Zero dBFS Defined

Robert Katz Leave a Comment

From: Jim Schley-May
10/28/00

My comments are:
Bob,

What is the precise and accepted definition of 0dBFS? Perhaps this is common knowledge, but I haven’t seen a definitive reference.

Definition 1: the mathematical evaluation of root mean square on the signal, with it’s values normalized to the range of +1 to -1. This method yields a result of -3dB for a full scale sine wave and 0dB for a full scale square wave. Sound Forge uses this method.

Definition 2: as in definition 1, but raised by 3dB. This method yields a result of 0dB for a full scale sine wave, and +3dB for a full scale square wave. Cool Edit Pro uses this method.

Which is it? I’m anxious to try calibrated monitoring levels as you’ve recommended, and I want to roll my own pink noise reference.

Thanks in advance for your reply, and I really appreciate your ongoing efforts to educate the audio world.
Hello, Jim…
It is a standard, as set forth (I believe) in AES-17.
Sound Forge is not following the rules of the standard AES-17 as set down and they are in error from the official standard by 3 dB. A couple of manufacturers have made this serious mistake. Basically the rule is as follows: The 0 dB reference for either peak OR RMS measurement is that of a sinewave at full scale. Or, to put it another way, if you wish to work with RMS measurements, the 0 dB reference for that is that of a sinewave whose peak value is full scale.
That’s the way the rule works! Even if it doesn’t seem logical to you; just think of it as a reference, and that it is IRRELEVANT that the RMS value of a sine wave happens to be 3 dB below its peak level. So what… you can (and the AES standard does) define your reference as 0 dB.

Which is it? I’m anxious to try calibrated monitoring levels as you’ve recommended, and I want to roll my own pink noise reference.

Gotcha. In that case, start with a very accurate peak-reading meter, and calibrate the sine wave to 0 dB. (measured on the peak meter). Then read the RMS value, and calibrate the RMS meter to 0 dB. This is the absolutely correct method for measuring the pink noise. At that point, if you wish to roll your own pink noise, then set it to -20 dB, RMS measured, below that full scale level.
We have a download with this pink noise signal at our downloads section!
Hope this helps,
Bob

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