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Author Topic: Examples of Diffuse Field Correlation (DFC) - hear what it's about  (Read 684 times)

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Offline Gutbucket

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I commonly mention the term diffuse field correlation (DFC) in posts here because it's an important aspect influencing the sound of our live recordings, especially audience perspective recordings which tend to have significant ambient content.  Yet I suspect not everyone here understands what I'm talking about when using that term, and some may even roll their eyes when I bring it up as that term is admittedly so very acoustics-geek-tech sounding!

What is it?  Well in short, correlation is one of several possible measures of similarity between signals.  We need good correlation between the Left and Right channels for the main elements on which the recording is focused in order to achieve good solidity and stereo imaging - these are the direct field components of the sound.  In contrast to that, the diffuse field components consist of sound arriving from all directions equally - primarily the ambient and reverberant components of the recording such as the sound of the room, audience reaction, etc.  In practical terms this can be though of as the sound arriving from all other directions outside the direct-sound main window (in Stereo Zoom terms all sounds arriving from outside the SRA), and this diffuse component is optimally captured and reproduced with low correlation between signals. 

A good goal is producing recordings which have a highly correlated direct sound component, yet low correlation of the diffuse sound component.

Why and what does that mean in less academic terms?  In subjective terms, low DFC is commonly described as sounding "big, wide, open, ambient, spacious, diffuse, airy, voluminous, 3-dimensional," etc.  Whereas high correlation might be described as sounding "more narrow, flat, 2-dimensional, but more defined, sharp and precise". 

Here's a link to a page at Helmut Wittek's hauptmikrofon website with sound sample examples of the same source recorded using several different microphone setups- http://www.hauptmikrofon.de/audio/diffusefield.html.   All of these are coincident microphone setups using Schoeps microphones.  You can switch at any time between omni, X/Y mk4, X/Y mk41, Mid/Side (mk4/mk8), and Blumlein (mk8/mk8).

What I suggest is this-  Listen through headphones.  First listen with only one ear (doesn't matter which), by simply removing the headphone from your other ear.  Notice that all the samples sound pretty much the same.  Then listen with both ears and notice how different they sound from each other.  That difference is primarily due to the differences in diffuse field correlation of each microphone setup.

You can do the same thing listening over speakers, by connecting or disconnecting one speaker.  Don't sum the two channels to mono, that's doing something different, introducing other variables and complicating things.  You can however send either the left or right channel to both playback channels

The sample is a recording of ambient sound in a university hall and pretty much all diffuse ambience with minimal direct sound of significance, making the differences in DFC very clear.  By analogy, it's sort of like just the room and audience noise part of the recording, without the direct sound component from the stage or PA.
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Offline heathen

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Re: Examples of Diffuse Field Correlation (DFC) - hear what it's about
« Reply #1 on: August 04, 2017, 11:33:20 AM »
A good goal is producing recordings which have a highly correlated direct sound component, yet low correlation of the diffuse sound component.

In some situations (stack taping, a room/arena that sounds bad, etc) we might want to eliminate as much diffuse sound as possible, right?  In that instance, will diffuse sound be more or less noticeable when it has low correlation?

I guess what my question amounts to is whether high or low correlation of diffuse sound can have an effect on the degree to which we perceive it in relation to the direct sound?
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Offline Gutbucket

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Re: Examples of Diffuse Field Correlation (DFC) - hear what it's about
« Reply #2 on: August 04, 2017, 11:53:06 AM »
A good balance between direct and diffuse is best.  The most appropriate balance is somewhat subjective and there is leaway, but in many cases we don't really have as much control over the direct/ambient balance as we might like.  We can really only control it by recording position - how far we are from the source.  And to a much lesser extent by mic pickup pattern and angle, but only in a much more limited way.  Mostly it's proximity to the source.

No diffuse sound at all is like a straight, dry SBD with no 'verb at all on it.  That has good clarity (which is a big part of the battle when we are often swamped in diffuse sound), but no sense of space, location, liveness, there-ness.  The closest we get to that is a stack tape close to a very loud PA.  Even stack tapes close to moderately loud PAs can have significant ambient diffuse sound to them.  Anything from further away than right up at the stack will have significant diffuse sound component, regardless of how we record, and that's good.  We need to manage that as best we can in a crappy sounding place, but there are tricks to making it sound better than it might live.

Not so much a question of noticeable or not perhaps as much as acceptable and listenable or not.  What is there sounds better or worse.  Keeping the direct sound correlated and the diffuse stuff with low correlation allows us to more easily mentally separate the direct form the diffuse sound, so you can hear the direct sound and ignore the diffuse stuff even though it's still there at the same level.  That's related to the "cocktail party effect" and how we can focus attention selectively in the real world or in a sufficient stereo reproduction of it.
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Offline bombdiggity

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Re: Examples of Diffuse Field Correlation (DFC) - hear what it's about
« Reply #3 on: August 04, 2017, 07:35:49 PM »
I think I had some recent experience with this aspect though it's still slightly a mystery what happened that night. 

It was in one of my favorite rooms for sound and clarity and I have a usual spot about 5 rows back (which is actually also up since it has a steep slope). 

Almost everything I've recorded there has been really nice but I actually think the last one I should have been a little further back than usual.  Usually I run smaller groups there and have been very happy with the results but the last one was an 18 piece big band and that one seems a little dry (even to me).  Maybe they were just running a little too much of the PA, which usually isn't needed or a factor, that night, but it has a different feel than all the others from there.  I've really liked the other stuff I ran there (from duo to about 10 piece). 

It's a narrow room which may be the problem.  I ran the same 18 piece much closer in a very wide room and really liked that but it had a much more directional quality and I was ahead of the PA so it was a different feel. 

Live and learn I think.  The opener was a small group and that has more than enough diffuse sound. 

There's definitely an interaction between direct and diffuse and apparently that arc moves a bit from show to show, even in the same room.  It feels like I got caught a bit somewhere just around no man's land that night where the direct is satisfying but the diffuse not.  OTOH the marimba and piano are really clear and up front which was not always the case.  The horn sections usually rule the sound in this group. 
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Offline Gutbucket

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Re: Examples of Diffuse Field Correlation (DFC) - hear what it's about
« Reply #4 on: August 07, 2017, 10:19:26 AM »
OTOH the marimba and piano are really clear and up front which was not always the case.  The horn sections usually rule the sound in this group.

All speculation on my part but clear and upfront piano would seem to indicate strong PA reinforcement.  Horns are highly directional and loud, so the sound guy may have been balancing the PA mix against the sound of the horns in the room, pushing the levels of everything else and making compromises through the PA.  Kind of like the PA issues balancing with loud guitar amps in a small club.  The mixer has less control and the room often gets over driven.
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