On 11/19/2010 7:52 PM, Randy Yates wrote:
My question is this: What is the definition of dBFS?
Decibels relative to full scale. 0 dBFS is full scale,
everything else is down from there. -6 dBFS is half the
maximum number as with all the bits on.
If dBFS is defined as
dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave),
where XRMS is the RMS value of the digital data stream, and you're
generating a "digital square wave," then you are wrong. The digital
square wave can go to +3dBFS as defined above.
But it's not defined that way. In reality, you can't have
anything higher than 0 dBFS. That's where you run out of
numbers to express the amplitude. There's such a thing as
"intersample overload" where the input actually goes higher
than the 0 dBFS level between two adjacent samples, but
that's an anomaly.
It seems that there really is no standard definition. That's the
problem. It's not a matter of abstractness, but rather of
well-definedness.
The thing is that what you're concerned with in digital
recording is how much headroom you have. You can choose your
own headroom amount simply by choosing the analog reference
level that gives your desired headroom. If you're
compressing the **** out of everything, you don't need as
much headroom as if you're recording an orchestra or
something where you're unsure of the input dynamic range,
and you can choose a higher reference level. For most music,
20 dB of headroom is pretty safe, which is why the -20 dBFS
reference is fairly common.
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