XPost: rec.audio.pro, comp.dsp   
   From: mrivers@d-and-d.com   
      
   On 11/21/2010 9:02 PM, Randy Yates wrote:   
      
   > The question in my mind is this:   
   >   
   >    1. How is dBFS defined? If there is no formal definition, then   
   >    how is it used?   
      
   You seem to think that the formal definition - that it's a   
   SCALE and not an absolute value, is inadequate, but that's   
   all there is. Sorry.   
      
   It's like asking how Fahrenheit is defined. You can define   
   212 degrees F as the temperature at which water boils (under   
   specified conditions), but you can't define what "a   
   Fahrenheit" is.   
      
   You've also been explained how dBFS is used. Once again,   
   with feeling, any value below zero tells you how much   
   headroom you have. That's all. It doesn't tell you how loud   
   something sounds unless you add time and duty cycle (which   
   will tell you how much air gets moved at the end of the   
   chain), and you can't use it to determine how many volts   
   will come out the analog end or how many volts coming in   
   will get you to a specific number on the dBFS scale.   
      
   > Specifically, what I was wondering was this:   
   >   
   >    1. Is dBFS an instantaneous measurement or some sort of average?   
      
   The only significant number is instantaneous. You could   
   calculate an average when you know all the values within the   
   bounds over which you're calculating it. Although some   
   programs do that (and give you a number) because they CAN,   
   it really isn't of general use. I think that's why everyone   
   is dancing around an answer for you. We don't use "average   
   dBFS" so we don't care. There are other, more useful averages.   
      
   >    2. If dBFS is some sort of average, what type? RMS? RMS sine? RMS   
   >    square? Averaged magnitude? Any of an infinite number of ways to   
   >    define it?   
      
   Will you accept "no" as a temporary answer until you   
   yourself define how you'd like it defined, and for what purpose?   
      
   > I admit there is some ambiguity built into the question itself since, in   
   > my understanding and teaching, a dB (of WHATEVER) can always be traced   
   > back to a ratio of two powers, and power, as I understand it, is always   
   > an average [Note 1].   
      
   Classically, dB is a ratio of measured power to a reference   
   power. Mathematically, however, it's a ratio of anything you   
   want. It doesn't have to be power, and it can be an   
   instantaneous measurement. In the case where we use dBFS,   
   the reference is 0 dBFS, and the amplitude of any single   
   sample, as instantaneous as you can get, can be compared to   
   the reference for a single value of dBFS.   
      
   We can, in fact, even do better than that. Under certain   
   conditions, with or without less than theoretically perfect   
   filtering, it's possible to calculate a value greater than 0   
   dBFS given data from two or more adjacent samples. This is   
   NOT an average, however. And it's real. It's a source of   
   distortion in A/D conversion.   
      
   > Also, let me acknowledge that many folks have already answered my   
   > question.   
      
   Can we give it a rest, then?  What does Hitler have to say   
   about dBFS?   
      
   --   
   "Today's production equipment is IT based and cannot be   
   operated without a passing knowledge of computing, although   
   it seems that it can be operated without a passing knowledge   
   of audio." - John Watkinson   
      
   http://mikeriversaudio.wordpress.com - useful and   
   interesting audio stuff   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)