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  #1  
Old 05-26-2004, 04:39 AM
LEDD LEDD is offline
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Default 0 VU / -18dBFS

Hello!

As a new M-box user I did a search on this forum to see what the best level would be to record at.
I found that for the M-box -18dBFS = 0VU.

But what does this mean??

I used a 1000hz test tone and set it to 0VU on the BF meter bridge (calibrated to -18).
After that I opened my PAZ meter from Waves and I noticed that this 0VU indeed was -18dBFS, but ONLY when the meter was set to 'RMS'. The PEAK level of my 1000Hz tone was somewhere around
-14.9....

Is this correct? Does this mean I should record my AVERAGE level at -18dBFS? This means the peaks are much louder.....is that a bad thing?

I'm just a little confused!!
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Old 05-26-2004, 05:47 AM
Chris Cavell Chris Cavell is offline
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Default Re: 0 VU / -18dBFS

Quote:
Is this correct? Does this mean I should record my AVERAGE level at -18dBFS?
It's a good rule of thumb on any system, digital or analog...just check to see what the particular system is meant to be calibrated at. The most intuitive way to think of RMS (root mean square) is as average...although it isn't EXACTLY average in the way most people think of average.

Quote:
This means the peaks are much louder.....is that a bad thing?

Yes, the peaks will be louder and no, that isn't a bad thing at all. If you're peaks go too high you can still clip the converters...but chances are that you'll distort your pre's first. The way digital audio works, you can have your peak volume input all the way up to but not exceeding 0dBFS without encountering digital distortion...so going higher than -18 dBFS is not necessarily a bad thing at all...it's just a good rule of thumb to keep the average volume there while tracking. Running a test tone through any of your pre's prior to tracking and setting your output level of the pre to get the -18dBFS reading will help ensure that all of your meters, VU and Peak, will be sort of intuitive looking, kind of lined up...and help to get the headroom of the pre's in a range similar to what the digital inputs can handle without clipping for the peaks.
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Old 05-26-2004, 06:58 AM
where02190 where02190 is offline
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Default Re: 0 VU / -18dBFS

That's exactly waht it means, nominal signal inputs should be targeted for -18dbfs on the M-box. This leave adequate headroom before clipping for transient peaks while maintining optimum audio quality.
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Old 05-27-2004, 03:13 AM
LEDD LEDD is offline
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Default Re: 0 VU / -18dBFS

Right thanks for the answers!

Quote:
It's a good rule of thumb on any system, digital or analog...just check to see what the particular system is meant to be calibrated at. The most intuitive way to think of RMS (root mean square) is as average...although it isn't EXACTLY average in the way most people think of average.

Ok, now I am getting curious! If it isn't exactly average, then what is it?
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Old 05-27-2004, 05:14 AM
Chris Cavell Chris Cavell is offline
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Default Re: 0 VU / -18dBFS

OHHHH, you want me to pull that stuff out...okay, it's the definition of the average of the root of the square. Some would look at this and say, well, the root of the square is just the number...not so. In many cases, just taking the average can result in singularities and zero points, where taking the average of the root of the square does not. I know this must be confusing for a lot of people...it aint easy to describe, so I'll see if I can find some info on the web to gander at for explanation, but suffice it to say this: there is indeed a difference mathematically (in meaning and calculation) between the square root of the average square and the average.

here is basically what an average is

and here is RMS


I could probably dig up some of my old physics lessons in ps or pdf form if you really want an in-depth explanation of the differences...warning: math ahead, break out the aspirin and the wine!
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Old 05-27-2004, 05:21 AM
Chris Cavell Chris Cavell is offline
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Default Re: 0 VU / -18dBFS

All Right!, I found a page that should be fairly easy to grasp for anyone (math background or not) that shows how the average and the rms can differ: can be found here.

This one is very simple and should give a good idea of how the two can differ and basically give you an understanding of the concepts even if you don't understand the integral calculus.

Hope this helps,
-Chris
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Old 05-27-2004, 06:14 AM
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Default Re: 0 VU / -18dBFS

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Old 05-27-2004, 06:43 AM
Chris Cavell Chris Cavell is offline
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Default Re: 0 VU / -18dBFS

(I was rather surprised to find a description like this one...you can see all the direct relations to audio... )
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Old 05-27-2004, 07:23 AM
LEDD LEDD is offline
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Default Re: 0 VU / -18dBFS

Yes...that made it all a little clearer, thanks man!
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Old 05-27-2004, 07:58 AM
Chris Cavell Chris Cavell is offline
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Default Re: 0 VU / -18dBFS

Okay, so a little homework for you then:

What is the equation for average rms?




































































Ans: <sqrt(<x^2>)>

It's different too! (Which is why wavelab has all those different values for RMS...a peak rms, an average rms...etc...)
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