# pink noise, correlated vs uncorrelated



## CobraVin (Jul 31, 2007)

i know one is stereo, the autosound 2000 disc says to find an in between

my rta's are different, should i lean towards one or the other, thanks


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## ErinH (Feb 14, 2007)

isn't correlated where they are in phase, or am I wrong?

I listened shortly to them on a laptop computer and it seemed the uncorrelated was out of phase. _If_ that's the case, then I'd shoot for the correlated measurements. 

*just guessing here*

Now that I think about it, maybe the uncorrelated is designed to center with.  

I'd like to know the answer myself.


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## chad (Jun 30, 2005)

Think about the word correlation, that will explain it all.


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## ErinH (Feb 14, 2007)

chad said:


> *For example* a correlation meter shows phase difference between two channels.


This is supposed to help.

Why does it not?...


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## chad (Jun 30, 2005)

I edited, 

http://en.wikipedia.org/wiki/Correlation

Look at this section :Correlation and linearity:


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## CobraVin (Jul 31, 2007)

i know one is stereo, thats why i put that in there

but my question is, should i use one more than the other for eq ing or just split the difference, didnt know if a car environment leaned more towards one or another

the cd tracks that play left and right channels seperatly are un correlated, so im just trying to guestimate, thanks


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## chad (Jun 30, 2005)

I use correlated because I feed it a mono signal when tuning.

Sorry about the vauge response last night, Mrs, Chad was frisky and, well, you know. I'll try to get a better response typed up soon.


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## CobraVin (Jul 31, 2007)

actually the left and right tracks just say "pink noise 20hz to 20khz left channel only"

hey bro no need to appoligize for that, i understand completely, priorities,in fact i was just about to go to home depot to pick up some concrete for 15 sections of fence im putting up and noticed how quiet it is in my house, hey the baby is sleeping and the older one is in school, hmmm, i can still make it into work by 4, hopefully the wife is not napping, ....ummm, bye


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## CobraVin (Jul 31, 2007)

yeah oh well, at least i made it to home depot and work on time

ok so Chad likes correlated, anyone else?

i am measuring left and right seperatly but wondered when i play both sides with correlated vs uncorrelated which is more accurate to whats really going on, or should i just stfu and split the difference,thanks


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## squeak9798 (Apr 20, 2005)

From Kevin7909 on ECA:

_It does make a difference whether it's in stereo or mono. A large difference actually. Stereo pink noise is 'uncorrelated'. Therefore it is not associated with the pink noise on the other channel. This minimizes problems with standing waves, etc. Mono pink noise does not share this attribute. This is easy to test. Play stereo pink noise and try to set your time alignment. You can't.

The reason is that the left and right speakers must have a similar signal for us to compare the distance between them based on time differences. We will place the soundstage using SPL with stereo pink noise. Mono pink noise has the same signal through both speakers at the same time so we can compare the time differences between the two sources. Therefore, we can hear the soundstage moving across the vehicle with mono pink noise while changing the TA settings on our processors.

*Basically we set frequency response using uncorrelated (stereo) pink noise. We set time alignment by ear using correlated (mono) pink noise.* (bold emphasis mine)

Kevin_


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## TiHKaL (Feb 9, 2014)

Apologies for a thread from the dead, but I think there is an issue with "Correlated Pink Noise" on the Autosound CD.

I wanted to make a longer noise track, 1:40 wasn't enough. I loaded the track into Audacity to copy and paste a longer track. This is what I found:

They don't match. The L & R waves are the same but they are misaligned by a ~0.02ms.

Is this enough to be significant? It's the same throughout the whole track.










I made this by copying the left channel onto the right. Was it a waste of time?


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## Jepalan (Jun 27, 2013)

I copied this from Home Theater Shack, but it is the explanation that makes most sense to me...

Correlated versus uncorrelated applies to signals coming from multiple speakers at the same time while measuring. If correlated, then the same measurement signal is coming from the speakers in phase. This is not good for determining EQ settings, you will end up with phase cancellation at some frequencies, phase reinforcements at other frequencies, and will get a totally crazy frequency response curve. Move the measurement mic a few inches, and you get a totally different crazy frequency response curve.

By using uncorrelated pink noise, the measurement signal coming out of each speaker is random and completely unrelated to all the others, so there will be no phase cancellations or reinforcements, and you get a sane looking frequency response curve.

Which one to use when? Always use uncorrelated. The only use for correlated pink noise for taking measurements would be when you want to play a trick on someone who doesn't know any better and have them waste a bunch of time. That would not be nice.

Of course, you only run one speaker at a time while equalizing, so none of this applies to the normal measurement/equalization process. It can be useful/interesting to do it after equalization is complete, to get a curve showing how all the speakers are working together.


Read more: Measuring Car Frequency Response - Page 7 - Home Theater Forum and Systems - HomeTheaterShack.com


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## TiHKaL (Feb 9, 2014)

I understand what it's for.

"*We set time alignment by ear using correlated (mono) pink noise.*"

But in the case of the Autosound CD the Correlated Pink Noise is out of phase by ~0.02ms


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## Jepalan (Jun 27, 2013)

TiHKaL said:


> I understand what it's for.
> 
> "*We set time alignment by ear using correlated (mono) pink noise.*"
> 
> But in the case of the Autosound CD the Correlated Pink Noise is out of phase by ~0.02ms


By bad. I didn't read through to end carefully.
I don't have that CD, but looks like copying left onto right fixed the problem. 
I could be wrong here but isn't .02 mS less than 3-tenths of an inch error in driver placement? or half a wavelength of 20KHz? Would it matter?


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## TiHKaL (Feb 9, 2014)

344 x 0.02 = 6.88mm.

Alpine have it right.

Alpine imprint - 03 - Pink Noise.wav


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## 14642 (May 19, 2008)

I use both. It's especially important if you are tuning a system with an upmixer that does ambience and center channel processing. Correlated pink noise will be steered to the center mostly and uncorrelated will be played by all the speakers. For non-upmixer systems, it can also be helpful to use both. MUCH of the information in music is recorded in mono, so the measurement of the correlated signal will be approximately the frequency response of your center image and the response of the uncorrelated signal will be the frequency response of the stereo information.


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## dengland (May 25, 2014)

Jepalan said:


> I copied this from Home Theater Shack, but it is the explanation that makes most sense to me...
> 
> <SNIP>
> 
> Read more: Measuring Car Frequency Response - Page 7 - Home Theater Forum and Systems - HomeTheaterShack.com


Thanks for the reference. The paper referenced in Post #14 was a good read too.


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