On 19/01/2012 9:03 AM, Linda Seltzer wrote:
Why and under what circumstances is it advantageous to set up the Y axis
as dbFS rather than dbV, dbSPL,
out of the ones you mention, dBFS is the only one that has any meaning
in the digital domain -- since as we've established, the others don't
have a well defined mapping from sample values to volts or sound pressure.
or a linear scale?
That would be the same as asking why use a logarithmic scale at all. (1)
because it has some perceptual relevance, (2) because it's a convention
that is commonly followed so people are used to reading dB graphs.
If you use a dB scale
that is referenced to the lowest volume rather than to the loudest level,
then the issues of a clipping level all go away.
Well you can't reference the dB scale to silence since that is -inf dB.
What would be "the lowest volume"? an LSB? Then you'd get vastly
different peak values for a 24 bit and 16 bit spectrum. A 24 bit signal
is not 48dB louder than a 16 bit signal so it doesn't really make sense
to reference off the LSB.
What do you learn from a
graph where the Y axis is in dbFS that is different from what you would
learn if you use dbV or dbSPL?
I can't think of much.
But it makes no sense to use dbV or dbSPL for a digital system unless
you have a well defined mapping from 0dBFS to a reference voltage and/or
sound pressure level.
Is there anything perceptually that would cause one to plot the data one
way or the other?
Is there anything you would learn about, in terms of problems,
distortions, etc., when using one type of dB over another?
Obviously the frequency weighted measures are weighted for a reason (you
can look it up).
If you were coupled to a physical/analog system you might be more
interested in the relation to analog than if everything is numerical.
For example if you were driving an analog circuit and you knew how it
behaved you might be interested in dBV because you don't want to drive
the circuit to clipping (or you do). Similarly for acoustically coupled
systems.
There are other people doing all of their graphs this way and I haven't
been able to discern from them the reason.
I would expect it to be standard practice for DSP algorithms that have
no well defined voltage/SPL relation to the analog domain.
You havn't put forward an alternative that makes sense in the digital
domain.
Ross.
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