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Measuring 0 dBFs reference level on Universal Audio Apollo or any digital gear !

January 20, 2014

Hi all,

this post is to clarify some of the notions I feel are definite must know in this  digital recording era !

It is intended to further a video presentation made by Fab Dupont on PUREMIX.NET on recording levels concepts.

In my mind this is basically what we were tought when I did Audio engineering school, try by practice to grasp real world concepts that are essential to proper recording with in my time analog gear and now days digital gear.

I feel that as digital has furthered us away from electricity it is a needed thing to go back to it to better understand recording properly music !

I ll touch properly gain staging Apollo for recording, 0 dBFs convention and headroom in the Apollo and in general.

Before I complement what Fab already explains in the video on recording levels, here is the link

to the PDF file of complete blog post on Measuring 0 dBFs reference level with pictures !

Fab already answered the question regarding any rig be it -16 dBFs,-18dBFs or U.S type -20 dBFs...

You should record at an average -18dBFS !

Now more specifically on an Universal Audio Apollo, here is how:


Fab towards the of his video talks about Geek level II notions:

He shows us a picture of the spec of a bad converter and talks about how bad the specs it showing us the maximum output of the converter !

This is Rule No1 – the rule is :
The loudest your D/A can output is 0 dBFs , if you know your maximum output level you know the 0 dBFs level , hence the reference !

We now have a new definition the difference between the maximum output level of your converter (without clipping) and + 4 dBu is your converter headroom !

Let's REWIND, again said another way : Headroom is the distance in dB before clipping (0dBFS) compared to a 0 Vu level ie +4dBu

Fab goes on to explain that if the maximum output level is + 10 dBu , since we know the reference
0 Vu input is an input level of +4 dBu , the headroom in this case is 10 dBu – 4 dBu = only 6 dB !

Hence for that crappy converter 0 dBFs is referenced at – 6 dBFs !

You know your headroom and your 0 dBFs spec, by just by looking at your converter output spec !

Case in Point applying what we learned to Apollo now, the specs reads Hardware manual page 30 and 31.

Maximum Output Level 18 dBV

So how do we go from dBV to dBu ?


Well you must read, this :

www.sengpielaudio.com/calculator-db-volt.htm

it should help and clarify a few things about levels and voltages!

And absolutely also this (the clearest resume of it all...):

www.tangible-technology.com/audiobasics/levels/level.html


TO RESUME the answer is 18 dBV = 20.218487499 dBu

So in short Apollo ‘s converter are referenced to -20 dBu !

If you read some more you'll know this is normal since this a U.S gear and this what U.S pro Gear use as 0 dBFs reference ie -20 dBFs

But now we have Geek level number III (ready for the headache :) )


here is an extract from UAD system manual on Page 81

Operating Levels
Except as noted in Table 3 below, the internal operating level of most UAD Powered Plug-Ins is typically –18 dBFS. 0 dBFS is calibrated to +4 dBu with 18 dB of headroom, so 0 dBFS is the equivalent of +22 dBu in the analog do- main.

Table 3. UAD Powered Plug-In Internal Operating Levels
UAD Powered Plug-In Typical Internal Operating Level 0 dBFS Equivalent

Most
–18 dBFS
+22 dBu

Ampex ATR-102 Teletronix LA-2A Limiter Collection
–12 dBFS
+16 dBu

Manley Massive Passive
–14 dBFS
+18 dBu

Neve 33609 (selectable)
–14 dBFS –18 dBFS –22 dBFS
+18 +dBu +22 +dBu +26 +dBu

Studer A800
–12 dBFS
+16 dBu

Some more reading of interest on the subject:

www.uaudio.com/blog/digital-versus-analog-metering/

www.gearslutz.com/board/music-computers/...ng-uad-plug-ins.html

support.presonus.com/entries/22234380-St...metering-dbFS-vs-dbu

 

And now let's go to GEEK LEVEL Number IV

Don't take my word for it

I will detail how you can learn the proper way ie learn to measure this value by yourself !

So how do we measure the 0 dbFS reference level electrically on the Apollo and prove
it is set at -20 dbFS !


You will need an electronic voltmeter (DMM) ,

the definition of 0 db Vu = + 4 db u = 4 * 0,775 mV = 3,1 V peak to peak in volts on a scope and 1,228 Volts RMS on your DMM If it reads correctly true rms ( at 1khz)

1 dbu where u stands for unloaded ( by the way 0,775 volt produces 1 mWatt of power with a 600 ohms load )

A signal generator ( any output of your sound card or alike) ,
OR create a pure tone with one of your signal generator plugs a sine at 1 khz.

Route it to ouput 1 of your Apollo and adjust the output to read 1,228 V AC ( true rms) on your voltmeter between ring and sleeve (ground) of a TRS ( jack) cable !
      


Then feed ( back) that signal into one of the apollo inputs .

Record and playback and measure the output levels.


try it on your Apollo and you will be in for a surprise !

They are three or four pitfalls out there for YOU!

On my daw to get an output at +4dBu I set the level of the signal generator plug at – 10dB !

 



I then read between tip and sleeve of my stereo patch jack 1.227 V, (this is the voltage level for +4 dBu remember).

Since I am a perfectionist I also plug an XLR to unbalanced TRS cable in that same output from my say Focusrite ISA One line input that so happens to have an old style VU and a 0 dBFs led display !...

Fine I read 0 Vu (and by the way -18 dBFs on the led display ,ie it's not aligned at the same reference level remember Focusrite is “british”)



Now , If I record the signal and play it back same thing happens the reverse way I get a -10 dB reading on the DAW output meter and a + 4 dBu between tip and ring!

But wait a minute here , if you do the math
going down to 0 -4 ,
going down to -10 = -4 -10 = -14 dB

So where the hell is my -20 dB figure ??

Pitfall Number 1 (measuring voltages on Balanced versus Unbalanced)

remember I told you I have a stereo patch cable ! Well good thing !

Now try measuring voltage between tip and ring and not tip and sleeve !

Well you will find double the voltage 2.454 Volts.

Why ? because if you read the apollo manual it says :

“The individually addressable line-level analog outputs use balanced 1⁄4” TRS phone jacks “


Pitfall Number 2 (Power versus Voltage dB difference)

I am sure you have heard somewhere that 3 db power difference doubles the power, so what about voltage ?

Here we have a doubling of the voltage.

Then how do we relate that in dB ?

Well the rule say it is 3 dB for doubling the power and 6 dB for doubling the voltage. (you will find the formulas in my references).

HAHA so since we are dealing with a doubling of the voltage we are in fact dealing not with +4dBu but with +4+6 = +10 dBu !

Hence going down from +10 to 0 = -10 – 10 (sig gen level) = -20 dBFs

Now wait a minute before you said you connected a ISA one and measured 0 dB VU !

Well remember I said I used an XLR to unbalanced TRS cable ? , there exactly lies the problem !



The ISA ONE VU measured the same voltage as my digital multimeter !
Ie 1.227 Volts remember, that is +4 dBu !

This is because of my unbalanced TRS cable and this TRAP Number 3

So the moral of this story is:

Balanced is different from Unbalanced : the rule - 6 db going from Balanced to Unbalanced.

Know your cables and how the pins are connected inside an XLR and a TRS when reduced from Balanced to Unbalanced....

Watch out witch cables you use when measuring voltages......

Remember that Voltage and Power relate to each other by a factor of two,


+20 dB Voltage increase multiply voltage by 10
+10 dB Power increase multiply Power by 10

+3 dB power increase power by a factor of two.
+6 dB Voltage increase Voltage by a factor of two.

Now you should have the method and the tools to measure 0 dBFs on ANY gear,

and grow independent from master and manufacturers !

Et voila

Eric

PS: You will need also a good VU Meter plug to make all this easy ! klanghelm.com/VUMT.html

 

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