Topic: Suggestions for a center channel omni to run into my MixPre3

[edtyre]

Looking for something to run into channel #3 on my MixPre3
For open taping i might as well run 3 channels. I'll be running Schoeps MK4V's
for the outside pair. Cap + body or fixed. What should i do? AT, AKG's i was thinking.

[kindms]

Looking for something to run into channel #3 on my MixPre3
For open taping i might as well run 3 channels. I'll be running Schoeps MK4V's
for the outside pair. Cap + body or fixed. What should i do? AT, AKG's i was thinking.

ed

we have liked a card as center, fig 8 is fun to play with as well if you have one

[kindms]

ah didnt realize the MK4V were cards

interesting setup

[noahbickart]

Mk22.

[gewwang]

I'm going to run subcards(dpa4028) as the stereo pair and card(dpa4023) as the mono.

[Gutbucket]

Think about the primary thing you wish to achieve from the addition of a third channel.  That could be any one or a combination of several different things:

A more extended bass response
A solid center stereo image, which preferably doesn't sacrifice image width
A focused center with less ambient material in the middle compared with the outer edges of the playback image
An increased sense of depth and/or space without the loss of forward presence.

[Gutbucket]

A single center omni will extend bass response of a pair of cardioids, and that's the first thing most folks think about when considering adding an omni. If only interested in that bass extension aspect, a low-pass filter on the omni will keep your near-spaced cardioid configuration from being affected above the low bass region and preserves the stereo qualities you're familiar with using a standard near-spaced stereo microphone configuration.

If the center omni is not low-pass filtered, imaging, recording timbre, and the ambient pickup will also be altered along with bass extension.  That may be good or not good, depending.  The recording will sound more ambient and reverberant by making the overall pickup more omnidirectional.  Yet at the same time the stereo imaging will become less wide because of the introduction of additional common information to both channels.  And their will be more comb-filtering due to the close proximity (without true coincidencce) of having three microphone locations interacting with each other, with smaller mics spacings between the center and side mics than what one had between just left and right in the two-channel config.

To offset the reduced imaging width and to reduce the comb-filtering and it's associated timbrel effects, increase the spacing and/or open up the angle between the Left and Right mics.  How much depends on what center channel pickup pattern you've chosen and how much center channel level you are going to use (along with other things such as the particularities of the venue, sound system, and your setup location). I strongly recommend using more spacing whenever possible when adding a full range center mic between left and right microphones, partly because more angle between mics is not enough on it's own and is quite often not wanted anyway. In terms of just imaging, if its absolutely necessary to stick with a standard near-spaced two-channel mic configuration spacing between left and right microphones, a full 180 degrees between left and right mics is not only reasonable, it's not quite enough!   If you can't space the left/right stereo pair mics more than you would for a 2-channel config, consider raising or lowering the omni by 2' or 3' on the same stand.  That might sound like an odd suggestion, but the idea is to introduce sufficient distance between the three points in space to decorellate the reverberance sufficiently at medium and high frequencies (since we are using no low pass, remember?).  Sufficient spacing reduces the comb-filtering timbrel affects and produces a more open sounding reverberant pickup.

[Gutbucket]

It may be simpler to first think of the implications of using three identical cardioids, and extend that case by substituting an omni for the center cardioid.   

The arrangement of three cardioids can be thought of as two standard near-spaced stereo configurations which happen to share one microphone in the center.  The two stereo near-spaced configs are ideally joined seamlessly along the shared central edge without either too much pattern gap or too much overlap.  Imagine two DIN configurations (pair of cardioids spaced 20cm / angled 90-degrees) next to each other sharing a center mic.  That positions the left and right mics 28cm apart, pointing 180 degrees away from each other.  In terms of stereo imaging, that's a reasonable 3-mic arrangement when all three channels are used at the same level, and believe it or not, it's often appropriate outdoors. Don't want to point your left and right microphones at the side-walls? Using less angle between mics ideally means introducing more spacing between them to compensate for reducing the angles.


Fortunately we also gain a good degree of flexibility by introducing a third channel.  So we can vary the level of the 3rd channel against the other two to find and appropriate fit even if the configuration isn't perfectly correct to start with - something we can't do when limited to 2-channels.  We can also EQ the center mic differently from the left/right pair if we want to mess with doing so, which is a super powerful tool, especially when using different pickup patterns for left/right and center, as it can help achieve a smooth blend of image and timbre across the soundstage.

[Gutbucket]

I'm going to run subcards(dpa4028) as the stereo pair and card(dpa4023) as the mono.

I like this idea because it's an interesting adaptation of spaced omnis with a directional center mic.  In terms of imaging, the subcards need not be spaced as far as the omnis if pointed 180 degrees apart. Or space them as far as omnis and limit reverberance and ambient pickup somewhat by pointing them forwards.

we have liked a card as center, fig 8 is fun to play with as well if you have one

An 8 in the center sort of does something similar but from the inside out, allowing a somewhat narrower omni spacing because the center pickup is more narrowly focused, so there is less narrowing pattern overlap.


Bass extension is most related to pickup pattern. The stereo quality of the bass is going to be most effected by the spacing between the mics regardless of their pattern.  The imaging of the midrange and above is a combination of pattern, spacing and mic angle.

[Gutbucket]

What to do when you can't use more spacing than your normal stereo config, but still want to use that center mic?

Low pass a center omni, or point the center mic directly backwards regardless of it's pattern!

Pointing it backwards produces the maximum achievable angle difference between the center mic and the otherwise too narrow-spaced stereo pair.  Even an omni would benefit from this by providing mixdown control over ambient "air" at high frequencies. Bring up the center with the mids and highs cut to reinforce as much low bass as you want, then bring up some highs in that channel for open ambient "air" while leaving the midrange scooped to reduce boominess and reverberance.

A wider pattern like subcardioid will provide somewhat more extended bass, as well as somewhat more user control over the  the room ambience, because it is picking up less direct sound from in front.  A rear-facing cardioid or supercard probably won't extend the bass any, but provides maximum ambience control as it maximally excludes the direct sound from in front as much as possible.  EQ may not be strictly necessary to reap the benefit of a rear-facing mic, but is very helpful when doing this.

[Gutbucket]

And finally, for anyone using a single center cardioid or supercard in between subcards or omnis, consider using an X/Y or M/S pair in the center instead, as long as you have the mics to do it.  Most recording channels come in even numbers, so running 4 channels isn't much more hassle than running 3.  A coincident stereo center keeps things simple acoustically - there are still only 3-points in space with respect to the comb-filtering.  The nice thing is that it provides imaging flexibility.  You can control the smooth image blend between the sharp-imaging coincident center pair out into the ambient bed of the spaced pair, not only by simply adjusting center level (and EQ if you do that), but also with with center panning and stereo width adjustment on the coincident pair. 

I suggest keeping the center X/Y angle relatively narrow to retain a good forward focus and center clarity.  You'll get plenty of overall image width from the spaced pair.  Keeping the X/Y pair angled narrowly limits ambient pickup in the center and also effectively acts as a PAS center config so you get maximum PA clarity from your recording location.  Dialing in the perfect amount of coincident center width afterwards simply snaps everything into place.  Its easy and exciting to hear that happen.

[jcable77]

Lee if you wrote a book I'd buy multiple copies.

[tom the taper]

how bout a mk3.  I got 1 extra.

[edtyre]

how bout a mk3.  I got 1 extra.

Thanks Tom, check your messages :-)

[edtyre]

Thanks Lee for all the info brother. I'm going to try both facing backwards and running a few feet higher than the cards.
Having the additional channel to play with cannot hurt anything, just take more time 

I bought an AT 4022 omni

[Gutbucket]

Have fun with it Ed, and thanks for the good excuse!

[Gutbucket]

Hiya Ed,

If interested, I just posted more details on the "why" behind the idea about mounting a non-low-passed center channel omni significantly higher or lower than your stereo pair here- Relationships between spaced omnis and center mic

Warning though, that post along with the rest of my contributions to that thread are pretty much TL:DR for most.

[edtyre]

Here's an example of a pair of forward facing MK4V's along with a rear facing AT 4022 Omni panned center and mixed into the two stereo tracks.
I didn't track out the audio just used it in this video my friend shot.

https://www.youtube.com/watch?v=PtGXNd_EXY4

[heathen]

Sounds great, at least IMO.  It would be cool to hear a comparison clip (even if it's just a minute or so) of just the Schoeps vs. the Schoeps + AT...if you're up to the effort of that.

[edtyre]

Sounds great, at least IMO.  It would be cool to hear a comparison clip (even if it's just a minute or so) of just the Schoeps vs. the Schoeps + AT...if you're up to the effort of that.

Sure, here
https://www.dropbox.com/s/avbgo5u143f3b7x/Archive.zip?dl=0

[Gutbucket]

Hi Ed,

If you end up using that setup regularly, and aren't averse to EQ, you might play around with EQing just the rear-facing omni channel with a curve resembling the equal loudness curve (boosted bass, deeply scooped mids with maximum reduction of the upper mids, boosted highs).  That helps the omni contribute what it's best at (bass extension down low + rear facing ambient air and openess up top) while keeping it out of the way of the MK4 stereo pair through the midrange, which is where the addition of the omni tends to collapse image width and sometimes muddy things up somewhat.    Also helps to minimize the listener's attention being pulled toward the audience members behind the recording position acting up or talking loudly, without overall reduction of omni level.  That way, you may find you are able to dial in more omni level without problems if you want to do so.  Once you find a general curve that works, you can apply the same curve to the rear facing omni each time, only tweaking it further if necessary.  I find this can be quite useful with my own rear facing ambiance mics.

[heathen]

A single center omni will extend bass response of a pair of cardioids, and that's the first thing most folks think about when considering adding an omni. If only interested in that bass extension aspect, a low-pass filter on the omni will keep your near-spaced cardioid configuration from being affected above the low bass region and preserves the stereo qualities you're familiar with using a standard near-spaced stereo microphone configuration.

Sorry to bump this old-ish thread but I'm thinking about trying a 3-channel setup with a card pair and an omni.  The omni will be used solely to supplement the low end, so I will likely use a low-pass filter in post.  My question is, when using an omni for this purpose and with a low-pass filter, does it matter at all where the omni is placed with respect to the cards (within reason, of course)?  Are there no phase issues or the like at the very low frequencies?  And what if the omni is just clamped lower on the stand than where the cards are at?

Another thing I'm thinking about trying is similar to a Straus Packet, with a pair of mini omnis set up right under the cards in the same config...but that's for another thread.

[Gutbucket]

Necessary proximity to the cardioids depends on the frequency ranges being summed.  Low frequencies have long wavelengths.  If you are low-passing the omni you can probably put it anywhere on your stand without phase interaction at the crossover point.  And actually if you want to play with using the omni for higher frequency ambience addition as well as extending the bass response of the cardiod pair and don't want to change your cardioid stereo configuration, I can make an argument for mounting the omni as far above or below your cardioid pair as possible (say a few feet or more).  That will intentionally randomize the phase differences of much of the higher frequency ambient pickup, while not affecting the phase of the longer wavelength bass frequencies.

If you are aiming for an overall flat response - with the omni picking up exactly where the response of the cardioids fall off at low frequencies, you'd tailor the omni rolloff to be the inverse of the cardioid's response so the combination sums flat.  But in the real world who's to say flat is what you really want.  The bottom end would probably benefit from EQ anyway.  Mess with the combination afterwards to find what works.

For making a Strauss-packet (what Schoeps Polar-Flex does), you need the mics as coincident as possible so the phase difference is minimal throughout the entire frequency range.

[heathen]

Necessary proximity to the cardioids depends on the frequency ranges being summed.  Low frequencies have long wavelengths.  If you are low-passing the omni you can probably put it anywhere on your stand without phase interaction at the crossover point. 

Not that I'd necessarily do this, but just for the sake of discussion, could the low-passed omni be below head level of the crowd without any ill effects?

My idea with this is just to supplement the bass to account for the slight roll-off of the cards...flat "on paper," but subject to what sounds best in post.

[Gutbucket]

If you are discarding everything from the low mids up then I don't see why not.  That low bass goes right around human sized things. 

You might notice some increase in low bass level as the mic gets closer and closer to the floor.  That's the boundary effect in action, and the boundary layer is relatively deep at very low frequencies.  Ever notice how much louder the very low bass is if you sit on the ground rather than stand in the same place at an outdoor music fest with large subs?   That's the same boundary layer effect thing going on effecting the lowest frequencies.  Similar to how the omni needs to be to more and more coincident to the other mic(s) if you want ever-higher frequencies to remain in-phase, a microphone taking advantage of boundary layer effect needs to be super-close  to the boundary (ideally flush-mounted to it) if it is intended to leverage the boundary effect without combing up to the high-frequency limits of hearing, but the effect is still active at low frequences up to a significant distance from the boundary, which in this case is the ground.

[heathen]

Thanks for the insight as always.  I'll mess with this at some point in the future and hopefully not botch things too badly.