

Handel- Lute concerto- Bach edit.mp3 (11.31 MB, 849 views)


Last edited by studer58; 2 weeks ago at 04:06 PM..
Old 2 weeks ago
  #20
The imagery sounds correct and defined.

What did you think about the result yourself, from what you heard of the original at the concert recording?

Are the positions of the instruments represented well in the recording?
Old 2 weeks ago
  #21
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Quote:
Originally Posted by Geoff Poulton View Post
The imagery sounds correct and defined.

What did you think about the result yourself, from what you heard of the original at the concert recording?

Are the positions of the instruments represented well in the recording?
Without any frequency filtering (hi/lo passing) there are definite limits to how far the omni mic can be pushed, before it starts both accentuating the room ambience AND pulling the sound-field into the centre, in mono-ish fashion.

In this regard it's no different to the balancing act necessary when balancing mid and side components of an M-S recording.

With the hi/lo passing, it's possible to push the omni a little higher, before it becomes noticeable as the '3rd wheel'

As I've presented it here, it's still a very typical ORTF recording...in a fairly ambient church...hence, one could perhaps have wished for a little more voice (and lute/theorbo) definition, via the use of spot mics. However, I didn't have time to deploy spots...but still, the recording isn't a failure without them ?

Overall, I prefer a spaced pair of omnis over this single, central fella...but it's a workable result nevertheless....with the caveat of not pushing its level too far up in the mix balance.
Old 2 weeks ago
  #22
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Okay - so I made a recording last night; the chamber orchestra didn't have any basses which was a bit disappointing, but they did have a reasonable cello section.

Mics were placed ~3.5m high, about 4-5m away from the orchestra (they wouldn't let me put them any closer for visual reasons). The orchestra was seated flat, so no tiered seating. Acoustic was quite wet and echoey, in a church - see the pic for a 'microphone's eye view' (camera was about 10cm lower than the mics).

In the end I didn't get to do any M/S recording as per posts #5 and #21 :-( . That said, I did manage to have a few spot mics (not in any of these mixes).

I've attached 8 audio files;
  1. ORTF only - as reference
  2. Omni only - as reference
  3. 245Hz crossover
  4. 200Hz crossover
  5. 170Hz crossover
  6. 150Hz crossover
  7. 120Hz crossover
  8. 300Hz crossover

(Mics: Omni - Studio Projects C4 w/omni cap; ORTF: AKG C480B pair w/cardioid cap)
Attached Thumbnails
ORTF + Central Omni-miclocation.jpg  
Attached Files

1 ORTF Only.mp3 (441.1 KB, 658 views)

2 Omni Only.mp3 (441.1 KB, 631 views)

3 Tri-245Hz.mp3 (441.1 KB, 630 views)

4 Tri-200Hz.mp3 (441.1 KB, 624 views)

5 Tri-170Hz.mp3 (441.1 KB, 639 views)

6 Tri-150Hz.mp3 (441.1 KB, 624 views)

7 Tri-120Hz.mp3 (441.1 KB, 640 views)

8 Tri-300Hz.mp3 (441.1 KB, 624 views)

Old 2 weeks ago
  #23
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I don't really understand what you are after here. I like the ORTF just fine, but for me the others just add mono rumble and confusion to the sound while spoiling the feeling of space that is in the-ORTF only file. Have you tried any of the variations of "Boojum-Norman" or center-stress etc. (ORTF+ 2 omnis)?
Old 2 weeks ago
  #24
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Now I'm listening on the laptop what makes no sense at all to check the low end, but is atually quite cool to hear the clarity differences between the versions. Actually how lower the crossover how more precise the sound remains. More omnis in this particular recording gives more blur in the lower register I feel.
Will check some other time on the better tools :D Otherwise it sounds nice!
Old 2 weeks ago
  #25
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Stradivariusz's Avatar
Listening on monitors now.
It's very interesting comparison. I don't like the ORTF pair on its own, sounds sharp and bottom poor. The 300Hz crossover gives less focused sound but due to this somewhat enlarges the ensemble, altough sound is maby to mono in the lows. I think 170Hz is a good compromise. I also like to hear the low bass spread in stereo field I think that using two omnis could give more natural stereo image. Of course this experiment would be even more interesting with at least ONE double bass in the orchestra!
Old 2 weeks ago
  #26
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Quote:
Originally Posted by studer58 View Post
I made a concert recording using this mic configuration tonight, using mic array as pictured 2 posts earlier than this ie post #17 (ORTF plus central OM-1).... without any spot mics to confuse the situation.

Omni is low passed and ORTF high passed.... around 160 Hz in each case.
ORTF pair : 0dB OM-1: -7.7dB
No added reverb, just the church sound au naturel

I'll post a few audio segments edited together, to display how it captured the acoustic and performance ...see below
Nice!
Old 2 weeks ago
  #27
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Quote:
Originally Posted by Stradivariusz View Post
Nice!
Thank you....and just to add confusion to the model, in the last piece (with the soprano, after the lute concerto) the cello was seated directly behind the singer, and then the double bass was located directly behind the cello, all central....so it's one of the rare situations where all the bass frequencies are exactly centred, in the stereo picture

However, if you push up the Omni mic in the blend...remember you are not just increasing the low bass, but you're also simultaneously bringing in more room/space ambience too (it's a package deal you can't escape from !)

You need to make a choice whether you want the Omni mic or the ORTF pair to define the direct/ambience ratio of your recording.
Old 2 weeks ago
  #28
Gear Addict
 

Over the years, I have taught many young engineers about using ORTF+Omni method. I encouraged them to experiment with this configurations to mainly recover some low end. At the times, also to increase some intelligibility to the solo vocalist standing in the middle.

Two weakness of ORTF setup are lack of low end and center intelligibility.

For the low end only, I would add one omni in the middle of ORTF, pointing it up, 90 degrees off the stage, do a low pass at about 140Hz. You push up the omni level until the real low end is in you mix. The actual amount of omni you use, as well as the crossover frequency, crossover sloop has to do with the cardioid used in ORTF, the hall it is in and amount of low end you prefer to be in your mix. I would consult with the published data of the cardioid and do a comparative microphone test yourself in the hall; do a measurement of your cardioid vs an omni. You will immediately see the low end deficiency in cardioid mics. This test could guide you in terms of the omni usage in this case. Lastly, cardioid mic could have very out of phase response at the low end; you may need to experiment with the omni phase. This is a similar issue with speaker crossover design; often you need to reverse the tweeter and woofer phase in relationship to the midrange in order to have them play nice with each other.


All microphones, not just cardioid microphones, suffer from narrowing pickup pattern as the frequency goes up. The bigger the diaphragm the worse the problem. There is always a high frequency hole in the middle of ORTF setup regardless what cardioid microphones you use. Most of microphones will have high end being 10dB or more lower output than the lower frequency at 60 degree off axis, which is what ORTF calls for, 60 degree off center. You need a microphone pointing in the middle to recover that hole. You could use the same omni for low end to help with the high frequency as well. In this case, I would have the omni pointing at the sound source, split the signal into two subgroups, one with lowpass filter and one with high pass filter, both panned to center. The high pass filter would be something like 5000-8000Hz. The level needed for high end recovery usually is much less than what is needed for the low end. Don’t worry about it being omni since all microphones become cardioid at that frequency. Again, consult with the cardioid mic data to aid you with center omni usage. Although, absolute phase also is at play here but it is nowhere near as audible because the wavelength is so short here.


This ORTF+Omni method is a very cheap way to experiment, especially if the goal is only to extend the low end. Any cheap omni can be used here; the low pass filter gets rid of all the audible noise in the omni.



Best regards,

Da-Hong
Old 2 weeks ago
  #29
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Quote:
Originally Posted by Freddo View Post

I've attached 8 audio files;
  1. ORTF only - as reference
  2. Omni only - as reference
  3. 245Hz crossover
  4. 200Hz crossover
  5. 170Hz crossover
  6. 150Hz crossover
  7. 120Hz crossover
  8. 300Hz crossover

(Mics: Omni - Studio Projects C4 w/omni cap; ORTF: AKG C480B pair w/cardioid cap)
#8 sounds the nicest, has a relaxed elegance;
Old 2 weeks ago
  #30
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The LPF at 120Hz sounds best to me. It has the proper distribution of frequencies that you'd hear in the room.

I'd even go lower to 100Hz and below.

LPFs set higher than this are smearing the clarity of the mids.

The 'ORTF only' pair sounds good but unnaturally thin to me. Has it been high passed or is this the natural response of the mics?
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