On Sat, 20 Nov 2010 09:21:25 +0000 (UTC), glen herrmannsfeldt
wrote:
In comp.dsp Eric Jacobsen wrote:
(snip, someone wrote)
Nonsense. All you've given is the meaning of the acronym, not an
engineering definition of the unit. This is similar to stating the
definition of RMS is "root mean square."
(snip)
I think you're asking what color the sky is, and people are telling
you "blue", but you're expecting a wavelength or something, so you're
not accepting the answer.
As you know, dB measurements are always relative to some reference
level. With dBFS the reference level is Full Scale of the converter
or number system or whatever. The ratio of the level measured to the
Full Scale level provides the argument for the logarithm, and the
scaled result is dBFS.
But there is more to it than just the reference. Well, if you
just measure one sample then that is all, but for a signal
of some duration, it is more complicated. I can, for example,
compute RMS for a whole CD track. I could also compute the
mean of the absolute value, the geometric mean of the absolute
value, or many other mathematical functions of the samples.
If I have a sine that reaches peak at exactly a sample point,
and reaches full scale at that point, then RMS is 5 log(2),
or about 1.5dB lower. For mean absolute value, 10 log(2/pi),
or about 1.96dB lower.
Yup. If you have a different measure, you'll get a different value.
Why is that confusing? People in our area don't normally confuse
amplitude and power, or even various ways to measure power. Why does
it become so difficult when you plug it into a simple equation?
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com