| 
  • If you are citizen of an European Union member nation, you may not use this service unless you are at least 16 years old.

  • You already know Dokkio is an AI-powered assistant to organize & manage your digital files & messages. Very soon, Dokkio will support Outlook as well as One Drive. Check it out today!

View
 

Correlation

Page history last edited by Randy Coppinger 12 years, 7 months ago

If anything here is confusing, inspiring or absolutely incorrect your comments would be much appreciated.  This is a work in progress and your help improving the information is requested.  Thanks!

 

< The Cocktail Party Effect

 

Of all the stereo concepts, Correlation seems the most difficult to understand and achieve.  Correlation is actually a mathematical way to look at two sets: the more they have in common, the more they correlate.  For example, if two sets of numbers match exactly, their Correlation value is 1.  But if they share none of the same numbers, their Correlation value is 0.  Most stereo mixes fall somewhere between 1 and 0, which is to say, some of the signal in both channels is the same and some is different.

 

As we have seen, if we want to Localize a discrete sound between speakers, the information in the two channels must be very similar.  We can differentiate by volume, delay and/or EQ to help position it, but it still has to be pretty similar if we want to know where it is.

 

On the other hand, Early Reflections and Reverb do not benefit from the sameness we need for locating sound.  In person we tend to experience Ambience as highly De-correlated, with wildly different timing, volumes and frequency.  So it makes sense that we should desire De-correlated Ambience on speakers too.  On one hand Localization works well with Correlated sounds.  On the other, our sense of Spaciousness seems to work better when the information is highly De-correlated.

 

The actual math isn’t so much what’s important.  We could just as easily describe the differences in terms of interaural Coherence – what is the same and what is different about the information presented to ears?  The point is we need similarity for accurate Localization, and by contrast, we need the Ambience to be very different from the direct sound.  In addition, the Ambience should be De-correlated from itself when you compare the left signal to the right signal.

 

What does this mean practically?  Well, if you are recording a drum kit in a room and you are using “room mics” to capture the Ambience -- to present the listener with the spaciousness of that room -- Correlation is the key.  If you have a lot of direct sound in the room mics, that Correlates with the direct sound in the mics aimed directly at the kit.  The more you are able to separate the sounds of the room from what is in the mics aimed at the kit, the more De-correlated they will be, making the presentation more Spacious.  And if you are using two room mics, the more De-correlated those two mics are from each other, the more compelling your presentation of that space. 

 

When we start looking at Mic Arrays, consider how similar and how different the information picked up by each of the two mics.  Room mic arrays that have a lot of differences (in time, volume & frequency) between the two mics would be better choices for your drum kit room mics.  Likewise the more a recording engineer can isolate those room mics from the direct sound of the drum kit, the better.

 

Floyd Toole has suggested that the size of the Sweet Spot – the area between the speakers where Phantom Center is stable and the stereo presentation is most effective – is all about Correlation.  Toole suggests that if the direct sound image is very highly Correlated and the Spaciousness in the recording is very highly De-correlated, the Sweet Spot is larger.  So if we want to make a stereo presentation that sounds good as we move around in front of the speakers, thinking about Correlation is important.

 

We first understood stereo simply as two channels with some difference between them.  While this is true, Correlation refines our understanding.  Compelling stereo seems to require a balance of things that are similar and things that are different, depending on the kind of stereo presentation we have in mind.

 

Phantom Center?  Highly correlated, virtually 1.  Big, wide Ambience?  De-correlated.  And when these two different kinds of signals are contrasted by their high Correlation and high De-correlation, the stereo presentation is effective for a larger area between the speakers.

 

Listening On Speakers >

 

Audio Wiki front page

Randy Coppinger - who I am and other stuff I'm doing.

 


Comments (0)

You don't have permission to comment on this page.