dBFS & VU

[Rolo Tomasi]

I've read elsewhere that -14 dBFS is equivalent to 0 VU. Does that sound about right?

 

Is there a way to calculate what Logic's dBFS would be be in VU terms?

[Jope]

I've read elsewhere that -14 dBFS is equivalent to 0 VU. Does that sound about right?

 

Erm, to be honest, it doesn't... dBFS is a scale for peak amplitudes in relation to the maximum in a given digital resolution, and VU is a kind of RMS scale referring to the electrical power of a signal. A dBFS meter might show the same value for different waveforms where the VU meter would show different values (or the other way around). What is worse, there are different interpretations of both technical terms. Have a look into Wikipedia, search for dbfs and for vu, it's well explained there.

But may we know what technical problem hides behind your question? Maybe someone could give you a more useful answer.

[fader8]

I've read elsewhere that -14 dBFS is equivalent to 0 VU. Does that sound about right?

 

Sure. But so does -12, -18 and -20dBfs.

 

There's no actual hard calibration standard for Zero Volume Units and its relationship to headroom below 0dbfs. You can think of this as more of a "house rule" rather than a standard. For example, K-system digital meters allow you to select which method you'd like to use. There are accepted norms for various applications, eg broadcast video, feature films, DVD, etc., and some of those are actually standardized, (somewhat).

 

If I recall correctly, ProtoolsHD systems ship with 0VU at -18dBfs which translates to +4dbm at the analog outputs. For Logic, you can get a number of metering plug-ins now that do K-system metering allowing you to choose your "0VU" reference. Inspector XL, UAD Precision Limiter, etc.

 

Hope that helps?

[guavadude]

For example, K-system digital meters allow you to select which method you'd like to use. There are accepted norms for various applications, eg broadcast video, feature films, DVD, etc., and some of those are actually standardized, (somewhat).

 

 

Anyone have a list of the somewhat standardized references?

[LukeDunn]

OK.. I could be wrong (and correct me if I am)

 

But like Jope said, dbFS is a peak value and VU is a RMS (Root Mean Square) value. Peak value being the highest point in the signal (like the peak value of a sine wave) and RMS being the average.

 

And if dbFS is a peak value, and VU is a RMS value.... If you wanted to view the output of logic in VU (RMS) you could insert the level meter plugin on your output and set it to RMS values.

 

 

Luke

 

edit.... I found this link that kind of explains it

 

http://www.bcae1.com/voltages.htm

[Rolo Tomasi]

Thanks for the replies. All very knowledgable and helpful.

 

ProtoolsHD systems ship with 0VU at -18dBfs which translates to +4dbm at the analog outputs

.

 

A gentleman on the Apple forum says the same is true for Logic. I'm not sure I understand the last part of what you said though:

 

which translates to +4dbm at the analog outputs

 

What is the difference between VU and dbm?

[fader8]

which translates to +4dbm at the analog outputs

 

What is the difference between VU and dbm?

 

This is simply referring to what the electrical level is at the outputs of your interface relative to what your virtual signal level is in the software. Depending on your particular interface, this might be adjustable.

 

Remember that VU also implies a certain meter ballistic. Calibrating a VU meter to a steady state signal doesn't mean it will respond the same as a digital meter when monitoring complex audio signals.

 

It all comes down to what you're producing for, your personal preferences, and what kind of external gear your driving signal with. Then you'd pick the appropriate metering solution. If you're looking for something more authoratative but still written in laymans terms, Bob Katz's Mastering Audio does a decent job of overviewing this. Beyond that, there's a ton of AES Journal articles available that delve into this subject in excrutiating detail.

 

Be careful if you're just googling for this kind of info. There's as much bad misinformation out there as there is good!

[musick]

...dbFS is a peak value and VU is a RMS (Root Mean Square) value. Peak value being the highest point in the signal (like the peak value of a sine wave) and RMS being the average.

 

And if dbFS is a peak value, and VU is a RMS value.... If you wanted to view the output of logic in VU (RMS) you could insert the level meter plugin on your output and set it to RMS values.

 

Also db's are always a ratio. dbFs is the ratio related to digital Full Scale, vu or dbm reference to a specified analog signal level.