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Old November 22nd 10, 03:05 PM posted to rec.audio.tech,rec.audio.pro,comp.dsp
Mike Rivers
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Default dBFS

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
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