Phase (polarity) changes between different MIR venues

[Bat]

Hello there,

I’ve noticed a phase (correlations) alert using different venues in MIR 3D.

Real recording, two clean electric guitar panned with MIR L/R and slightly Front/Rear.

With Synchron Wide (HOA DnMx and 115°cardio) the correlation meter is “green” most of the time between +0.75 and +1.

With Pernegg (same mic setup) is the opposite: “orange” most of the time between -0.75 and -1.

I tried to randomly flip the polarity of the mics but the changes are minor.

With the ears I can feel this “flip” when I change venues but both sounds good, different but good.

Maybe I can notice a listening that is a little more fatiguing with the "wrong" polarity.

I’ve read that this polarity warning may indicate a problem, but not always.

With MIR plugin bypassed (but active) same story, with plugin off (dry guitars) the correlation is fixed +1.

Pro Tools with MIR in insert.

Thanks, cheers

Bat

 

Update: switching the main Pernegg mic from "Front of the nave" to "Middle of the nave" the polarity goes to correct values.

[Dietz]

Hi Bat,

as in real life, this depends entirely on the relationship between the source (and its chosen width) and the chosen microphone(s). This is evident from the example in your "PS" where the main microphone is significantly further away from the source if the " Middle of the nave" position is chosen.

Furthermore, it makes a difference if the L&R channels of the already positioned dry signal component are out-of-phase (which indeed might cause problems when listening in mono), or if only the resulting reverb contains a lot of decorrelated / out-of-phase information (which we mostly perceive as a pleasant, sought-after spatial envelopment).

Please post some examples (screenshots might do) to facilitate the discussion! :-)

Kind regards,

[Bat]

Hi Dietz,

below 3 screenshots: 

-1 Synchron (good correlation) 

and 2 Pernegg: 

-A corr. issue

-B fixed with small changes in source/mic positioning.

The secondary mic was always in phase, so the responsible was the positioning LR of the sources I guess.

I like this MIR behavior, it confirm the authenticity of the models, I just have to keep in sight the correlation meter , which should actually be done regardless ;-)

Thanks, best

Bat

[Dietz]

Well, that's quite obvious: In your "bad" examples you were moving the sources already beyond the (hypothetical) line that separates the areas with in-phase and out-of-phase imaging. :-)

For better understanding please look at my (overly simplistic*)) scribble in the attachment: You will see three equilateral triangles. The "M" marks the Main Microphones' positions, "L" and "R" shows the position of the channel(s) with the most extreme panning on your stage. In case of the red triangle, sources between L and R will produce mostly in-phase results, sources outside this area will produce out-of-phase results (especially for the dry signal). However, in case of the green triangle the L and R "borders" would rather be like outlined by the light-grey triangle, which means that the sources that sounded out-of-phase with M1 will be still in-phase with M2.

*) "overly simplistic", as in the end it totally depends on the chosen Output Format (read: virtual microphone array) how the individual capsules will pick up sound from the stage and the hall. Like in real life. :-) Keep in mind that MIR is "just" about natural halls, with natural distances and relations between its virtual items. 

HTH,

[Bat]

Hi Dietz,

For unknown reasons the last email notification was in the spam...

I see. 

Could be a good/practical idea to implement something like your scribble in some future  MIR releases? 

Or something linked to the “hotspots” that warn about potential bad polarities zones in relation with the microphones array?

Thanks

Best wishes

Bat

[Dietz]

That's a perfectly valid idea! But the thing is: there is no ultimate "right" or "wrong". :-) MIR 3D is "just" an empty room which lends itself to any type of instrument seating and many different recording approaches.

A source position that might cause a phase problem with one microphone setup (e.g. a so-called "Blumlein pair" of two crossed Figure-8 capsules) might sound perfectly fine for the same source as soon as the Main Microphone is used in form of an M/S array with an omni-capsule for the M(id)-component. Similarly, it will make a difference whether the source's stereo width is reduced to mono, or increased to 10 or 12 meters in case of a huge ensemble. ... all of these classical coincident stereo techniques can be modeled easily within MIR 3D, all kinds of source positions, rotations and sizes will have and influence, too.

Our options get even more diverse as soon as we leave the domain of 1st Order Ambisonics and proceed to the world of Higher Order Ambisonics. Here it makes more sense to map the source positions to a spatial "sphere" than to think in terms of simple virtual microphone capsules. Consequently, possible phasing problems will most likely occur with a completely different set of parameter choices.

Finally, all of these points are moot when mixing in surround or 3D. While phasing between individual speakers is still possible on a technical level, it will be much less of an issue due to the obvious fact that all the other speakers will counteract any audible problems.

Bottom line: While I absolutely understand your quest for "once and for all" solutions, it rather makes sense to think through some basic audio engineering aspects to exploit MIR 3D to its full extent. There are full-blown, tried-and-tested Venue Presets with "Roles" for 450+ sources available which can serve as staring points for your own creations. And I'm happy to tell you that many, many Venue Presets for "empty" stages are about to be released in an update in the near future. :-)

Kind regards,

[Bat]

So an alert "Hey buddy check out your damn polarity!" is not practicable :-) Too much variables and sometimes an healty moderate out of phase is part of the real (recording) life. Isn't it?

Actually I rely on my ear and some basic knowledge/experience and I really appreciate MIR as a training tool also. It's like having the keys of the Synchron Studio, as well as many others venues. Bless the technology! After all, the problems and possibilities connected with sound fascinate so many people because it is such a huge and very difficult field. Maybe sailing is just as difficult.

I look forward to the new presets for empty stages.

Thanks, all the best

Bat

[Dietz]

@Bat said in [Phase (polarity) changes between different MIR venues](/post/317333):

So an alert "Hey buddy check out your damn polarity!" is not practicable :-)

:-D ... the most likely equivalent would be a goniometer (also known as a vector- or stereo-scope) in your listening bus, which you would want to keep an eye on:

-> https://media.vsl.co.at/images/VS_Pro_Goniometer_960x720.jpg

In fact, I have my metering visible on a dedicated screen all the time, the goniometer being one of them (albeit the RME Audio one, as it works so nicely with my audio hardware):

-> https://photos.app.goo.gl/d9vCybTonKNvNQSP9

[Bat]

Nice and clean 5.1 (7.1?) setup.

I've a TC Clarity meter but was on Lufs most of the time, now I've learned to push that button switching to VScope... 

(RME is great, all I had from this brand was excellent, now I'm thinking about a Pro multichannel converter in order to use ADAT I/O's of my actual interfaces)